HYDROGEOLOGICAL SURVEY REPORT
Telephone Number 0722- 875980 P.O. Box 5392 00100 NAIROBI
Report Reference Number WD/WRP 001/11/548 Date 16th October 2014
|1.0 GENERAL INFORMATION
1.1 Site Location…………………………………………………….
1.2 Water Demand…………………………………………………..
2.0 BOREHOLE DRILLING
2.1 Drilling Methods…………………………………………………
2.2 Plumbness and Alignment………………………………………
3.0 BOREHOLE DESIGN
3.1 Borehole Logging……………………………………………….
3.3 Screen Material………………………………………………….
3.4 Slotted Metal Pipe…………………………………….………….
3.6 Design and Construction Factors………………………………..
4.0 DEVELOPMENT OF BOREHOLES
4.1 Development Methods…………………………………………..
4.2 Other Methods…………………………………………………..
5.0 GROUNDWATER CHEMISTRY
5.1 Water Quality………………………………………………………
5.2 Water for Irrigation………………………………………………..
6.0 WATER QUALITY PROTECTION
6.1 Groundwater Pollution…………………………………….……..
6.2 Sources of Danger………………………………………………….
6.3 Groundwater Protection……………………………………………
6.4 Health Criteria…………………………………………………….
6.5 Groundwater Protection Zones…………………………………….
7.0 GROUNDWATER MODELS
7.1 Data Requirement…………………………………………….…..
7.2 Groundwater Recharge and External Factors…………………….
7.3 The Safe Yield or Reserve………………………………………..
7.4 Impact of Borehole Drilling………………………………………
7.5 Groundwater Flow Velocity……………………………………….
8.0 EFFECTS OF GEOLOGIC BOUNDARIES
8.1 Geomorphic Controls on Groundwater……………………………
8.2 Occurrence and Movement of Groundwater………………………
8.3 Geologic Controls in Groundwater……………………………….
9.0 GROUNDWATER EXPLORATION
9.1 Surface Geophysical Methods…………………………………….
9.2 Electrical Resistivity Method…………………………………….
9.3 Selected Exploration Method…………………………………….
10.0 EVALUATION OF PUMPING TEST DATA
10.1 Step Drawdown Tests……………………………………………
10.2 Water Level Recovery Data……………………………………..
10.3 Estimating Local Aquifer Parameters……………………………
- GENERAL INFORMATION
- Site Location
Land Reference Number Nairobi/Tassia Block 97/1428/017 is located on Map Sheet Number 148/4 in Tassia area of Nairobi County. The site is about 1.5 kilometres west of Embakasi Village. The elevation of the site is approximately 1624 metres above mean sea level. A copy of the map indicating the approximate location of the site has been presented below where the scale of the original map is 1:50,000. The coordinates of the selected site are as follows:-
|36o 53′ 49.2″ East and 1o 18′ 20.8″ South
37M 0265995 ; 9855576
APPROXIMATE SITE LOCATION
1.2 Water Demand
The Client intends to construct a residential building in the said parcel of land and the Client is not connected to any water supply system. The Client will require approximately 35 cubic metres of water per day. A small quantity of water can be obtained from the roof catchment method. In order to meet the Client’s daily water requirements, a large roof surface area and storage facilities are required. Due to the seasonal nature and the quantity of rainwater expected to be tapped from this method, the roof catchment method is not a viable option. Due to various factors, a dam cannot be built to tap the seasonal runoff.
It is against this background that the Client wishes to explore the possibility of sinking a borehole in order to have a convenient source of water. Water from the borehole is to be used for industrial and domestic purposes. The objective of the survey was to identify the most suitable borehole drilling site or otherwise.
2.0 BOREHOLE DRILLING
2.1 Drilling Methods
Various drilling methods have been developed because geologic conditions range from hard rocks such as granite and dolomite to completely unconsolidated sediments such as alluvial sand and gravel. Particular drilling methods are dominant in certain areas because they are most effective in penetrating the local aquifers and thus offer cost advantages.
The drilling procedure depends on the depth and diameter of the well, type of formation to be penetrated, sanitation requirements and the principal use of the well. No single drilling method is best for all geological conditions and well installation.
Well construction comprises four or five distinct operations: drilling, installation of casings, placing a well screen and filter pack, if required, grouting to provide sanitary protection, and developing the well to ensure sand-free operation at maximum yield.
The driller is expected to report changes or fluctuations in the static water level at various depths during the course of drilling. Whenever possible, pumping tests may be carried out determine the specific yields of the aquifers encountered at various depths.
In the Cable Tool Percussion, the borehole is stabilized during the entire drilling operation. Recovery of reliable samples is possible from every depth unless heaving conditions occur. The well can be constructed with little chance of contamination.
Because of size, drilling rig can be operated in more rugged, inaccessible terrain or in other areas where space is limited. The well can be drilled in formations where lost circulation is a problem. The well can be bailed at any time to determine the approximate yield at that depth.
In the Direct Rotary Drilling method, penetration rates are relatively high in all types of material. Minimal casing is required during the drilling operation. Rig mobilization and demobilization are rapid. Well screens can be set easily as part of the casing installation.
Grouting is the act of injecting certain substances into the void of earth materials to reduce or eliminate their permeability, consolidate them, or increase their strength. Grouting or cementing well casing involves filling the annular space between the casing and the drilled hole with suitable slurry of cement or clay. The length of the borehole section to be grouted depends on the water well codes, aquifer structure and water quality.
In some formations, where poor-quality aquifers are interspersed with high quality water zones, the poor-quality aquifers are cemented off. Materials used for grouting include cement, bentonite, hydrated lime and synthetic materials such as polymers. It is important that grout should be mixed thoroughly and should be free of lumps.
To assure that grout will provide a satisfactory seal, it is necessary to place it in one operation before setting begins. The ideal result is a uniform sheath of cement around the casing for the entire vertical section to be grouted. Tight places and dead spots result where casing is not properly centered and touches the wall of the hole, causing channeling of the slurry.
2.3 Plumbness and Alignment
A borehole should be straight and plumb, although in practice any borehole of substantial depth may not be perfectly straight or perfectly plumb. A well bore may be straight but not plumb. A deviation from plumbness of two-thirds the well’s inside diameter per 30 metres is reasonable, considering the difficulties of drilling in earth materials. Straightness of the well bore is important, because it determines whether or not the casings and a properly sized pump can be installed in the well to the desired depth.
3.0 BOREHOLE DESIGN
A well is a hydraulic structure which, when properly designed and constructed, permits the economic abstraction of water from a water-bearing formation. Successful wells are designed and constructed by using materials that will provide an efficient well with a long service life, taking maximum advantage of the hydrogeological conditions, and applying the principles of hydraulics in a practical way to the analysis of wells and aquifer performance.
3.1 Borehole Logging
This is a crucial exercise in determining the design and construction of an efficient borehole where samples are collected after a depth of at least every 2 metres. A borehole log provides important information related to the actual geological conditions of the penetrated rock formation in its natural status at various depths. A geophysical log is a detailed description of the character and thickness of the various strata at the borehole and serves as an indicator of the water quality by measuring the apparent resistivity of the materials surrounding the well bore.
The samples are thoroughly mixed in order to obtain a representative sample the collected sample is kept in a sample box for the purpose of correlation and logging. Other methods investigating aquifers utilize geophysical instruments in the borehole. Borehole data collection and analysis is done during the drilling phase to facilitate screen emplacement.
The data is used to supplement data from a surface survey and are also used to determine the thickness of formations, zones of highest porosity and water quality. Geophysical borehole logging techniques and properly collected rock samples enable an experienced Hydrogeologist to identify the precise depths at which water was struck, the vertical extent or thickness of the aquifer and therefore be in a position to decide the borehole’s construction details.
Electric logging in uncased boreholes filled with water offers several important advantages. These include locating the top and bottom of each distinct formation, determining the relative water quality, and differentiating clean sand strata from silty sand and from sand strata with clay stringers. Borehole techniques include caliper, flow meter, temperature, resistivity, conductivity, spontaneous potential (SP), natural gamma, gamma-gamma, neutron, acoustic (sonic) and the TV Camera.
The techniques are also useful for verification of the efficacy of borehole construction techniques. Each of these methods is particularly suitable in describing certain stratigraphic, lithologic, and other aquifer properties. This permits the installation of the screen in the most desirable position more accurately than merely relying on the cutting’s log. Consequently, borehole logging should be done by an experienced Hydrogeologist.
The casing serves as a housing for the pumping equipment and as a vertical conduit for water flowing upward from the aquifer to the pump intake. Selection of casing material is based on water quality, well depth, cost, borehole diameter, and the drilling procedure. Steel is used most commonly, but thermoplastic materials can be used especially in areas where groundwater is highly corrosive and the depth of the well is less than 300 metres.
Corrosion resistance, light weight, relatively low cost, easy installation, and resistance to acid treatment make plastic (PVC) casings desirable for many installations where high strength is not required. Plastic casings must be centered in the borehole before backfilling or filter packing is completed. Any voids in the backfill or filter pack material may lead to sudden collapse of formation materials against the casing, causing it to break.
Caution is required in selecting materials for any well deeper than 90 metres, especially for large diameter wells. Plastic casings exhibit one or other characteristics that may present a hazard to drinking water quality in areas where groundwater contamination has occurred. If volatile organic chemicals exist in groundwater near a borehole, but above the intake section of the well, some of these chemicals still might move into the discharge by passing through the wall of the casing. It appears that plastic casing can be permeable in the presence of certain chemicals.
3.3 Screen Material
The choice of material used to fabricate screens depends on the water quality, potential presence of iron bacteria and strength requirements. Water quality analysis show whether groundwater is corrosive or incrusting or both. It is therefore important to use a well screen fabricated from corrosion-resistant material. Incrusting water deposits minerals on the screen surface and in the pores of the formation just outside the screen. These deposits plug both the screen openings and the formation.
A screen permits water to enter the well from the saturated aquifer, prevents sediment from entering the well and serves structurally to support unconsolidated aquifer material. The value of a screen depends on how effectively it contributes to the success of a well. The screen should have no clogging slots, sufficient column and collapse strength. It should also be easily developed and resistant to corrosion.
The screen should be easily developed and be able to control sand pumping in all types of aquifers. Head losses through the screen and incrusting tendency should be low or minimal. Due to various natures of aquifers and groundwater chemistry, the screen design must accommodate these varying physical and chemical characteristics. Screens with the following characteristics provide the best service in most geological conditions: –
- Slot openings should be continuous around the circumference of the screen, permitting maximum accessibility to the aquifer so that efficient development is possible.
- Slot openings should be spaced to provide maximum open areas consistent with strength requirements to take advantage of the aquifer hydraulic conductivity.
- Individual slot openings should be V-shaped and widen inward to reduce clogging of the slots and sized to control sand pumping.
- Screen construction methods should permit the use of a wide variety of materials that are compatible with differing groundwater environments to minimize corrosion and incrustation.
- If constructed of metal, screens should be of single-metal construction to minimize galvanic corrosion.
- Screens must be sufficiently strong to withstand stress normally encountered during and after installation.
- A full series of fittings should be available to facilitate screen installation and well completion operations. In most geological formations, the drawdown is a function of the open area. The lower the open area, the greater the drawdown for a certain yield.
Iron bacteria produce accumulations of slimy material of gel-like consistency, and oxidize and precipitate dissolved iron and manganese. It causes plugging of pores in water-bearing formations and openings in well screens but has no effect on our health. The combined effect of the growing organisms and the precipitating minerals can plug a well almost completely within a short time.
The design of a borehole supplying potable water should include features which provide continuous sanitary protection. Contaminated water from the surface drainage or low-quality water encountered in the well can move downward through the annulus space between the casing and borehole wall. Thus, the annulus around the casing must be sealed off with a cement or bentonite grout.
3.4 Slotted Metal Pipe
A pipe with slots, produced by one of several means, is used in water wells as a screening device. Slot openings may be cut with a saw or oxyacetylene (cutting) torch or punched with a chisel-and-die casing perforator. Steel slotted pipe is not corrosion resistant and most methods of perforation tend to hasten corrosion attack on metal when the water is aggressive. Jagged edges and slot surfaces are susceptible to selective corrosion. In general, the use of slotted steel pipe will increase maintenance costs and may significantly
reduce the life of a well. Important limitations of perforated pipe are: –
- Openings cannot be closely spaced. Percentage of open area is low. Size of slot openings varies significantly. Openings small enough to control fine sand or medium sand are difficult or impossible to produce.
3.5 Design and Construction Factors
Slotted casings allow entry of water in a borehole whereas plain casings block entry of water in a borehole. Please note that the design of a borehole is absolutely independent from a quotation. A typical borehole design not drawn to scale has been presented below.
| Annular space to be filled with gravel (optional)
Final Borehole Depth
The design of a borehole primarily depends on the water column available in a borehole and not the depth(s) at which water is reported to have been struck. Casing and diameter of the open hole vary between 130 and 305 millimetres. The diameter (or width) and length of the screen slot should provide sufficient open area so that the entrance velocity of water does not generally exceed 0.03m per second. The borehole’s design and construction is very crucial to the success of the borehole i.e. the borehole’s efficiency and it’s lifespan and depends on the following:-
- Choice of a well screen with insufficient open areas makes entrance velocities too high, resulting in greater-than-normal entrance (head) losses.
- Poor distribution of screen openings cause excessive convergence of flow near the individual openings, and may produce twice as much drawdown as necessary.
- Insufficient length of well screen, resulting in partial penetration of the aquifer, distorts the flow pattern for some distance around the well. Improperly sized filter packs or those made from angular or plate-like materials can restrict flow into the well screen. Particle shape, size, and grain size distribution affect the hydraulic conductivity of the pack.
- Inadequate development of a well may leave so much drilling fluid and small particles in the formation around the screen that the original permeability is reduced.
Improper placement of the well screen may put it at a depth that does not correspond to the best water-bearing stratum. The principal objectives of good design should ensure water that remains sand free, a borehole that has a long life span, reasonable long-term costs as well as the highest yield with minimum drawdown consistent with aquifer capability and good quality water with proper protection from contamination. Important hydrogeological information required for the design of an efficient high capacity well includes stratigraphic information concerning the aquifer and overlying overburden as well as:-
- Water quality, transmissivity and storage coefficient values for the aquifer.
- Current and long-term water balance conditions in the aquifer.
Dimensional factors, strength requirements and costs associated with well construction and maintenance also play an important role in establishing the particular design parameters. The intake of wells is generally screened to prevent sediment from entering with the water and serve as a structural retainer to support the loose formation material.
4.0 DEVELOPMENT OF BOREHOLES
All drilling methods cause some plugging of fractures or crevices in rocks. Borehole development is designed to maximize the well yield by repairing damage done to the formation by the drilling operation so that natural hydraulic properties are restored. Development also alters the basic characteristics of the aquifer near the borehole so that water will flow more freely to a well. These objectives are accomplished by applying some form of energy to the screen and formation.
Well development is confined to a zone immediately adjacent to the well, where the formation materials have been disturbed by the well construction procedures or adversely affected by the drilling fluid. The undisturbed part of the aquifers just outside the damaged zone may be reworked physically during development to improve it’s natural hydraulic properties. All new wells should be developed so as to achieve sand-free water at the highest possible specific capacity. Development procedures have the following beneficial purposes:
- Reducing the compaction and intermixing of grain sizes produced during drilling by removing fine materials from the pore space.
- Increasing the natural porosity and permeability of the previously undisturbed formation near the borehole by selectively removing the finer fraction of aquifer materials.
- Removing the filter cake or drilling fluid film that coats the borehole, and remove much or all of the drilling fluid and natural formation solids that have invaded the formations so that the well will yield sand-free water.
4.1 Development Methods
Borehole development methods include overpumping, backwashing, mechanical surging, air development, high-velocity air or water jetting, and a combination of high-velocity water jetting and simultaneous pumping. Overpumping and Mechanical Surging is the simplest method of removing fines from water-bearing formations. Overpumping by itself seldom produces an efficient well or full stabilization of the aquifer, particularly in unconsolidated formations because most of the development action takes place in the most permeable zones closest to the top of the screen.
In this method, water flows only in one direction, towards the screen, and some sand grains may be left in a bridged condition, resulting in a formation that is only partly stabilized. Water is forced to flow in and out of a screen by operating a plunger up and down in the casing, similar to a piston in a cylinder. Surge blocks sometimes produce unsatisfactory results in certain formations, especially when the aquifer contains many clay streaks or mica, because the action of the block can cause clay or mica to plug the formation.
4.2 Other Methods
The best development results are obtained by a combination of overpumping, followed successively by mechanical surging and simultaneous water jetting/air-lift pumping. Overpumping is the least effective of the three methods. One of the best methods used to clean rock holes is the water jetting/air-lift pumping method in which inflatable packers are used to isolate the zones that supply water to the well.
Aquifer simulation is done when the aquifer does not yield enough water even after development procedures have been applied. It can also increase the yields far beyond those obtained through typical well development. Blasting and bailing techniques are used in an attempt to reduce sand pumping and enhance yields from wells after other development methods have been applied. If a rock is massive, with few joints or faults, the volume of water available is often inadequate.
Explosives are sometimes used in an attempt to develop greater specific capacity. Good results can be obtained if blasting procedures are appropriate for the rock type and the size and depth of the well. However it is difficult to predict whether the shooting operation will produce beneficial results. Extreme care must be used when blasting a well by taking precautions against a potentially significant seismic shock.
5.0 GROUNDWATER CHEMISTRY
The chemical nature of groundwater is determined by the unusual ability of water to dissolve a greater range of substances or minerals than any other liquid. The slow percolation of water through the ground results in prolonged contact of water with minerals in the soil and rock. Many of these minerals are dissolved slowly as groundwater passes over them, and in time, a quasi-chemical equilibrium can be reached between the groundwater and the minerals in the soil and rock. Groundwater chemistry changes with depth and lateral distances as water flows along in the subsurface environment, increasing in dissolved solids and most major ions. The longer water remains underground, and the farther it travels, the more it resembles sea water.
5.1 Water Quality
The main purpose of a water quality analysis is to determine the suitability of water for a proposed use. The three main classes of water use are domestic, agricultural and industrial. A supply intended for public use may include all the three classes and accordingly require a standard of quality that is generally higher than that needed for any one class. Water for use in a particular industry may require a quality that is substantially higher than the one required by a public supply. Occasionally, more than one water-bearing formation is encountered in a well and the composition of the respective water may vary drastically.
Under these conditions, it may be desirable to exclude the poor-quality water from the principal supply by grouting. Bacteriological quality of a water supply is determined by analyzing for coliform bacteria. The coliform group of organisms is used as an indicator of dangerous contaminant levels.
5.2 Water for Irrigation
Water quality problems in irrigation include salinity and toxity. Excessive salinity occurs when there is an accumulation of salts in the topsoil especially in arid or semi-arid regions with extensive flat terrains. Some waters contain high enough concentrations of certain elements to retard or even eliminate the growth of some plants. Boron, chlorides and sodium are common toxic substances.
Sodium has far-reaching effects on soils. Of particular interest is the ratio of sodium to calcium and magnesium. When sodium-rich water is applied to soil, some of the sodium is taken up by clay: the clay gives up calcium and magnesium in exchange. This reaction is called Base Exchange and alters the physical characteristics of soil and can even lead to growth retardation.
Even worse, high concentrations of sodium salts can produce alkali soils in which little or no vegetation can grow. In order to measure the effect of sodium ions, the sodium effect is calculated as the Sodium Adsorption Ratio (SAR). Development of excess sodium in soils will result from irrigation water that has a high SAR value (18 or above); values below 10 indicate little danger of a sodium problem.
6.0 WATER QUALITY PROTECTION
All water that seeps into the ground is contaminated to some degree even before entering the subsurface environment. For instance rain water picks up carbon dioxide, minerals, bacteria, and inorganic contaminants such as oxides of sulphur and nitrogen from the atmosphere and soil. Once in the ground, any water percolating through soils near sources of pollution can become heavily contaminated with dangerous solvents or chemical residues.
Percolation of water from septic systems and refuse disposal sites poses a serious threat to the preservation of groundwater quality. In properly designed and constructed systems, the movement of contaminated discharge through finer grained soils is quite effective in removing pathogenic bacteria over short travel distances. The occurrence and widespread migration of chemicals in the subsurface environment may ultimately become the most serious threat to groundwater quality.
Unfortunately many of these substances are not absorbed into soil particles and can travel great distances in the subsurface environment. Protection of groundwater quality depends on the well design and the methods and materials used to construct the well. Some of the deficiencies in well construction are:-
- Insufficient or substandard well casing. Use of well pits, inadequate seal between the well casing and the borehole.
- Poor welding of casing joints and lack of sanitary protection at wellhead.
6.1 Groundwater Pollution
Groundwater can be polluted by poisonous or pathogenic substances or by other detrimental changes in it’s quality especially through Poisonous substances such as compounds of lead, cadmium, chromium, cyanide, fluoride or mercury. Chemicals for plant protection, herbicides, pesticides and plant growth regulators as well as:-
- Sewerage, refuse or garbage. Detergents, fats, petroleum products. Acids, alkalis and salts.
- Colouring agents such as dyes, paints and aromatic substances. Metabolic and decomposition products of micro-organisms and fertilizers. Acids, alkalis and salts.
6.2 Sources of Danger
Pollution substances from the source of danger can reach the groundwater and the point of abstraction by several different ways e.g. seepage through sinks, infiltration, leaching, sluicing and capillary action. A borehole should certainly be located as far away as possible from all the sources of danger such as Plants and installations, especially those which release radio active substances or those that can impair the quality of water, sewerage waste, gaseous and particulate emissions etc. Other sources of danger are:-
- Transportation, utilization, storage and deposition of garbage, refuse or scrap metals. Hospitals, sanatoriums, hotels and cemeteries.
- Manufacture, transportation, usage, storage and deposition of substances which can impair the quality of water
- Sewage seepage into the ground through septic tanks, injection of sewerage or other dissolved and undissolved substances into the ground and surface water.
- Pipelines for substances which can impair the quality of water. Polluted water bodies, parking and washing of motor vehicles.
- Washing and leachates from the soil, organic fertilizers (liquid manure, barnyard manure, sludge, garbage compost) and mineral fertilizers. Use of chemical substances for plant protection, herbicides, pesticides and growth regulators.
6.3 Groundwater Protection
Permanent measures for the protection of groundwater are required in the immediate vicinity of the proposed borehole and also it’s catchment area. The danger of contaminating groundwater quality is correspondingly increased by factors such as negligence, population density and over abstraction. Groundwater is an important source of drinking water and therefore aquifer protection to minimize deterioration of their quality is necessary because of the increasing chemical diversity of potential groundwater pollutants manufactured, used and disposed of by mankind and:-
- The widespread disposal of domestic, agricultural and industrial effluents to the ground, especially due to the high cost of alternative arrangements.
- The enormous increase in the application of fertilizers and pesticides to agricultural land. The potentially insidious health effects associated with the pollution of groundwater supplies, consequent upon slow but persistent increases in concentration of contaminants with uncertain toxicology.
- Rehabilitation of polluted aquifers will always be expensive and may often prove impracticable, leading to abandonment of valuable groundwater resources at considerable economic cost.
Groundwater movement, and pollutant migration from the land surface to production boreholes, tends to be a relatively slow process in many aquifers. This means that it can take many years, even decades, before the full impact of a pollution episode, involving a persistent contaminant, becomes fully apparent in groundwater supplies.
6.4 Health Criteria
It will not be practicable to prevent all pollution. Water quality standards and guidelines for potable or other uses thus, in effect, become the design criteria for groundwater pollution protection. The guidelines are based on two separate criteria; health implications (toxic, carcinogenic, mutagenic effects), which are of primary importance. Aesthetic grounds (taste, odour, colour) are of secondary importance providing the consumer will accept the water.
Water quality standards presented in this report are based on the WHO Standards as a guide where TH denotes the Total Carbonate Hardness (Maximum, 500 mg/l) and TDS denotes the Total Dissolved Solids. The maximum pH is 9.2.
Among the inorganic constituents of health significance, by far and away the most widespread is nitrate, because of its high mobility and stability in aerobic groundwater systems. Others, generally of natural origin, such as fluoride, arsenic and selenium, occur quite widely and may be mobilized by imposed stresses on aquifers. The hazardous heavy metals tend to be immobilized by precipitation, or other processes, but migrate significantly in groundwater systems of low pH. Numerous inorganic constituents listed in the guidelines on aesthetic grounds occur widely at elevated levels in groundwater; often naturally, sometimes due to pollution.
Most notable amongst these are chloride, iron, manganese, sodium, and sulphate. Organic constituents that appear from current evidence to represent the greatest threat to groundwater quality are some of the chlorinated alkanes, alkenes and benzenes, which are relatively mobile and persistent in groundwater. The bacteriological standards for drinking water are also of direct relevance because groundwater is often consumed with minimal disinfection.
6.5 Groundwater Protection Zones
Zone 3 is the outer protection zone and generally extends up to about two kilometres from the abstraction works. This zone serves the purpose of groundwater protection from far-reaching impairments especially from non-degradable chemical pollution or those degradable only with difficulty and from radioactive pollutants.
Zone 2 is the inner protection zone and serves the purpose of groundwater protection from pollution, which results from various human activities and installations and are especially hazardous because of their closeness to the point of abstraction. This zone extends from the boundary of Zone 1 upto a line which marks the distance that the groundwater will cover to reach the point of abstraction in 50 days. Zone 2 can be left out when deeper lying sealed aquifers are tapped that are covered for the 50-day-line-to-the point of abstraction distance by sufficiently thick impervious layers.
Zone 1 is the abstraction area and is meant to should ensure protection to the immediate vicinity of the point of abstraction from pollution. Zone 1 generally extends 10 metres in all directions in case of wells or 10 metres in the direction from which the groundwater flows in case of springs. However, Zone 1 should at least extend so far as to permit the use of organic fertilizers in Zone 2. The following are not allowed in this Zone:-
- Installations, activities and proceedings mentioned for Zone 2 and Zone 3.
- Vehicle and pedestrian traffic. Any type of agriculture. Organic fertilizers.
- Use of chemical substances for plant protection, herbicides, pesticides and growth regulators.
7.0 GROUNDWATER MODELS
A groundwater model is used to understand why a flow system is behaving in a particular observed manner and to predict how a flow system will behave in future. Models can be used to analyse hypothetical flow situations in order to gain generic understanding of that type of flow system. The term model refers to any representation of a real model.
In studying a groundwater flow system, a conceptual model is initially developed. No matter how much field work is performed in describing a flow system, a model may not fully describe all the minute details of the real system. There are many types of dynamic models of groundwater flow. These are physical scale models, analog models and mathematical models.
No matter how much field work is performed in describing a flow system, a model may not fully describe all the minute details of the real system. There are many types of dynamic models of groundwater flow. These are physical scale models, analog models and mathematical models.
Salinity is the simplest measure of the basic aquifer characteristics and a useful tool in defining boundary conditions in groundwater models. A Groundwater Model is a useful tool in the management of groundwater resources. Some knowledge of the amount of natural recharge to an aquifer is mandatory in groundwater development and management. A water budget for the recharge area of an aquifer is a very useful means of determining groundwater recharge.
Many of the parameters used for a hydrologic budget are measured directly: precipitation, stream flow, transported water and reservoir evaporation. Groundwater inflow, outflow and change in storage are computed from the hydraulic aquifer characteristics and measured potentiometric data.
7.1 Data Requirement
It is necessary to have a data base which provides adequate information to apply the requisite equations. All models start with a groundwater flow model. You need to know the physical configuration of the aquifer. This includes the location, areal extent and thickness of all the aquifers and confining layers; the locations of the surface water bodies and streams; and the boundary conditions of all the aquifers.
Hydraulic properties that need to be known include the variation of transmissivity or permeability and storage coefficient of the aquifers, the variation of permeability and specific storage of the confining layers, and the hydraulic connection between the aquifers and surface water bodies. Hydraulic energy, as indicated by water table or potentiometric surface maps and the amounts of natural aquifer recharge and natural streamflow are also needed.
In order to model stresses on the natural groundwater flow system, the modeler must know the locations, type, and amounts through time of any artificial recharge, such as results from recharge basins and wells or return flow from irrigation, as well as the amounts and locations through time of groundwater withdrawals from wells.
Changes in the amounts of water flowing in the streams and changes in the water levels of surface water bodies should also be known. A model is initially calibrated by taking the initial estimates of the model parameters and solving the model to see how well it reproduces some known condition of the aquifer.
Once a model has been calibrated, the model is finally verified. This is usually done by history matching. Model field verification can then be performed by actually stressing the aquifer to see if the model correctly predicts the response of the aquifer as it is stressed. No conclusive groundwater modeling has ever been done in the country.
A Groundwater Modeling Project for Nairobi and it’s environs was executed in 1997. The project was meant to address groundwater apportionment and management. Unfortunately, the project was not concluded due to various difficulties encountered during the course of implementation e.g. the complex heterogeneous nature of the local rock formations and lack of a credible data base.
7.2 Groundwater Recharge and External Factors
Destruction of ecosystems has resulted to unpredictable weather conditions such as prolonged drought or abnormal floods. Climatic change has disrupted the water cycle and by extension aquifer recharge through infiltration. Over abstraction, if not checked, may lead to adverse hydrogeological conditions. Poor disposal of effluent may affect the quality of groundwater.
Groundwater recharge depends on various factors such as the field capacity etc. and may be estimated by using hydrographs obtained from observation boreholes only in static hydrogeological conditions and not in dynamic conditions i.e. in a situation where the existing boreholes in the relevant area are also used as observation boreholes over a period of time. Groundwater recharge can be determined from the baseflow.
Baseflow from a basin is an indirect measure of recharge as it represents the drainage of groundwater from aquifer storage after groundwater recharge has occurred. A simple method of estimating groundwater recharge utilizes stream and or borehole hydrographs from two or more consecutive years. The hydrographs would indicate fluctuations in the static water levels, rising during the period of recharge and falling when the recharge is less than the amount of groundwater abstraction. Hydrographs for the quantity of water available (excluding runoff) over time in rivers is also necessary.
Under certain conditions (Meyboom, 1961), the total volume, Q, of baseflow which would be discharged during an entire uninterrupted period of groundwater recession can be computed from the equation where, K’=initial value (at time t) in respect of the first recession, m³; K”=the time period for one log cycle (s). The duration of the recession is important.
The difference between the total potential groundwater discharge, at the beginning of a specified recession period, and the amount of actual groundwater discharge, during that recession period, gives the remaining potential groundwater discharge. The difference between the remaining potential groundwater discharge, at the end of any baseflow recession, and the total potential groundwater discharge, at the beginning of the next recession, is a measure of recharge that takes place between the recessions.
A water budget (Fetter 1994) for the recharge area of an aquifer is a very useful means of determining aquifer recharge. In the water budget method, the aquifer does not have to be in equilibrium in order to use it. Many of the parameters are measured directly. Groundwater inflow, outflow and change in storage are computed from the hydraulic aquifer characteristics and measured potentiometric data.
The amount of evapotranspiration is typically estimated using an appropriate formula such as the Penman method. Basinwide groundwater recharge may be determined by a water budget analysis, where, Groundwater recharge can be estimated from the following equation:-
(Precipitation + surface water inflow + imported water + groundwater inflow + Industrial use + municipal use + domestic use + irrigation use) – (cooling water evaporation + irrigation water evapotranspiration + water exported in products +sewerage discharge into surface waters +evapotranspiration +reservoir evaporation +surface water outflow +exported water + groundwater outflow) ± changes in surface water storage
7.3 The Safe Yield or Reserve
For the purpose of groundwater management, determination in the allocation plan for a given aquifer or part thereof, the spacing of boreholes or wells to be equipped with motorized plant may be guided by the existing borehole or well spacing, individual aquifer characteristics including water quality, existing aquifer use, and existing bodies of surface water. The quantity of water available for exploitation (reserve) depends on whether groundwater abstraction has exceeded the safe yield of an aquifer or not. The safe yield of an aquifer system is only one facet of a groundwater management program.
The safe yield (Fetter 1994) is the amount of naturally occurring groundwater that can be withdrawn from an aquifer on a sustained basis economically and legally, without impairing the native groundwater quality or creating an undesirable effect such as environmental damage. Due to various constraints, determination of the safe yield is not a simple matter. Groundwater models have been applied to four general types of problems: groundwater flow, solute transport, heat flow, and aquifer deformation. Computer models of groundwater flow systems are ideal tools for estimating the series of safe yield values
There are numerous cases where boreholes have been drilled close to one another and no overlap of the area of influence cases has been reported. Boreholes have been drilled very close to hand dug wells and the wells still have water. The occurrence of mutual interference of several closely spaced boreholes (Karanth, 1993) does not mean that no additional boreholes can be drilled within the area of influence, the area may be distorted by local geological conditions e.g. faults.
Under certain conditions, the radius of influence R for an unconfined aquifer can be estimated from the following equation where H=static head measured from bottom of aquifer, m; h=depth of water in the borehole while pumping, m
r=radius of borehole, m; b=thickness of aquifer, m
Q=1.366(H²-h²) and for confined aquifers, Q=2.73 Kb(H-h)
Log R/r Log R/r
The minimum spacing can only be determined by proper hydrogeological research supported by drilling, and where necessary, pumping tests and economic consideration. Theis (1957) derived a basic formula, which introduces the economic factors to be considered in proper spacing of boreholes in heavily exploited groundwater basins i.e.
rº =116 CQ
The equation is based on the proportionality between the spacing and the average transmissivity values between any two boreholes for a given locality where, rº =spacing between two discharging boreholes; C= cost to raise 1 m³/day; Q=discharge of each borehole in m³/day; k =capitalized cost per year for pipeline, borehole maintenance; T=transmissivity of the aquifer, m²/day
Freeze (1971) has shown how a computer model can compute a maximum basin yield. To date, there is no conclusive evidence based on research to support any negative effect on the safe yield if a borehole is drilled in the Client’s land. Analysis of the groundwater available for apportionment depends on the rates of groundwater recharge and groundwater abstraction at any given time.
Verified groundwater models are considered to be necessary for the purpose of predicting the behaviour of aquifers when subjected to certain injection or abstraction conditions. Due to lack of research, the current rates of groundwater recharge and groundwater abstraction in our aquifers are not known and therefore, the amount of groundwater available for apportionment is not known. To safeguard the safe yield, the maximum groundwater abstraction rates nay have to be restricted based on research results.
7.4 Impact of Borehole Drilling
During pumping, water flows toward the well from every direction. As the water moves close to the well, it moves through imaginary cylindrical sections that are successively smaller in area. As the pump removes water, an area of low pressure develops near the borehole. Because the water level is low in the pumped well than at any point in the water bearing formation surrounding it, water moves from the formation into the well to replace water being withdrawn by the pump.
When pumped, all wells are surrounded by a cone of depression. The size and shape of this cone depends on the pumping rate, pumping duration, aquifer characteristics, slope of the water table, and recharge within the cone of depression. Factors contributing to excess drawdown or inefficiency are related primarily to choices made in the design and construction of wells. Water resources have been known to be a source of conflict worldwide. Who are the complainants? Are the complains actually genuine?
To demonstrate the seriousness of this statement, spirited campaigns have sometimes been mounted against people intending to have boreholes drilled in their land. Some of the complainant’s boreholes or hand dug wells are located as far as one kilometre away from the proposed borehole drilling site. According to the complainants, a borehole drilled in the applicant’s land would deplete water in their dug wells or boreholes. Water resources, either surface or groundwater belong to all of us and therefore require our collective efforts in conservation and management.
Drilling, equipping and maintaining a borehole is expensive and has to justify the reason why a borehole is required by the Client. A borehole is urgently required by the Client for the purpose of serving as the only practical and convenient alternative source of water. Do we have irrefutable hydrogeological reasons to support any expected negative impact(s) of drilling a borehole in the Client’s land?
The density of boreholes in the area is low and the possibility of any adverse effects on the local aquifer entirely depends on the subsurface geological boundaries or generally, the flow path. Any adverse effect is practically nonexistent unless proved otherwise by irrefutable facts and figures. Water abstracted from the Client’s borehole is meant for domestic use and elaborate effluent disposal mechanisms already exist.
Therefore water abstracted from the borehole cannot impair the quality of water in the existing boreholes. The amount of groundwater available for apportionment is not known and therefore, abstraction of groundwater in certain areas should not be based on the water demands based on the groundwater application forms.
7.5 Groundwater Flow Velocity
The terminology groundwater flux does not exist! The flow of groundwater has been described under various terminologies e.g. Darcy flux velocity (Darcian velocity), bulk velocity and specific flux and is the apparent velocity per unit area at which water would move through an aquifer, if the aquifer were an open conduit. Groundwater flow, whether laminar or turbulent require a hydraulic gradient and has no relationship with the topographical water catchment boundaries. Salinity in groundwater depends on the velocity of flow and is directly related to geological boundaries.
Groundwater flow velocity can be measured by placing a tracer such as dye or salt in one borehole and noting the time of it’s arrival in a second borehole downstream from the first. Borehole yields are directly affected by flow velocities, but velocity in an aquifer is difficult to study. Groundwater flow may be estimated from the following equation where V=velocity of flow, K= hydraulic conductivity, ή=porosity and L=distance along flow path between points h’ and h located at observation boreholes
8.0 EFFECTS OF GEOLOGIC BOUNDARIES
The relationship between geomorphic and geologic factors (Karanth) governs the movement of water from the time it reaches the land surface till the time of leaving it. Of primary importance is the occurrence and distribution of aquifers and their relationship with associated relatively impermeable beds that act as non leaky or leaky confining layers and barriers to groundwater movement.
The geologic structure has a marked influence on the lateral and vertical extent of aquifers and associated less permeable rocks, artesian pressure in aquifers and quality of groundwater. An integrated study of the geology and evolution of land forms is useful to understand the occurrence of porous and permeable zones. Faulting may affect the hydrogeological conditions of the area in which the fault is located.
8.1 Geomorphic Controls on Groundwater
Geomorphic features control, in a large measure, the distribution of precipitation and the amount of precipitation that contributes to runoff and groundwater recharge. The important factors governing the development of landforms are resistance to erosion and the geologic structure of the underlying rocks, climate and vegetative cover. The measurement of landforms i.e. morphometry, is gaining importance in evaluating hydrologic parameters of drainage basins, some of which are useful in understanding groundwater situations.
Morphometric parameters such as drainage density and slope characteristic provide a basis for evaluation of runoff and groundwater potentials of basins. The drainage area determines the total quantity of water available in a basin. Drainage density is measured from a topographical map or air photograph, using an opisometer or chartometer for determining the length of the stream channels and a planimeter for the basin area.
All conditions being similar, a high drainage density is indicative of more runoff than infiltration from precipitation. In general, a low network of drainage courses in a basin is indicative of the presence of highly resistant or highly permeable rocks on the surface, while a high density characterizes hilly terrain and areas underlain by weak or impermeable rocks.
Slope has a dominant effect on the contribution of rainfall to stream flow and to groundwater reservoir in as much as it controls the duration of overland flow, infiltration and subsurface flow. The slope conditions also control the depth to the water table, distribution of head and artesian pressures in aquifers. Slopes indirectly control the infiltration capacity of soils.
The water table is a subdued replica of the topography and is only true in a regional context. The topography of the area is made up of a flat terrain which gradually descends towards the north east. The drainage pattern in the locality is generally towards the north east. Black soils are found in the area due to the poor drainage conditions in the locality.
Climatic changes have made it difficult to quantify the amount of rainfall. However, the mean annual rainfall is expected to vary between 450 to 650 millimetres. The rainfall frequency and intensity in the locality is unevenly distributed throughout the year and basically falls in two seasons. The first rainy season has greater rainfall intensity and is normally expected between March and April. The second rainy season has a lesser intensity and is normally expected between October and December. The vegetation cover comprises of scattered trees and grass.
8.2 Occurrence and Movement of Groundwater
The hydrogeology of the area depends on various factors such as the nature of the parent rocks, structural geological features, degree of weathering, and whether the locality is connected to the area of recharge or not. A weathered or fissured rock formation normally has good aquifer potential. Groundwater occurs in the vadose zone and the zone of phreatic water or zone of saturation. The vadose zone or zone of aeration has three types of water i.e. soil water, intermediate vadose water, and capillary water. Soil water provides water for plant growth. The region immediately below the soil water zone is the intermediate zone.
Although most of the water in this zone is moving downward, some of it is retained, but no in situ use of it exists and it cannot be recovered for use. Capillarity holds water in the smallest spaces between soil particles. When the water holding capacity of the capillary forces is exceeded, water begins to percolate downwards to the zone of saturation under the force of gravity.
Depending on the origin, there are two types of groundwater that exist in the subsurface environment. These are connate water and meteoric water. Connate water is the water that was trapped in sediments at the time of deposition. Meteoric water is the water that penetrates the rock through subsurface infiltration. Groundwater is found in one continuous body or in several distinct rock or sediment layers at any one location.
The thickness of the groundwater bearing zone depends on the local geological boundaries, the degree of weathering or fracturing, porosity or availability of pores or openings in the rock formation, recharge and movement of water from areas of recharge to points or areas of discharge. Water exists in aquifers under two completely different physical conditions namely confined and unconfined aquifers. Confined groundwater is isolated from the atmosphere at the point of discharge by impermeable geologic formations and the confined aquifer is generally subject to pressures higher than atmospheric pressure.
An aquifer performs two important functions i.e. a storage function and a conduit function. The interstices of a water- bearing formation act as storage sites and are part of a network of conduits. Groundwater is constantly moving through these conduits under the local hydraulic gradient. Thus water contained in any aquifer is in temporary storage, and if not used, will be discharged as rivers, streams and springs or into lakes or oceans.
Openings in aquifers comprise of openings between individual particles in sandstone and sand and gravel formation, crevices, joints, faults, and gas holes in igneous and metamorphic rocks and solution channels, caverns and vugs (opening) in limestone and dolomite. The shape of the openings in the rock, their size, volume and interconnection all play a vital role in the hydraulic characteristics of an aquifer.
8.3 Geologic Controls in Groundwater
The nature, distribution and structure of geologic formations control the occurrence, movement, quality and availability of groundwater. Depending on the mode of formation, there are three types of rocks namely, sedimentary rocks, igneous rocks and metamorphic rocks. Sedimentary rocks were formed through transportation processes and therefore the porosity of these rocks is the highest among the three rock types.
Igneous rocks were formed by the cooling and solidification of molten materials derived from the earth’s interior, mainly though volcanic activity. Metamorphic rocks are derived by the alteration of other rocks due to heat and pressure such as from sandstone to quartzite and limestone to marble. Metamorphic rocks have very low porosity as they were formed by interlocking of crystals. Metamorphosed crystalline rocks generally have very little, if any, primary porosity except for minute disconnected voids within the crystals.
Rocks which lack porosity do not transmit water and are hence impermeable. Metamorphic rocks have very low hydraulic conductivities. Consequently, metamorphic rocks are the least porous. Please note that hydrogeological conditions in metamorphic rocks are often unpredictable. A hydrogeologic boundary could be the edge of the aquifer, a region of recharge to a fully confined artesian aquifer, or a source of recharge, such as a stream or lake. Boundaries are considered to be either recharge or barrier boundaries. A recharge boundary is a region in which the aquifer is replenished.
A barrier recharge boundary is a region in which the aquifer is replenished or the edge of the aquifer, where it terminates, either by thinning or abutting a low permeability formation, or has been eroded away. A recharge boundary can be simulated by a recharging image well located an equivalent distance away from the recharge boundary but on the opposite side. Boundaries have the most dramatic impact on the drawdown of a pumped well for the aquifer with no source of vertical recharge.
The drawdown proceeds as a function of the logarithm of time as the well withdraws water only from storage in the aquifer. The effect of a recharge boundary is to retard the rate of drawdown. Change in drawdown can become zero if the well is supplied entirely with recharged water. The effect of a barrier to flow in some region of the aquifer is to accelerate the drawdown rate. The water level declines faster than the theoretical straight line. Geological boundaries have had significant hydrogeological implications where, contrary to expectation, only a small quantity of water is struck in a borehole or no water at all.
What happens if one has no previous knowledge about the location of the subsurface geological boundaries in a particular area? Would the hydraulic characteristics of a borehole drilled in my neighbour’s land be of any use to my particular parcel of land under investigation? What are the implications of quantifying the anticipated borehole yield or quality of water based on existing borehole data?
Suppose the stated anticipated borehole yield or quality of water ends up being misleading? The following cases illustrate possible repercussions associated with quantifying unknown borehole yields or quality of water before drilling:-
- Borehole serial numbers 11161 and 11734 belong to neighbours whereas borehole 11604 is located less than two hundred metres away from borehole 11734. The boreholes were drilled in a locality where rocks of the Basement System suddenly rise close to the ground surface resulting to the thinning of the overlying igneous rocks. Suppose borehole 11604 was drilled first and boreholes 11161 and 11734 were drilled later? What would have been the repercussion of quantifying the anticipated yields of boreholes 11161 and 11734 based on the yield of borehole 11604?
Borehole serial numbers 12254, 13448 and 13891 were drilled in the same farm and are located close to one another. Suppose borehole 13891 was drilled first and boreholes 12254 and 13448 were drilled later. What would have been the repercussion of quantifying the anticipated yield of boreholes 12254 and 13448 based on the yield of borehole 13891? See summarized borehole data in the table below where Δs is the drawdown in metres. The definitions of D and Q are presented in Chapter 10.
- Clay is impermeable and does not transmit water. Boreholes drilled in localities where thick clay has been encountered tend to have low yields. A borehole was drilled in Kitengela Township to a depth of about 210 metres. What was the anticipated yield of this borehole? Contrary to expectation clay was encountered almost throughout the course of drilling and only a negligible quantity of water was reported to have been struck. The borehole was declared dry and subsequently abandoned. A successful borehole serial number 16235 was drilled later and yet the two boreholes are barely 30 metres apart.
- A weathered or fissured rock formation may not necessarily contain water. Consequently, the productivity of an aquifer depends primarily on whether the investigated site is connected to the area of recharge or not. In the cases presented below, successful boreholes have been drilled less than 50 metres away from unsuccessful boreholes. To demonstrate the complexity of hydrogeological conditions in metamorphic rocks, several cases where different results were obtained have been listed as follows:-
- Lele, Naresho, Sultan Hamud and Olopolos areas in Kajiado County.
- Kathome area in Machakos County and Kayata area in Makueni County.
- Lwalenyi Ranch area in Taita-Taveta County.
- Lobira area of Eastern Equtoria in South Sudan.
- Yambio area of Western Equtoria in South Sudan.
During the course of drilling a borehole in Sultan Hamud sand was encountered at a depth of 74 metres. Upon encountering sand, water in the borehole disappeared. A replacement borehole was drilled to a depth of 65 metres. A similar situation occurred in Yambio.
Geologic and geomorphic barriers or boundaries sometimes do have extraordinary or dramatic implications. Lake Naivasha was initially reported to have fresh water despite being land locked and has no visible outlet. Could there be any relationship between the lake and the Olkaria geothermal field? Is the thermal gradient known? Hot springs do occur at certain localities where they are least expected. What could be the reason why the hot springs are located where they are? Deep boreholes have been drilled at certain localities where water is either warm or hot. What could be the cause of such abnormal temperatures?
Why has Lake Magadi never dried up despite being land locked and located in an arid area where temperatures are almost always unbearable to an ordinary visitor and the evaporation rate is always high? Where does Lake Magadi derive its water from? Could it be possible that Lake Magadi is actually located along the outlet flow path of Lake Naivasha?
Ewaso Ngiro river water suddenly disappears at Lorian swamp. Where does the river water go? Could there be any relationship with the so called Merti aquifer? Mzima Spring dramatically sprouts at a location where it is indeed least expected. The spring is a classic example of a Karst aquifer and is most likely related to the Chyulu Range of Hills. Recent vesicular Basaltic rocks are the major rocks in the locality.
9.0 GROUNDWATER EXPLORATION
The main categories of groundwater exploration techniques are hydrogeological maps, geophysical surveys conducted at the ground surface, borehole sampling procedures and geophysical logging of bores. Geophysical instruments provide information on the physical and chemical character of the subsurface environment. Geophysical exploration methods are used either before or during well construction and obtain information on the character of formation and on the presence and chemical characteristics of groundwater.
Some methods are extremely useful in determining the effectiveness of well construction. Certain methods are conducted at the surface and include seismic refraction, seismic reflection, gravity, magnetic, resistivity, radar, and electromagnetic techniques.
The signals received from any geophysical method are in response to a particular physical (geologic) property. Some geophysical methods measure a particular property directly, such as the gamma-ray emissions from a clay-rich sediment or the gravity difference between rocks. For others, an electrical signal or acoustic wave may be induced and the formation response measured.
Geophysical prospecting methods can be divided into several categories: mechanical, gravimetric, magnetic, electrical, nuclear, thermal, and acoustic. The method(s) selected will depend on the type of information needed, whether the test will be conducted at the surface or in the borehole, whether the borehole is cased or uncased.
9.1 Surface Geophysical Methods
These provide specific information on the stratigraphy and structure of the local geologic environment as well as the aquifer properties. Stratigraphic data may include information on the types and extent of superficial materials, and the nature and extent of the underlying bedrock.
Faults, fissures, folds, karstic terrain and igneous rock intrusions (dikes) are common structural elements that can be located and defined by geophysical methods. In some instances, the rock type, thickness, dip, and structural character may be defined and assumptions may be made on the presence of groundwater, it’s rate and direction of movement, and it’s chemical quality.
9.2 Electrical Resistivity Method
This is a major geophysical tool used in groundwater exploration efforts. Resistivity, the inverse of electrical conductivity is the resistance of the geologic medium to current flow when a potential (voltage) difference is applied. For a given material with a characteristic resistivity ρ, the resistance R is proportional to the length L of the material being measured and inversely proportional to it’s cross-section area A i.e. ρ= (RA) ÷ L
The ability of a rock to conduct electrical current depends primarily on the amount of open spaces between particles (porosity), the degree of interconnection between those open spaces and the volume and conductivity of the water in the pores. The presence of water and its chemical characters are the principle controls on the flow of the electric current because most rock particles offer high resistance to electrical flow. Thus resistivity decreases as porosity, hydraulic conductivity, water content, clay and water salinity increase.
Fresh compact rocks have higher resistivities than saturated sand or gravel. Resistivity values are obtained by the electrical sounding method or the electrical profiling method. Electrical sounding involves vertical exploration. In this procedure, a series of stations is established and careful depth soundings are taken. By evaluating the resistivity values at different electrode spacings, an understanding of the subsurface materials can be developed.
This method is especially useful for estimating the depth to water bearing strata or estimating the thickness of selected formations.
The electrical profiling method investigates materials at only one depth by selecting numerous stations. Resistivity measurements are taken for the same depth at each station. These values produce a numerical picture of the subsurface materials at the chosen depth across a horizontal plane. Electrical profiling is most often used in searching ore deposits, faults or fault zones, evaluating sand and gravel deposits, delineating boundaries and for finding dipping contacts of different earth materials.
The depth of electrical penetration is governed by the spacing of the electrodes i.e. the larger the separation, the deeper the penetration. Electrode spacing is progressively increased to determine the variation in resistivity with depth. If the resistivity changes with increased electrode spacing, it suggests that the formation changes as the depth increases.
9.3 Selected Exploration Method
According to the geological map enclosed in Geological Report Number 98, no fault is located close to the investigated land. Geological reports cover a wide area and only insitu rock specimens were collected for analysis. No rock outcrops were exposed at the ground surface. In a locality where no rock outcrops are exposed at the ground surface, would description of the local rock formations and structural geological features such as folds, joints etc. be realistic?
There are various groundwater exploration techniques. The existing hydrogeological maps, cross sections and the geological logs of the nearest boreholes have been preferred. This method was selected because geophysical data alone was expected to provide little definitive information. Geophysical groundwater exploration methods were therefore not considered to be necessary for the following reasons: –
- There exists sufficient hydrogeological information that can be used to predict the groundwater conditions for the locality more accurately.
- Geophysical equipment used for the exploration of groundwater require a sufficient lateral space so that the applied electric current can penetrate deeper into the subsurface. For instance, in order to probe a depth of 250 metres, a 500 metre straight line is needed. Electrical conductors such as underground pipes, electric cables, subsurface water tanks etc. are known to affect the accuracy of the resistivity measurements. Due to lack of space, a geophysical survey could probe up to a depth of 20 meters.
- Further hydrogeological information has been obtained from cross section number 11 which covers the area. See Hydrogeological report compiled by E.A.L. Gevaerts. Geological log of the boreholes along the section indicates that the stratigraphical sequence in the plot is likely to be as follows:-
|DEPTH INTERVAL(M)||EXPECTED GEOLOGICAL FORMATION|
Tuffs of the Athi Series
10.0 EVALUATION OF PUMPING TEST DATA
Measurements of water levels after the pump is stopped (recovery) are extremely valuable in verifying the aquifer coefficients calculated during the pumping phase of the test. The pumping test should be conducted for a continuous period of 12 to 24 hours, depending on the type of aquifer. The accuracy of a drawdown data taken during a pumping test depends on maintaining a constant yield during the test. The drawdown is carefully measured in the pumping well and in one or two properly placed observation wells. Drawdown readings are taken at appropriate time intervals. Determining changes in barometric pressures; stream levels affect the drawdown data. Comparing recovery data with drawdown data taken during the pumping portion of the test.
The test should be conducted for 12 hours for a confined aquifer and 24 hours for an unconfined aquifer during the constant rate test. A simple and accurate method for determining the pumping rate is to observe the time required to fill a container of known volume. This method is practical, however for measuring only relatively low pumping rates. A commercial water meter is more reliable when measuring abstractions over a long duration. Its only disadvantage is the unavoidable delay in obtaining values at the start of the test when the pumping rate is being adjusted to the desired level.
10.1 Step Drawdown Tests
These tests assist to examine the performance of wells having turbulent flow. The well is pumped at several successively higher pumping rates and the drawdown for each rate or step is recorded. The entire test is usually conducted within a day and calculations are simplified if all the pumping times are the same for each discharge rate. The water level should be allowed to recover to the static level between each step. Usually 5 to 8 pumping steps are used, each lasting 1-2 hours.
10.2 Water Level Recovery Data
When pumping is stopped, well and aquifer water levels rise towards their pumping levels. The rate of recovery provides a means for calculating the coefficients of transmissivity and storage. The time-drawndown measurements taken during the pumping period and the time-recovery measurements taken during the recovery period provide two different sets of information from a single aquifer test.
10.3 Estimating Local Aquifer Parameters
Credible pumping test data plays an important role in the determination of aquifer characteristics. Evaluation of pumping test data is especially useful in groundwater modeling and can be used to supplement geophysical data obtained from surface groundwater exploration methods. Evaluation of pumping tests (Karanth, 1993) is based on assumptions and generalizations regarding the hydraulic properties and dimensions of an aquifer and head distribution. While analyzing aquifer test data, it is essential to verify as to what extent the assumptions on which a formula is based are applicable in the case considered. Most of the assumptions are not realistic and are as follows:-
- The aquifer is of a simple model. It is rarely so. The aquifer is homogeneous and isotropic. Alluvial and marine formations are stratified and hard rocks are extremely heterogeneous and anisotropic.
- Transmissivity is constant in time but in some cases changes in permeability may be brought about by chemical reaction and precipitation.
- Dupuit’s assumption for phreatic groundwater. The velocity of flow is proportional to the sine and not the tan of the angle of slope of the phreatic surface, and the flow is not horizontal and uniform near the phreatic surface.
- Thiem’s assumption for phreatic groundwater. Transmissivity is assumed to be constant, but the assumption is permitted only under certain conditions.
- The flow through the aquifer is assumed to be horizontal, and the flow through the covering and underlying layers vertical. This assumption is justified only if the vertical permeability of the semi pervious layers is less than that of the aquifers.
- The storage coefficient is constant and the release from storage is instantaneous. Slow release of water from falling water tables and irreversible compression of some formations do not conform to assumptions.
- Covering or intercalated layers of smaller permeability are assumed to have a uniform hydraulic resistance. All formulae are based on the assumption that these layers are continuous and of uniform hydraulic resistance, but most confining layers are impersistent and have varying hydraulic resistance.
- The borehole efficiency is assumed to be 100 percent. This assumption is rarely attained in borehole design and construction unless the borehole is open. Darcy’s law is valid. The law is only valid for laminar and not turbulent flow.
Darcy’s law or the law of linear resistance has been expressed in various forms and is only valid for laminar flow and invalid for turbulent flow. Reynolds Number has been used as an attempt to determine whether flow will be laminar or turbulent. However determination of whether flow is laminar or turbulent is difficult. Beside the above, there are other assumptions which have been made to derive solutions to specific flow problems.
Although it would appear that the assumptions on which the formulae are based require an aquifer model too ideal to be met in nature, data of aquifer tests conducted under non-ideal situations have also been analyzed successfully by making a judicious choice, from amongst the various formulae, of the one that takes into account the deviations from the ideal.
Mathematicians have developed such complex solutions for specific well and aquifer conditions that practical application of the theory is nearly impossible in light of all the geologic and hydrologic uncertainties. Furthermore, certain geologic and aquifer environments are so complex that reliable analytical solutions for flow patterns are almost impossible.
For a well performance test, yield and drawdown are recorded so that the specific capacity can be calculated. These data, taken under controlled conditions, give a measure of the productive capacity of the completed well and also provide information needed for the selection of the pumping equipment. This is only possible if the pumping test was conducted for the intended purpose. Aquifer tests are useful for predicting the effect of the new withdrawals on existing well, the future drawdown in a well at different discharges and the radius of the cone of influence for individual or multiple wells.
There are two types of aquifer tests i.e. the constant-rate test and the step-drawdown test. In the constant-rate test, the well is pumped for a significant length of time at one rate. The results from properly conducted tests are the most important tool in groundwater investigations. An aquifer test is a specific type of pumping test designed primarily to evaluate borehole characteristics. An aquifer test is a controlled field experiment to determine hydraulic properties of aquifers and associated rocks by observing groundwater flow in response to pumping, changes in head along streams, changes in the rate of recharge etc.
Normally an aquifer test is conducted with the aid of one pumping well but occasionally multiple well tests are conducted. For instance, multiple boreholes have been pumped simultaneously and the drawdown response observed over a wide area. Multiple borehole tests are conducted to verify predictions based on single well tests.
The following information on aquifers and confining layers can be obtained from pumping tests:-
Transmissivity and Hydraulic Conductivity if the aquifer thickness is known, Storage Coefficient, Specific Yield in unconfined aquifers, Leakage Factor of semi confined aquifers, Drainage Factor of semi confined aquifers and Hydraulic Resistance, Distance, direction and nature of impermeable barrier and recharge boundaries.
Certain pre-test studies should be conducted for successful execution of the test. Geologic, hydrologic and hydraulic setting of the aquifer system should be evaluated to enable proper understanding of the boundary conditions controlling the flow domain. The study would indicate the information gaps and help in locating the test site as well as the layout of observation boreholes, in selection of the type of drilling required, and in the designing of test well and observation wells.
In many cases, pumping test data can be interpreted in more than one way. Even in the best controlled test, the data may be confusing unless all hydraulic and geologic factors are taken into account. The analyst must be able to identify unreliable data so that the calculated values for transmissivity and storage coefficient will correctly predict aquifer performance.
The analyst must also keep in mind several other factors during data interpretation. These include the effect of the hydrogeologic character of the aquifer on the data, errors in data acquisition, potential omission of important environmental impacts, well design and construction practices, pumping rates, and the theoretical limits of the Jacob theory. In some cases, data from pumping tests are difficult to analyse, and there may be a high degree of uncertainty regarding the actual values of aquifer transmissivity and the storage coefficient, and hence the real hydraulic potential of an aquifer.
The extreme heterogeneity of many geologic formations leads to data which seem to defy rational explanation. Some aspects of well hydraulics are complicated, and few Hydrogeologists have mastered all phases of the subject. Equilibrium and nonequilibrium well equations used to compute hydraulic characteristics under certain conditions in aquifers are based on research by different Hydrogeologists in different flow regimes in divergent hydrogeological conditions e.g., steady or nonsteady conditions, aquifer geometry and boundary conditions e.g. double and multiple boundary systems, whether the aquifer has been partially or fully penetrate, and areal methods e.g. numerical analysis and flow net analysis.
Boreholes have been drilled at localities where drilling difficulties have been encountered. What are the implications of borehole construction where drilling difficulties are encountered during the course of drilling? Drilling difficulties have sometimes led to wrong borehole construction. For instance, three boreholes drilled in such circumstances were constructed as follows:-
|Plain Casings (m): 0-200
Slotted Casings (m) —
Open Hole (m): 200-315
|Plain Casings (m):0-127;130-166;178-197;203-209
Slotted Casings (m): 127-130;166-178;197-203
Borehole capped at the bottom.
|Plain Casings (m): 0-3;21-27;39-45;57-63;75-93;105-117;123-141
Slotted Casings (m): 3-21;27-39;45-57;63-75;93-105;117-123
Aquifer parameters are basically useful in groundwater modeling. A groundwater model has a wide range of applications and depends on the accuracy of data used in groundwater modeling. Where sufficient data is available, a groundwater model is used to supplement surface geophysical methods only where the behaviour of the local aquifer is well understood. Challenges in estimating local aquifer characteristics are as follows:-
- Lack of research and a credible data base required to accomplish the task. Therefore the accuracy of the figures presented in this report is not guaranteed. Aquifer systems are known to be complex and consequently analysis of any aquifer system should be based on the qualifications and practical experience of the analyst. Obtaining credible pumping test data has also been a problem. The behaviour of the local aquifer is currently not known. A groundwater model attempts to provide practical solutions in groundwater exploration, exploitation, development and management.
- Nature of aquifer. Has the aquifer been fully penetrated or not? Is the aquifer geometry known? Nature of flow. Are flow conditions at equilibrium or transient? Is flow steady, unsteady or radial? Is aquifer confined, semiconfined or unconfined? Is flow turbulent or laminar? Is aquifer leaky or not? Is storage elastic? Is there storage in aquitard? Was the annular space packed with gravel during the pumping test? Was the pumping test conducted under dynamic conditions? Are there any other factors?
- Geologic and morphological barriers or boundaries play a key role in estimating aquifer characteristics. The existing boreholes were not drilled for the purpose of estimating aquifer parameters. The horst rocks are anisotropic and consequently, aquifer characteristics of a borehole drilled in my neighbour’s land may not be relevant to the hydrogeological conditions in my land!
- Borehole logging techniques have not been used in designing and construction of the boreholes. The design and construction of practically all existing boreholes has been based on quotations issued by the drilling contractors and where applicable, conditions imposed by the relevant Authority responsible for regulating groundwater apportionment. In a pumping test, the rate of recovery plays an important role in estimating the local aquifer parameters. Suppose no drawdown is recorded during a pumping test?
- Due to lack of official identification documents and prior notification, the existing boreholes were not accessible. The problem has further been compounded by the reality that the exact location of a significant number of the existing boreholes is not known. The existing boreholes are supposed to be plotted on topographical or cadastral maps for ease of reference.
- Identification of the existing borehole owners in the field and the serial numbers is difficult due to the fact that very few people would be willing to provide borehole information to strangers.
Completion records of some of the boreholes are not available. Suppose my borehole is indeed registered in a company name and yet my neighbour knows only one of my names? The slow pace of issuance of Authorization and extension of already issued Authorizations has further aggravated the situation which may have led to desperate measures.
- A pumping test is conducted as part of the drilling process. Thereafter estimating the quantity of groundwater abstracted from the existing boreholes is a difficult task. Please note that in the absence of a water meter, almost all the borehole owners are unable to quantify the amount of groundwater abstracted from their boreholes.
1) Pumping Test Data.
A transcript obtained from Water Resources Management Authority indicates only the name of the borehole owner, the borehole coordinates and the distance from the selected site. Information contained in the transcript is therefore not useful for estimating aquifer characteristic. Aquifer characteristics presented in this report are based on single borehole pumping test data contained in the borehole completion records. In the table below BSN is the Borehole Serial Number; Distance (DIST) from the site in metres; Depth (D) in metres; WSD is/are the depth(s) at which water was struck in metres, Initial Static Water Level (ISWL) in metres and Q is the Tested Yield in cubic metres of water per Day.
|BSN||D/B||DEPTH||WATER STRUCK DEPTHS (m)||ISWL||Q|
|122 ; 133 ; 198
147 ; 217
130 ; 160 – 175
104 ; 137 ; 164
124 ; 178
78 ; 116 ; 199
Because of turbulent well loses as the water enters the well, the drawdown inside the well is significantly greater than the drawdown in the formation just outside the well. Use of time-drawdown data from a single well pump test will understate the formation transmissivity. This can be overcome by measuring the recovery of the water level in the well after the pump has been shut down.
Time-recovery data can then be plotted and the aquifer transmissivity determined. Either the Theis or Jacob method can be used. In the table presented below, t was the time (minutes) after the pump was shut down, DWL was the Dynamic Water Level (metres) at a specific period of time and REC was the respective rate of Recovery (metres).
2) Aquifer Transmissivity
Aquifer transmissivity (T, square metres per day) is the amount of water which can be transmitted horizontally through a unit width by the full saturated thickness of the aquifer under a hydraulic gradient of 1. Due to the complex nature of aquifers, this assumption is only valid in some cases. The transmissivity of an aquifer may be estimated from pumping test data obtained from observation boreholes or from a single borehole based on the complicated Theis method which is based on master curves obtained from known aquifer characteristics, W(u) or from the simplified Jacob straight line method. Based on the following formula, determination of aquifer transmissivity requires at least one observation borehole, where
T= 0.159Q In(R/r) and
H= head at observation borehole located at a distance R from the pumping borehole; h= head at observation borehole located at a distance r from the pumping borehole.
An aquifer test may be made even if there are no observation wells. In this case, the drawdown must be measured in the pumping well. However, a plot of drawdown versus time for the pumping well can be used to determine aquifer transmissivity. The transmissivity of an aquifer may be estimated from pumping test data obtained from a single borehole using the Jacob straight line method where Δ(hº-h)=drawdown per log cycle of time, in metres, where
T= 2.3 Q or T= 0.183Q
The Jacob straight line method can be used to determine Δ(hº-h). In a pumping test, data obtained for the initial 10 minutes in either the drawdown or recovery test is almost always unreliable. Pumping test data usually stabilizes after a duration of about 10 to 15 minutes. Recovery versus Time graphs are based on pumping test data presented in the borehole completion records. See the following recovery versus time graphs.
Based on the Recovery versus Time graph, aquifer transmissivity at the specific locality in which the boreholes were drilled can be estimated as follows:-
3) Specific Capacity
Well efficiency is the ratio of the actual specific capacity at the designed well yield after 24 hours continuous pumping to the maximum specific capacity possible, calculated from formation well characteristics and geometry. In this method, it is possible to identify how much of the total head loss is attributed to natural losses in the formation and those caused by well construction damage to the aquifer and by installation of screen and filter pack.
From the Dupuit equation for steady flow, and that all other factors remaining constant, increasing the well diameter enhances the yield only marginally i.e. about 10%. In some cases however, it may be worthwhile to increase the diameter to obtain 15 to 25% more water, depending on the cost factors involved.
The Specific Capacity (C, square metres per day) is the yield of a borehole (Q) per unit drawdown i.e. Q÷Δs where Δs= PWL – ISWL, where PWL is the Pumping Water Level and depends on the quantity of water held in storage released by the aquifer and the capacity of the pump used for the pumping test. Based on the borehole completion records, the specific capacities at the exact localities where the boreholes were drilled have been summarized as follows:-
4) Hydraulic Conductivity
Hydraulic Conductivity (metres per day) is the rate at which water moves through a certain medium and can be measured directly in the laboratory using a cylindrical rock sample held in the permeameter chamber. For confined aquifers, Hydraulic Conductivity or the Coefficient of Permeability (K) can be estimated from the equation, where b= aquifer thickness, m
K=T/b and for unconfined aquifers K= 0.159 Q In(R/r)
Determination of the actual thickness of the aquifer can only be accomplished by geophysical borehole logging techniques; borehole logging techniques do not appear anywhere in the borehole completion reports. In the rotary drilling method, only the top of the aquifer can be reported accurately. In the percussion drilling method, water is used as a coolant and during the course of drilling it is difficult to establish the depths at which water was struck in a borehole. See obvious disparities in the water struck depths reported in the borehole completion reports. Description of the rock samples reported in the borehole completion records is not useful either.
It is therefore difficult to determine the actual vertical thickness of the aquifer. Consequently, the hydraulic conductivity is difficult to determine. Potentiometric surface represents the level to which water will rise in tightly cased or uncased wells. If the head varies significantly with depth in the aquifer, then there may be more than one potentiometric surface. The water table is a particular potentiometric surface for an unconfined aquifer. The Potentiometric surface cannot be used to determine thickness of an aquifer.
A slug test can be performed in a small diameter monitoring well. This type of test can be used to determine the hydraulic conductivity of the formation in the immediate vicinity of a monitoring well. A known volume of water is quickly drawn from or added to the monitoring well. The rate at which the water level falls or rises is measured. These data are then analyzed by an appropriate method.
However, for the purpose of visualization, the aquifer has been assumed to be confined and the thickness of the aquifer (b) to be the difference between the depth at which the main aquifer (MA) was reported to have been struck and the final borehole depth (D).
5) Storage Coefficient
The Storage Coefficient (Storativity) and or Specific Yield is the volume of water released from storage or taken into storage per unit of aquifer storage area per unit change in head. There are energy losses as the water rushes into the borehole, so that the head in the aquifer is higher than the water level in the pumping well. For this reason, aquifer storativity cannot be determined from a single borehole test. To estimate the Storage coefficient, the radial distance from the circular section of the cone of depression to the borehole should be known and therefore observation boreholes are needed. Storativity, S, is small and can be estimated from the equation
S=storativity, dimensionless, r= the radial distance from the circular section of the cone of depression to the observation borehole, m; tº = time where the straight line intersects the zero-drawdown axis, days.
The pumping test data of borehole serial number 20597 has been used to illustrate how to estimate storativity between two or more observation boreholes. So, imagine a situation where borehole serial number 20597 was used as an observation borehole during the pumping test of a new borehole drilled in the Client’s land. Suppose borehole 20597 is located 1800 metres away from the new borehole. r would have been equal to 1800 metres Based on the recovery versus time graph of borehole 20597, T=4.96 and tº = 5.8 minutes or 0.6 ÷1440 day, therefore the Storativity, S would have been equal to
(2.25 x 4.96 x 5.8) ÷ (1800² x 1440) =?
A borehole is meant to supply water to an area where a convenient source of surface water is nonexistent. Consequently, a hole without water is a waste of resources. Drilling a borehole should justify its intended purpose by being cost effective. The total open area in a slotted casing is small and varies between 5 and 20 percent of the total length of the casing. Gravel packed in the annular space of a borehole blocks a high percentage of the casing slots and understates aquifer parameters. Please note that the pore space in the annular space of a borehole packed with gravel is small and depends on the roundness of the gravel.
Gravel packed in a borehole reduces the open area in a casing slot and by extension, the amount of water entering the borehole. Corrosive water, silt or iron bacteria may block a casing slot permanently. Clogging of casing slots reduces a borehole’s efficiency, lifespan, the quantity of water entering a borehole and increases the entrance velocity. Borehole 3783 initially had a water column of 56 metres and the total length of slotted casings was only 25 metres. Could there have been drilling difficulties? Caving is reported to have occurred between a depth of 49-62 metres and between a depth of 85-98 metres. The total length of casings inserted in the borehole was 98.5 metres. This implies that about 8.2 metres of the borehole was not cased. The borehole initially had the following characteristics.
The borehole was cleaned 5 years later. A pumping test conducted after cleaning indicates that the borehole yield had dropped drastically to 87 cubic metres of water per day. Initially, the static water level was 50.60 metres and five years later, the static water level had dropped to 72.63 metres. Why had the water level dropped? The slope (Δ(hº-h) obtained from the recovery versus time graph is the same 5 years after the pumping test was initially conducted. The low transmissivity and specific capacity values calculated from a pumping test conducted 5 years after the initial pumping test clearly indicate that some of the casing slots may have clogged. Five year later, the borehole had the following characteristics:-
The human body is small and technology has enabled scanning of the live functions of any part of the human body in their normal operational status. Direct observation of groundwater movement and determination of other related aquifer parameters is impossible. It is therefore important to note that geophysical data alone provides little definitive information. Consequently all data obtained by geophysical prospecting methods must be correlated or verified with the actual rock samples retrieved from the borehole.
The stratigraphical succession of a given locality depends on the accuracy of the description of rock samples retrieved from boreholes and not necessarily on the geophysical data obtained in the field. Where boreholes exist, the lithologic conditions of a new borehole can be predicted using cross sections. In the surface geophysical vertical electrical resistivity groundwater prospecting method, low apparent resistivity values are often associated with prospects of striking water, either saline or not, and clay.
High apparent resistivity values are often associated with fresh rock formations or diminishing groundwater prospects and yet clay of various thicknesses has also been encountered in localities where clay is not expected. A borehole was drilled at a locality where the apparent resistivity values obtained in the field varied between 860 and 393 Ohm.metre. The borehole was drilled to a depth of 170 metres and contrary to expectation, a thick clay formation was encountered from a depth of 105 metres to a depth of 165 metres. The borehole had a low yield and was subsequently abandoned.
The prospects of striking groundwater within the project area vary significantly depending on various factors such as the porosity of the parent rocks and the degree of interconnection of crevices. Fissures or crevices in anisotropic rocks are generally limited in extent mainly due to the compactness of the rocks as a result of their resistance to weathering processes. The complexity of groundwater occurrence in these rocks has been demonstrated in cases where two boreholes are close to one another and yet their yields vary significantly.
Due to various natural factors, groundwater prospects in metamorphic rocks are quite unpredictable. For planning purposes, the number of boreholes expected to be drilled should always be at least 30 percent higher than the proposed boreholes. For instance, if 10 boreholes are required, the projected number of boreholes to be drilled should be at least 13 boreholes. In metamorphic rocks, the amount of fracturing or fissuring decreases with depth. Consequently, the maximum recommended depth in metamorphic rocks is about 150 metres or rarely to a depth of 180 metres.
Groundwater does not belong to individuals, organizations or licensed water service providers. Water belongs to all of us collectively and therefore, owning a borehole does not mean that I own groundwater in my immediate vicinity. In a productive borehole, groundwater must have flowed through other peoples land and these people also have a right to sink a borehole. There are numerous cases where boreholes have not been affected by boreholes drilled close to the existing boreholes or hand dug wells.
Some of the boreholes or dug wells are even less than 10 metres away from the boreholes. There is no land without surface boundaries throughout the country. Land is either owned by individuals or collectively by communities or by the government. Imagine a situation where every piece of land is fenced off by a vertical impermeable geological boundary to a depth of say, a hundred kilometres from the ground surface.
This implies that there would be no movement of groundwater and therefore productive boreholes or even rivers would not exist. What is more important is for us to conserve the areas of groundwater recharge and how we share the responsibility of managing the available groundwater resources. Prudent management of groundwater resources means that the rate of recharge is higher than the rate of groundwater abstraction.
Groundwater movement in the subsurface does not recognize our land surface boundaries and neither does it recognize the topographical water catchment zones. Groundwater movement is strictly governed by the hydraulic gradient, local geology, the degree of weathering or fracturing, porosity or availability of pores or openings in the rock formation, recharge and movement of water from areas of recharge to points or areas of discharge. Some knowledge of aquifer recharge and other related f actors are necessary for the purpose of groundwater management.
The rate of inflow-outflow = ± storage is currently not known and consequently the quantity of water available for apportionment is indeed difficult to determine. The main constraints in groundwater exploration and management are mainly based on lack of research and a credible data base.
A pumping test exceeding a duration of 12 hours is likely to be a waste of water and possibly a nuisance to my neighbours. Imagine a situation where the discharge of a borehole is about 1 to 50 cubic metres of water per hour. During the pumping test conducted for a duration of 24 hours, the total volume of water abstracted from the borehole would vary between 24 and 1200 cubic metres of water during the entire duration of the pumping test.
Where does this amount of water go during the pumping test duration? Would it not be a waste of water if this quantity of water is not used for any other purpose except determining the specific capacity of the borehole? The pump capacity and the depth at which the pump intake would be set can be determined from a pumping test conducted over a relatively shorter duration.
Where possible, the quality of water should be determined before the pumping test by using a portable water testing kit so that elaborate measures are taken to make maximum use of water abstracted from a borehole during the pumping test. For instance, water abstracted from the borehole can be stored or injected back to the aquifer in the nearest existing borehole for purpose of performing a slug test. Based on the prevailing hydrogeological conditions, it has been deduced as follows: –
- The selected borehole site may have several water bearing formations or aquifers. The water bearing materials or weathered zones are expected to be encountered at various depths. Please note that subsurface geological boundaries play an important role in the hydrogeological conditions of a particular area. The location of the boundaries is currently not known and therefore the actual yield and water quality can only be determined once a borehole has been drilled.
The quantity of water in the completed borehole or the borehole efficiency is likely to depend on various factors such as the depth of the borehole and borehole design, casing and screen material used, the total length of screen or total pieces of slotted casings inserted in the borehole and the procedures used to construct the borehole.
- The depths at which water may be struck and the static water levels are expected to be quite erratic. This implies that the piezometric pressures may fluctuate and therefore, the hydrogeological conditions in the area may not be isotropic or uniform. The area does not have a reliable water supply network.
- The local water undertaker has been unable to supply sufficient water to the Client. Due to the expected increase in the water demand, the existing sources of water would be unable to cope with the situation. Consequently, the only viable long term alternative available to the Client is to sink a borehole.
From the foregoing discussions, it is now recommended as follows: –
- The Client’s attention is drawn to the provisions of Rule Numbers 72 and 73 contained in Part IV of the Water Resources Management Rules dated 28th September 2007. Kenya Gazette Supplement Number 92. Legal Notice Number 171.
- No groundwater exploration method can be used to determine the quantity and quality of water before drilling. It is therefore not realistic to quantify the anticipated borehole yield or the quality of water available in a borehole before drilling. A borehole is to be drilled at the selected site to a minimum depth of 180 metres or a maximum depth of 210 metres. The selected borehole drilling site is known to the Client.
- Some boreholes are reported to have been drilled fairly close to the proposed borehole drilling site For the purpose of groundwater conservation, any aquifer struck between the ground surface and a depth of 100 metres should be sealed off with plain casings or any other acceptable means of eliminating or minimizing potential groundwater interference between the boreholes. The static water levels of the existing boreholes should be monitored during the pumping test.
- The borehole should be protected from possible sources of contamination by grouting a certain length of the borehole from the ground surface. For instance, grouting may be done from the ground surface upto the borehole’s static water level or even below this level depending on the local conditions by using plain casings and clay or bentonite.
- Water in a borehole is always turbid during the course of drilling and therefore the velocity of flow and the groundwater flow path can be estimated. This requires prior arrangements with the existing borehole owners, where applicable.
- The borehole must be developed adequately. Thereafter the pumping test of the completed borehole is to be conducted for a maximum period of 12 hours. The purpose of this test is to determine the actual quantity of groundwater that can be abstracted from the borehole, the abstraction capacity of the pumping equipment to be installed and the depth at which the pump can be installed. A groundwater recovery test is necessary in order to determine the time required for water to rise up to it’s natural or initial static water level before pumping commenced.
- A sufficient quantity of groundwater must be collected in a clean plastic or glass container towards the end of the pumping test. The collected water is to be submitted to the nearest competent water testing laboratory for analysis. The analysis is meant to determine the suitability of water for the intended purpose by analyzing the physical, chemical and bacteriological characteristics of groundwater.
Please note that water quality conditions in any given area are quite dynamic and not static. Clogging of the casing slots may occur over a period of time. Pumping tests should therefore be conducted periodically even after the borehole becomes operational.
- Upon completion of the borehole, a water meter and an airline are required in order to facilitate monitoring of the groundwater abstraction and the static water level measurements in the boreholes, respectively. An airline consists of a long open plastic tube or several pipes.
These pipes are connected together and are normally attached to the pump’s drop pipes when the pump is being installed. A water meter and the airline are required for the purpose of determining the relationship between the rate of groundwater abstraction and the static or dynamic water level in the borehole at any given time.
- The design and construction of a borehole should ensure that the water column available in the borehole is utilized fully. Supervision of the work by an experienced Hydrogeologist is recommended. Borehole drilling completion records are very important for future reference. Upon completion of the final contract payment, the drilling contractor is required to submit the borehole completion records to the Client and to the groundwater managing Authority. The completion records must among others, indicate the following:-
- Borehole owner and borehole serial number
- Location of borehole; land reference number, locality name, coordinates etc
- Name of Drilling Contractor and Contractor’s License Number
- Date(s) of borehole drilling and completion of drilling process
- Diameter of open borehole and the precise borehole depth
- Geological log (description) of the penetrated rock formations presented by a Geologist or a Hydrogeologist.
- Depths at which water was struck and the static water level
- Borehole construction details i.e. total length of casings inserted and location of slotted casings, casing diameter, slot length and width, size of gravel etc
- Tabulated pumping and recovery test results indicating the depth at which the pump intake was set and the time intervals
- Water quality analysis results presented by a Chemist.
Adsorption: The attraction and adhesion of a layer of ions from an aqueous solution to the solid mineral surfaces with which it is in contact.
Advection: The process by which solutes are transported by the motion of flowing groundwater.
Agglomerate: A volcanic breccia formed by disruption of a solidified crust or hardened plug of lava. Blocks may fit together as a loose mosaic or be completely disordered.
Alluvium: Sediments deposited by flowing rivers. Depending upon the location in the floodplain of the river, different sized sediments are deposited.
Andesite: A dark coloured, fine grained extrusive rock that, when porphyritic, contains phenocrysts composed primarily of zoned sodic plagioclase and one or more of the mafic minerals with a groundmass composed generally of the same minerals as the phenocrysts.
Anisotropy: The condition under which one or more of the hydraulic properties of an aquifer vary according to the direction of flow.
Aquiclude: A low permeability unit that forms either the upper or lower boundary of a groundwater flow system.
Aquifer: Rock or sediment in a formation, group of formations, or part of a formation that is saturated and sufficiently permeable to transmit economic quantities of water to wells and springs.
Aquifuge: An absolutely impermeable unit that will neither store nor transmit water.
Aquitard: A low permeability unit that can store groundwater and also transmit it slowly from one aquifer to another.
Artificial recharge: The process by which water can be injected or added to an aquifer. Dug basins, drilled wells, or simply the spread of water across the land surface are all means of artificial recharge.
Bailer: A devise used to withdraw a water sample from a small diameter well or piezometer. A bailer typically is a piece of pipe attached to a wire and having a check valve in the bottom.
Barrier boundary: An aquifer system boundary represented by a rock mass that is not a source of water.
Basalt: A dark coloured igneous rock, commonly extrusive, composed primarily of calcic plagioclase and pyroxene; the fine grained equivalent of gabbro.
Baseflow: That part of a stream discharge from groundwater seeping into the stream.
Baseflow recession: The declining rate of discharge of a stream fed only by baseflow for an extended period. Typically, a baseflow recession will be exponential.
Baseflow recession hydrograph: A hydrograph that shows a baseflow recession curve.
Borehole geochemical probe: A water quality monitoring devise that is lowered into a well on a cable and that can make a direct reading of such parameters as pH, Eh, temperature, and specific conductivity.
Borehole geophysics: The general field of geophysics developed around the lowering of various probes into a well.
Caliper log: A borehole log of the diameter of an uncased well.
Capillary forces: The forces acting on soil moisture in the unsaturated zone, attributable to molecular attraction between soil particles and water.
Capillary fringe: The zone immediately above the water table, where water is drawn upward by capillary attraction.
Cementation: The process by which some of the voids in a sediment are filled with precipitated materials such as silica, calcite, iron oxide, and that is a part of diagenesis.
Collection lysimeters: A devise installed in the unsaturated zone to collect a water quality sample by having the water drain downward by gravity into a collection pit.
Confined aquifer: An aquifer overlain by a confining bed. The confining bed has a significantly lower hydraulic conductivity than the aquifer.
Confining layer: A body of material of low hydraulic conductivity which is stratigraphically adjacent to one or more aquifers. It may lie above or below the aquifer.
Connate water: Interstitial water that was not buried with a rock that has been out of contact with the atmosphere for an appreciable part of a geologic period.
Darcian velocity: See specific discharge.
Darcy’s law: An equation that can be used to compute the quantity of water flowing through an aquifer.
Diffusion: The process by which ionic and molecular species dissolved in water move from areas of higher concentration to areas of lower concentration.
Dipole array: A particular arrangement of electrodes used to measure surface electrical resistivity
Discharge: The volume of water flowing in a stream or through an aquifer past a specific point in a given period of time.
Drainage basin: The land area from which surface runoff drains into a stream system.
Drawdown: A lowering of the water table of an unconfined aquifer or the potentiometric surface of a confined aquifer caused by pumping of groundwater from wells.
Dupuit assumptions: Assumptions for flow in an unconfined aquifer that(1)the hydraulic gradient is equal to the slope of the water table,(2)the streamlines are horizontal, and (3)the equipotential lines are vertical.
Dupuit equation: An equation for the volume of water flowing in an unconfined aquifer; based upon the Dupuit assumptions.
Dynamic equilibrium: A condition in which the amount of recharge to an aquifer equals the amount of natural discharge.
Electrical sounding: An earth-resistivity survey made at the same location by putting the electrodes progressively farther apart. It shows the change of apparent resistivity with depth.
Evapotranspiration: The sum of evaporation plus transpiration.
Fault: A fracture or fracture zone along which there has been displacement of the sides relative to one another parallel to the fracture.
Field capacity: The maximum amount of water that the unsaturated zone of a soil can hold against the pull of gravity. The field capacity is dependent on the length of time the soil has been undergoing gravity drainage.
Flow net: The set of intersecting equipotential lines and flow lines representing two-dimensional steady flow through porous media.
Fossil water: Interstitial water that was buried at the same time as the original sediment.
Gamma-gamma radiation log: A borehole log in which a source of gamma radiation as well as a detector are lowered into a borehole. The log measures bulk density of the formation and fluids.
Ghyben-Herzberg principle: An equation that relates the depth of a salt water interface in a coastal aquifer to the height of the fresh water table above sea level.
Gneiss: A foliated rock formed by regional metamorphism, in which bands or lenticles of granular minerals alternate with bands or lenticles of minerals with flaky or elongate prismatic habit.
Gravity drainage: The downward movement of water in the vadose zone due to gravity.
Groundwater divide: The boundary between two adjacent groundwater basins. The divide is represented by a high in the water table.
Hantush-Jacob formula: An equation to describe the change in hydraulic head with time during pumping of a leaky confined aquifer.
Hazen method: An empirical equation that can be used to approximate the hydraulic conductivity of a sediment on the basis of the effective grain size.
Homogeneous: Pertaining to a substance having identical characteristics everywhere.
Horizontal profiling: A method of making an earth resistivity survey by measuring the apparent resistivity using the same electrode spacings at different grid points around an area.
Hydraulic conductivity: A coefficient of proportionality describing the rate at which water can move through a permeable medium. The density and kinematic viscosity of the water must be considered in determining hydraulic conductivity.
Hydraulic gradient: The change in the total head with a change in distance in a given direction. The direction is that which yields a maximum rate of decrease in head.
Hydrograph: A graph that shows some property of groundwater or surface water as a function of time.
Hydrologic cycle: The circulation of water from the oceans through the atmosphere to the land and ultimately back to the ocean.
Infiltration: The flow of water downward from the land surface into and through the upper soil layers.
Intermediate zone: That part of the unsaturated zone below the root zone and above the capillary fringe.
Intrinsic permeability: Pertaining to the relative ease with which a porous medium can transmit a liquid under a hydraulic or potential gradient. It is a property of the porous medium and is independent of the nature of the liquid or the potential field.
Isotropy: The condition in which hydraulic properties of the aquifer are equal in all directions.
Jacob straight line method: A graphical method using semi logarithmic paper and the Theis equation for evaluating the results of a pumping test.
Juvenile water: Water entering the hydrologic cycle for the first time.
Karst: The type of geologic terrain underlain by carbonate rocks where significant solution of the rock has occurred due to flowing groundwater.
Laminar flow: The type of flow in which the fluid particles follow paths that are smooth, straight, and parallel to the channel wall. In laminar flow, the viscosity of the fluid damps out turbulent motion.
Leachate: Water that contains a high amount of dissolved solids and is created by liquid seeping from a landfill.
Lithologic log: A record of the lithology of the rock and soil encountered in a borehole from the surface to the bottom.
Lysimeter: A field devise containing a soil column and vegetation; used for measuring actual evapotranspiration.
Model calibration: The process by which the independent variables of a digital computer model are varied in order to calibrate a dependent variable such as head against a known value such as a water table map.
Model verification: The process by which a digital computer model that has been calibrated against a steady state condition is tested to see if it can generate a transient response, such as the decline in the water table with pumping that matches the known history of the aquifer.
Perched aquifer: A region in the unsaturated zone where the soil may be locally saturated because it overlies a low permeability unit.
Perched groundwater: The water in an isolated, saturated zone located in the zone of aeration. It is the result of the presence of a layer of material of low hydraulic conductivity, called a perching bed. Perched groundwater will have a perched water table.
Phonolite: A fine grained rock primarily composed of alkali feldspar and with nepheline as the main feldspathoid.
Porosity: The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment.
Potentiometric surface: A surface that represents the level to which water will rise in tightly cased wells. if the head varies significantly with depth in the aquifer, then there may be more than one potentiometric surface. The water table is a particular potentiometric surface for an unconfined aquifer.
Primary porosity: The porosity that represents the original pore openings when a rock or sediment formed.
Pumping cone: The area around a discharging well where the hydraulic head in the aquifer has been lowered by pumping. Also called cone of depression.
Pumping test: A test made by pumping a well for a period of time and observing the change in hydraulic head in the aquifer. A pumping test may be used to determine the capacity of the well and the hydraulic characteristics of the aquifer. Also called aquifer test.
Radial flow: The flow of water toward a vertically oriented well.
Recharge area: An area in which there are downward components of hydraulic head in the aquifer. Infiltration moves downward into the deeper parts of an aquifer in a recharge area.
Recharge boundary: An aquifer system boundary that adds water to the aquifer. Streams and lakes are typically recharge boundaries.
Recovery: The rate at which the water level in a well rises after the pump has been shut down. It is the inverse of drawdown.
Resistivity log: A borehole log made by lowering two current electrodes into the borehole and measuring the resistivity between two additional electrodes. It measures the electrical resistivity of the formation and contained fluids near the probe.
Runoff: The total amount of water flowing in a stream. It includes overland flow, return flow, interflow and baseflow.
Saturated zone: The zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is the top of the saturated zone in an unconfined aquifer.
Schlumberger array: A particular arrangement of electrodes used to measure surface electrical resistivity.
Secondary Porosity: The porosity that has been caused by fractures or weathering in a rock or sediment after it has been formed.
Semiconfined Aquifer: An aquifer confined by a low permeability layer that permits water to slowly flow through it. During pumping of the aquifer, recharge to the aquifer can occur across the confining layer. Also known as leaky artesian or leaky confined aquifer.
Slug test: An aquifer test made either by pouring a small instantaneous of charge water into a well or by withdrawing a slug of water from the well. a synonym for this test, when a slug of water is removed from the well, is a bail-down test.
Soil moisture: The water contained in the unsaturated zone.
Specific capacity: An expression of the productivity of a well, obtained by dividing the rate of discharge of water from the well by the drawdown of the water level in the well. Specific capacity should be described on the basis of the number of hours of pumping prior to the time the drawdown measurement is made. It will generally decrease with time as the drawdown increases.
Specific discharge: An apparent velocity calculated from Darcy’s law: represents the flow rate at which water would flow in an aquifer if the aquifer were an open conduit.
Specific yield: the ratio of the volume of water a rock or soil will yield by gravity drainage to the volume of the rock or soil. Gravity drainage may take many months to occur.
Specific storage: The amount of water released from or taken into storage per unit volume of a porous medium per unit change in head.
Spontaneous potential log: A borehole log made by measuring the natural electrical potential that develops between the formation and the borehole fluids.
Steady Flow: The flow that occurs when, at any point in the flow field, the magnitude and direction of the specific discharge are constant in time.
Storativity: The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. It is equal to the product of specific storage and aquifer thickness. In an unconfined aquifer, the storativity is equivalent to the specific yield. Also called coefficient of storage.
Theis equation: An equation for the flow of groundwater in a fully confined aquifer.
Turbulent flow: that type of flow in which the fluid particles move along very irregular paths. Momentum can be exchanged between one portion of the fluid and another.
Trachyte: A group of fine grained, generally porphyritic, extrusive rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase, also, any member of that group
Tuff: A general term for all consolidated pyroclastic rocks.
Unconfined Aquifer: An aquifer in which there are no confining beds between the zone of saturation and the surface. There will be a water table in unconfined aquifer. Water table aquifer is a synonym.
Unsteady flow: The flow that occurs when, at any point in the flow field, the magnitude or direction of the specific discharge changes with time. Also called transient flow or nonsteady flow.
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Fetter, C.W., 1994 – Applied Hydrogeology
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Kruseman, G.P., de Ridder, N.A., 1992 – Analysis and Evaluation of Pumping Test Data
Linsley R.K., et al 1992 – Water Resources Engineering
Saggerson, E.P., 1991- Geology of the Nairobi Area, Report Number 98