Orwa Borehole Geotech Draft

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HYDROGEOLOGICAL SURVEY REPORT

ORWA BOREHOLE SITE INVESTIGATIONS ORWA LOCATION

WEST POKOT DISTRICT

CLIENT The Chairman, ORWA Water Project P.O Box Kapenguria

February, 2011

CONSULTANT Mr. Charles N. Kithome Bsc(Hons). Msc. Diploma Registered Hydrogeologist/Water engineer GRB,ERB,NEMA,ESRI P.O BOX 22294-00100, Nairobi, Kenya [email protected]

Orwa Community Water and Sanitation Project, West Pokot Region Borehole Site Investigations

SUMMARY

Background The consultants were commissioned by the Orwa Community Water and sanitation Project/ World Vision ORWA IPA to undertake investigations for suitable borehole site within ORWA village, Orwa Location of West Pokot district in different community designated areas with the objective of supplying safe and clean water to the target communities. The current report describes the objectives of the programme, the overview of the project area, the methodology and the expected output. The report also describes the results of the geophysical surveys carried between 16th November and 22th November 2010. Project objectives and scope The overall objective of the Project is to improve the water supply in various community areas within the domain of Orwa Water project in West Pokot district and ensure reliable and safe water by increasing the supply of potable water within the affected communities where distances to safe water points are long and in places there are no safe water sources at all and the communities have to depend on surface water. The specific objectives of this Study were to:-

• To undertake hydrogeological/geophysical investigation on the occurrence of groundwater in various community areas with a view to identifying suitable borehole drilling site.

• Identify the most promising site for the proposed borehole drilling and advise the client on the best drilling method.

• Present, to the client, a detailed qualitatative and quantitative report of the overall findings and advice on project investment viability and substantial groundwater abstraction feasibility

• Recommend the best method of the proposed borehole drilling

• Obtain the necessary groundwater water authorizations and permits on behalf of the client and integrate the component of an EIA /audit report ahead of the actual drilling

Project area Orwa IPA is located in West Pokot district, Rift Valley Province of Kenya. The Project is funded by World Vision Hong Kong. The programme started in 2008 as a result of water needs assessment findings carried out by World Vision Kenya in the region. The IPA area is composed of Endough location of Sook Division of West Pokot district and Sekerr and Parkoyo locations of Sigor division in Pokot Central district in the north rift zone, of Rift Valley province in Kenya with an approximate area of 797km2

The current water demand for the investigated community areas is not known due to lack of proper demographic data. It is however reported that the community anticipates about 20m3 of water daily to meet their envisaged purposes. Hydrogeological System The IPA is divided into two agro-ecological zones. The lower zones consist of altitudes of less than 1500m above sea level and characterized by dry weather with low rainfall. These areas are prone to extensive soil erosion resulting in the loss of top soil thus reducing crop yield and cause siltation to Turkwel Dam. The upper zones are located on altitudes between 1500m and 2100m experiencing fairly cooler climatic conditions enabling the practice of agro pastoralism. Agro pastoralism is the main occupation for the majority of the community members in this region. Crops such as maize, beans, and finger millet are grown. (WPDDP 2002-2008.)


The hydrogeology of an area is determined by the nature of the parent rock, structural features, weathering processes and precipitation patterns. The main constraint for aquifer development in basement areas is probably the lack of recharge: the interplay of the shallow weathering of the basement rock, the overlying thick laterite lenses and the sandy bars, both at shallow and deeper levels is very complicated. This largely influences the groundwater flow mechanism (with no geological surface manifestation): some sandy stringers are isolated in terms of recharge wedging out into clay matrix, whereas others are hydraulically connected with recharge mechanism but occur at variable depths. Groundwater occurrence in the Basement rocks is likely to be localized, and limited to relatively small and isolated pockets. However, depending on the parent material, water may be struck in the weathered top layers (regolith and saprock). The underlying fresh Basement is in most cases dry, and significant amounts of groundwater can only be expected in fractures (cracks, joints, fissures, and faults).

Geophysical Investigations Combined geophysical and hydro-geological fieldwork was carried out between 26th - 27th November,2010. The main aim of the geophysical investigations was to get an insight into the hydrogeological conditions prevailing within the selected areas designated by the community as well as identifying optimum borehole drilling sites in those particular community areas. These investigations were carried out in four (4) community areas identified previously by Orwa Water Project/World Vision ORWA IPA as areas that require intervention due high water demand. In total Four (4)no. VES soundings and One(1) Control VES along an existing borehole was executed for calibration purposes. A Garmin e-trex GPS Satellite Navigator and a Trimble Juno SB Mapper (Data logger) with GPS and ArchPAD software were used to obtain accurate geospatial information and co-ordinates of the measured/surveyed points and log the identified VESs as well as collecting secondary hydro-geological data. All electrical measurements were undertaken using a TERRAMETER SAS 300C with depth booster and LUND-Imaging. Data analysis was qualitatively plotted in the field on Bi-logarithmic graph paper and later detailed quantitative interpretations were undertaken in the office using Interpex-1D and Schlumberger as well forward modelling and inversion using LOKE software Results and Discussion

Vertical electrical soundings (VES) provide quantitative depth-resistivity information for a particular site. VES sites were selected at representative points in relation to anomalies picked by profiling technique as well as GIS-remote sensing technique (satellite imagery analysis). The geomorphologic observations combined with the satellite imagery analysis during the desk study and field reconnaissance phase was used as the criteria for selection of the profile sites. Locations for profiling were selected at locations mapped as having structural lineaments and geormophological interruptions. The VES measurements were executed in an expanding Schlumberger array, with electrode spreads of AB between 260 and 400 m. This separation gives fairly reliable interpretations down to a depth of respectively 65 to 100 m, but only approximate solutions for resistivity layering at deeper levels. Depths beyond this level are only indicative, and do not give the precise position of the interpreted layers. In general, most of the profiles were executed in an east – west direction as the fault/fracture zones generally trend north – south or northwest – southeast. In most cases the profiles which were carried out using schlumberger array did not exceed 500 m. They were carried out at 20m station intervals and an AB separation of 100 m. While carrying out this exercise, the target was low anomalies which in this case indicate weathered or weak zones considering that the bedrock in most places is fairly shallow. The points at which low anomalies were noted were marked for vertical electrical soundings (VES). It is worth mentioning that in some cases, other indicators like large trees (e.g. VES III (ORWA# 3), VES IV (ORWA # 4, clearly marked the fault/fracture zones and this in combination with the profiles gave very excellent results.


The VES measurements were executed on the anomalous points along the profile lines. The most distinct characteristic noted on these measurements is the presence of weathered zones between 50m and a maximum depth of 80 m bgl within the suspected aquifer zones. It is quite clear that these aquifers are discontinuous and are not necessarily all connected. However in some places, the resistivities observed from the measurements indicate the presence of clayey material at depth and this is also noticeable where gullies expose the stratigraphy though to a shallow depth of not more than 3 metres. In interpretation of the resultant curves, 6 – layer models were adopted based on the trend of the curves. It should be noted that although there was vastness of the area of investigations, there was a slightly distinct variation in the trend of curves, hence similar layered models. In general however, the most promising curves depict a high resistivity layer overlying the low resistivity regime (aquifer) in most of the layers. In theory, this indicates that the main aquifer does not derive its recharge locally but is connected to a wider, probably regional recharge system which would mean a more reliable and stable supply. It is expected that such an aquifer would be semi confined or confined depending on the recharge. Conclusion and Recommendations The results of the geophysical site investigations together with the recommended sites for drilling and other relevant alternatives are summarised in Tables1. It should be noted here that the recommended depths are the maximum depths but should sufficient water be encountered after striking the main aquifer but before attaining the final depth, then drilling can be discontinued. It is also recommended that proper construction of the borehole after drilling should be adhered to. This is one of the greatest problems affecting the boreholes that lack proper supervision. In particular, proper installation of casings and screens as well as installation of gravel pack is emphasized. After borehole construction where possible, proper test pumping should be carried out to determine the yield of the borehole and other aquifer parameters.

Table 1: The results of the geophysical site investigation SITE Name VES

No. Coordinates (UTM) WSL

(m) Max. Depth (m)

Prospective Yield(m3/hr) Longitude Latitude Alt.(m)

1 Orwa I 035027’ 19.78’’ 01032’ 52.7’’ 918 25 130 Negative

2 Orwa II 035029’ 19.78’’ 01039’ 33.8’’ 923 25-80 150 Fair

3 Orwa III 035029’ 5.33’’ 01039’ 31.8’’ 903 20-80 130 Fair to Good

4 Orwa IV 035028’ 57.8’’ 01039’ 30.5’’ 919 20-80 150 Good)


TABLE OF CONTENTS

PROJECT SUMMARY SHEET .................................................................................................... vi

ABBREVIATIONS ..................................................................................................................... vii

GLOSSARY OF TECHNICAL TERMS ..................................................................................... viii

1.0 INTRODUCTION ........................................................................................................... 9

1.1 General Information ..................................................................................................... 9

1.2 Location ........................................................................................................................... 9

2.0 WATER SUPPLY SITUATION....................................................................................... 10

2.1 Sources of water ........................................................................................................ 10

2.2 Population and Water Demand ..................................................................................... 10

3.0 CLIMATE, PHYSIOGRAPHY AND LAND USE ............................................................. 12

3.1 Climate....................................................................................................................... 12

3.2 Physiography ............................................................................................................. 12

3.3 Hydrology ................................................................................................................. 13

3.4 Land Use Physical Development .................................................................................... 13

3.4 Soils 14

4.0 GEOLOGY AND HYDROGEOLOGY .......................................................................... 14

4.1 Geological Setting .......................................................................................................... 14

5.0 HYDROGEOLOGY AND GROUND WATER RECHARGE AND DISCHARGE. ........ 15

5.1 Hydrogeology ............................................................................................................. 15

5.2 Recharge .................................................................................................................... 16

5.3 Discharge ................................................................................................................... 16

6.0 EXISTING BOREHOLES AND RECHARGE ................................................................. 16

7.0 AQUIFER PROPERTIES ................................................................................................. 16

7.1 Borehole specific capacities (S) Transmissivity (T) Coefficient .................................... 16

7.2 Hydraulic Conductivity (K) and Ground Water Flux ........................................................ 17

7.3 Assessment of Availability of Ground Water .............................................................. 18

7.4 Analysis of Reserve and Ground Water Level Evolution ............................................ 18

8.0 GEOPHYSICS .................................................................................................................. 18

8.1 Basic principles of the resistivity methods ........................................................................ 18

8.2 Vertical Electrical Soundings (VES).................................................................................... 18

8.3 Electrical Resistivity Method ........................................................................................ 19

9.0 RESULTS AND DISCUSSION ....................................................................................... 20

9.2 Results, Interpretations and Discussion ...................................................................... 21

9.3 Impacts of Proposed Drilling Activity. ........................................................................ 25

9.4 Impacts on Local Aquifer’s Quantity And Quality ....................................................... 26

9.5 Impacts on Existing Boreholes In The Area: ................................................................... 26

10 CONCLUSIONS/RECOMMENDATIONS .................................................................... 27

10.1 General ............................................................................................................................. 27

10.2 Conclusion. ................................................................................................................ 27

10.3 Recommendations: ..................................................................................................... 27

10.4 Further Recommendations: .............................................................................................. 28

11.0 APPENDIX .................................................................................................................... 29


Encls. (To Be provided by the client)

• Copy of the Certificate of Land Ownership • Group’s Registration Certificate

• Allotment Letter • Copy of group’s PIN and ID card.

• Copy of letter of No objection from Area Chief


PROJECT SUMMARY SHEET

Project Details

Client ORWA WATER PROJECT

P.O. BOX KAPENGURIA

Project Borehole at ORWA Location L.R. No. (Documents encls) Locality ORWA District WEST POKOT Selected Borehole Site

Map Sheet 183/3 Coordinates (GPS) 035028’ 57.8’’ 01039’ 30.5’’ Elevation (GPS) 903 m asl

Projected Water Demand 20m3/day Main Purpose of Water Use Domestic and Minor Irrigation Investigating Geologists/Engineers Charles N. Kithome/ Mutie Simon/Ituli J.T.


ABBREVIATIONS

(S.I. Units throughout, unless indicated otherwise) agl above ground level amsl above mean sea level bgl below ground level d day E East EC electrical conductivity (µS/cm) GPS global positioning satellite hr hour K hydraulic conductivity (m/day) l litre m metre N North PWL pumped water level Q discharge (m3/hr) S South sec second VES Vertical Electrical Sounding W West WSL water struck level µS/cm micro-Siemens per centimetre: Unit for electrical conductivity oC degrees Celsius: Unit for temperature Ωm Ohmm: Unit for apparent resistivity

ρa Apparent resistivity " Inch


GLOSSARY OF TECHNICAL TERMS

Alluvium

General term for detrital material deposited by flowing water.

Aquifer

A geological formation or structure, which stores and transmits water and which is able to supply water to wells, boreholes or springs

Conductivity

Transmissivity per unit length (m/day).

Confined aquifer

A formation in which the groundwater is isolated from the atmosphere by impermeable geologic formations. Confined water is generally at greater pressure than atmospheric, and will therefore rise above the struck level in a borehole.

Denudation

Surface erosion.

Evapotranspiration Loss of water from a land area through transpiration from plants and evaporation from the surface

Fault

A larger fracture surface along which appreciable displacement has taken place.

Fluvial General term for detrital material deposited within a river environment and usually graded.

Gneiss Irregularly banded rock, with predominant quartz and feldspar over micaceous minerals. A product of regional metamorphism, especially of the higher grade.

Granitization

The process by which solid rocks are converted into rocks of granitic character without melting into a magmatic stage.

Gradient

The rate of change in total head per unit of distance, which causes flow in the direction of the lowest >head.

Heterogeneous Not uniform in structure or composition throughout Hydrogeological Those factors that deal with subsurface waters and related geological aspects of surface

waters Hydraulic head Energy contained in a water mass, produced by elevation, pressure or velocity. Infiltration

Process of water entering the soil through the ground surface.

Joint Fractures along which no significant displacement has taken place Migmatite

Rocks in which a granitic component (granite, aplite, pegmatite, etc.) is intimately mixed with a metamorphic component (schist or gneiss).

Perched aquifer Unconfined groundwater separated from an underlying main aquifer by an unsaturated zone. Downward percolation hindered by an impermeable layer.

Percolation Process of water seeping through the unsaturated zone, generally intimately mixed with a metamorphic component (schist or gneiss).

Permeability The capacity of a porous medium for transmitting fluid. Piezometric level An imaginary water table, representing the total head in a confined aquifer, and is defined

by the level to which water would rise in a well. Porosity The portion of bulk volume in a rock or sediment that is occupied by openings, whether

isolated or connected. Pumping test A test that is conducted to determine aquifer and/or well characteristics Recharge General term applied to the passage of water from surface or subsurface sources (e.g.

rivers, rainfall, lateral groundwater flow) to the aquifer zones. Regolith

General term for the layer of weathered, fragmented and unconsolidated rock material that overlies the fresh bedrock.

Specific capacity

The rate of discharge from a well per unit drawdown.

Static water level The level of water in a well that is not being affected by pumping. (Also known as "rest water level")

Transmissivity A measure for the capacity of an aquifer to conduct water through its saturated thickness (m2/day).

Yield Volume of water discharged from a well.


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1.0 INTRODUCTION

1.1 General Information

The Chairman, Orwa Community Water Project assisted by World Vision Kenya, ORWA IPA requested the consultants to undertake a detailed hydrogeological survey for a borehole site investigations in ORWA area, ORWA Location of West Pokot District. Subsequently, hydrogeophysical surveys were undertaken within the community’s designated hotspots within different areas of the locality to identify the most suitable site for sinking the proposed borehole.

1.2 Location Orwa IPA is located in West Pokot district, Rift Valley Province of Kenya. The IPA covers Sekerr, Endough and Parkoyo locations with an approximate area of 630KM2. The IPA ’s topography is characterized by rugged hills, undulating valleys, rocky outcrops, incised gullies which form seasonal streams that drain into the many beautiful Valleys below. The topography comprises Sekerr hills in the highlands favourable for agro-pastoralism and vast plains on the low lands suitable for Pastoralism. Most of the population is confined along the highlands and the slopes while the plain is inhabited by few pastoralists. Streams drain from Sekerr hills to the valleys below. The area is relatively remote with very few access roads and a larger part can only be accessed on foot. The area is classified as a hardship zone because of its harsh environmental conditions.. The proposed borehole site is located in ORWA village, hinterland of ORWA Divison West Pokot District some about 5km west of ORWA township town and 35km west Kapenguria town. The map indicating the approximate location of the selected borehole drilling site is shown in Figure1.


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Figure 1: Location and Administration of the Borehole Survey Area

2.0 WATER SUPPLY SITUATION

2.1 Sources of water

Orwa IPA is relatively a water scarce area in the West Pokot District. Therefore, access to safe water for domestic and livestock use is a key challenge. (West Pokot District Vision and Strategy: 2005-2015). The most common water sources are water pans, shallow wells, rivers and seasonal streams. Most of these streams were dried up due to prolonged drought. Water quality in most of these sources is characterized by high turbidity levels due to extensive mining (gold digging) along the river bed as well as human and animal pollution. Parts of Mbara centre in Sekerr location is served by gravity piped water system from the Mtelo hills. However, all of these sources are not adequate, palatable and wholesome. The Orwa water committee assisted by World Vision Kenya deemed it wise to explore and exploit supplementary groundwater source to boost their existing supply. The objective of this investigation was therefore to establish the optimum location of a borehole which will be near and more reliable to the client’s envisaged needs.

2.2 Population and Water Demand

The current water demand for the investigated community areas is not known due to lack of proper or scanty demographic data. It is however reported that the mass influx of the Turkana Pastoralists is very high and this is evidenced by the heavy bush clearing in the area which is being carried out by people who are moving their livestock in search of water and pasture. The major water supply sources in most places however are surface water ponds and ephemeral rivers which run dry at the peak of the dry season. Once these sources are depleted, people have to walk long distances to places where boreholes have been drilled.


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According to Sphere Standards however, the longest distance to a water point should be 500 meters while a single hand-pump should serve 500 people. This means that due to the scattered patterns of settlement in the area, the coverage is far from complete and more will have to be done in the way of safe water provision. It has been estimated that a demand of about 8 cubic metres of water per hour suffices the community needs. Water from the proposed borehole is to be used for domestic and micro- irrigation purposes by the target community.


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3.0 CLIMATE, PHYSIOGRAPHY AND LAND USE

3.1 Climate

Orwa is characterized by landscapes with high altitude ranges with spectacular escarpments of more than 700m. The lower parts of the IPA area is located between 1500m- 2100 m.a.s.l and receives low bimodal rainfall of between 200mm and 1600mm. The long rains fall between April and August while the short rains occur between the months of October and February. There is however great variations in the amount of rainfall received. Annual rainfall ranges between 300mm and 700mm. The temperature ranges between 150C and 300C. (WPDDP 2002-2008). The groundwater recharge is through infiltration and subsequent percolation of part of the mean annual rainfall as well as regional lateral replishments from areas of higher elevation of the project area.

3.2 Physiography

The Orwa topography consists of rough hilly areas, deep and steep valleys, rock outcrops, natural gulleys that act as seasonal streams flowing down the valleys. It comprises of Mtelo hills and large tracts of plain land on the lower parts. All streams originates from the Mtelo hills to the west draining below the valleys and ultimately into Lake Turkana. The topography is undulating with basement and granite intrusive rock outcrops in many places. The general physiography of the area is attributed to prolonged period of weathering, deposition and volcanic intrusions. There are 3 dome-shaped hills namely Motong, Mtelo and Chachai which expose the country rock. These outcropping features are dominantly basement in nature and include gneiss, paragneisses as well as undifferentiated basement rocks. Figure 2 shows the topography of the surveyed area

Figure 3: Digital Terrain Elevation Model showing the Topography of the studied area

Borehole survey

Orwa Community Water and Sanitation Project, West Pokot Region

3.3 Hydrology

Maghany and Orwa stream which strad the survey area many, more parallel, seasonally flowing tributaries. The entire area belongs to the Turkwecatchment area and the waters infiltrated before that. The rivers in the suevey area are There is no discharge data available, but it is obviousrainy season with the rivers drying streams in the study area

Figure 3: Map showing the land use and infrastructure within the local area

3.4 Land Use Physical Development

Orwa area and the whole of the reeds and very little or no grass cover in most parts. cover the flat lowlands where the borehole survey was carried out. On the highlands around Chachai and mtelo hiils farming is practised bflowing riversections. Apart from the thickets, other vegetation types predominant in the dependent on the topography and soil typescovered areas, tree-less grass cover with occasional trees dominates the vegetation. The areas

Pokot Region Borehole Site Investigations

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and Orwa stream which strad the survey area drains into the Weiwei River more parallel, seasonally flowing tributaries. The entire area belongs to the Turkwe

finally flows into Lake Turkana if it has not evaporat

are seasonal with flow ceasing a few months after the rains.data available, but it is obvious that the floods are received during the

rainy season with the rivers drying a few months after. Figure 3 shows the catchment and

Map showing the land use and infrastructure within the local area

Physical Development

whole of the IPA is covered with thorny acacia plants, cactus, green reeds and very little or no grass cover in most parts. Grass for pasture and thick shrub cover the flat lowlands where the borehole survey was carried out. On the highlands around

farming is practised because of the good fertile soil Apart from the thickets, other vegetation types predominant in the

dependent on the topography and soil types and nearness to the river. In the ‘black cotton’ ass cover with occasional trees dominates the vegetation. The areas

Borehole Site Investigations

Weiwei River with more parallel, seasonally flowing tributaries. The entire area belongs to the Turkwell

finally flows into Lake Turkana if it has not evaporated and

few months after the rains. that the floods are received during the Figure 3 shows the catchment and

Map showing the land use and infrastructure within the local area

acacia plants, cactus, green and thick shrub

cover the flat lowlands where the borehole survey was carried out. On the highlands around ecause of the good fertile soil and longer

Apart from the thickets, other vegetation types predominant in the are . In the ‘black cotton’

ass cover with occasional trees dominates the vegetation. The areas


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adjacent to the black cotton grasslands are covered by different species of acacia while the hill-tops and riverrines are covered by bushes and woodlands.

3.4 Soils Orwa IPA falls within the Arid and Semi Arid lands (ASAL) characterized by complex soils consisting of rocky and sandy soils and with different drainage conditions, which have developed from alluvial deposits. Some of these soils are saline in nature and characterized by shallow and stony soils with rock outcrop and lava boulders. The area is adversely affected by lack of adequate rainfall with annual rainfall ranging from 300-700mm per annum (WVK-OARDP 2008).

4.0 GEOLOGY AND HYDROGEOLOGY

4.1 Geological Setting

The metamorphic Basement forms the most extensive group of rocks, and comprises mainly schists and gneisses. Several younger intrusive bodies, such as ring dikes and batholiths (mainly consisting of granites and rhyolites) are also part of this series. Unfortunately, the Basement Complex of Pokot has not been surveyed in detail, and remains largely undifferentiated. In the West Pokot and Central Pokot, they include gneisses, paraschists, paragneisses, syenites, granites, granodiorites and quartzites (a) Regional Geology These are even-grained, holocrystalline rocks composed chiefly of feldspars and quatrz with some ferro-magnesium minerals. They are mostly medium to coarse-grained in texture. The common types of granites found in the Region are hornblende granites, biotite granite, augite granite, e.t.c. Granites in the country are outlined as Granitoid Shield on the Geological Map. The granites are contaminated by inclusions of the older metamorphic rocks, the inclusions varying in sizes. The inclusions irrespective of size may be highly migmatized or remain undigested with sharp boundaries. The granitoid shield is considered one vast outcrop of Palingetic granite. The geology of the investigated area is presented in figure 4 (b) Structural Geology

Since a detailed geological mapping of this North rift Region has not been extensively carried out, little is known about the structure of the Basement formations in the area. Foliation

trends are reported to be constant over wide areas, and vary from 340 to 020° further south. This is in line with the drainage pattern and the direction of the major watercourses.

The downwarping of the central basin, and the rift-controlled evolution of the Great Rift valley, must have been accompanied by intensive folding of rocks. One would expect that regional tectonism of such a scale must result in structural features of hydrogeological significance.

(c) Geology of the study Site The alluvium sediments overlie the Basement Complex uncomformably. Fresh and compact bedrock occurs variably at very shallow depths (<10m) in some areas. The thickness of the alluvium sediments can vary within the floodplains to a depth of about 30m in some places.

Orwa Community Water and Sanitation Project, West Pokot Region

In the floodplains, the nature of the bedrock is largely concealed bbut along the eroded stream channels and within the hills, basement outcrops are a common sight. Due to the thick cover of weathered material and sediments (and the fact that the area remains largely unsurveyed), faults/fractures wand related to vaguely identified linear features like streams, depressions and vegetation changes on the ground. The prevailing trend of faults/fractures in the study area is roughly north - south and northwest-southeast. These major faults/fractures and the extensive deposition control the drainage pattern of the surface water in the area. These faults/fractures, where present, rarely reach the surface, but are instead covered by the younger alluvium.

Figure

5.0 HYDROGEOLOGY AND GROUND WATER

5.1 Hydrogeology

The hydrogeology of an area is determined by the nature of the parent rock, structural features, weathering processes and precipitation patterns.the location drains into the Weiwei River. The entire area belongs to the Turkwell catchment area and the water of both rivers finally flows into Lake Turkana if it has not evaporated and infiltrated before that. The only perennially flowing rivers in the study region are the above mentioned rivers. At the moment there are no discharge data available, but it is obvious that the discharge will be

Pokot Region Borehole Site Investigations

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In the floodplains, the nature of the bedrock is largely concealed by the overlying alluvium but along the eroded stream channels and within the hills, basement outcrops are a common sight. Due to the thick cover of weathered material and sediments (and the fact that the area remains largely unsurveyed), faults/fractures were identified using the satellite imagery and related to vaguely identified linear features like streams, depressions and vegetation changes on the ground. The prevailing trend of faults/fractures in the study area is roughly

southeast. These major faults/fractures and the extensive deposition control the drainage pattern of the surface water in the area. These faults/fractures, where present, rarely reach the surface, but are instead covered by the

Figure 4: Geology of the study area

HYDROGEOLOGY AND GROUND WATER RECHARGE AND DISCHARGE

The hydrogeology of an area is determined by the nature of the parent rock, structural features, weathering processes and precipitation patterns. The central and western part of the location drains into the Weiwei River. The entire area belongs to the Turkwell catchment area and the water of both rivers finally flows into Lake Turkana if it has not evaporated and infiltrated before that.

erennially flowing rivers in the study region are the above mentioned rivers. At the moment there are no discharge data available, but it is obvious that the discharge will be

Borehole Site Investigations

y the overlying alluvium but along the eroded stream channels and within the hills, basement outcrops are a common sight. Due to the thick cover of weathered material and sediments (and the fact that the

ere identified using the satellite imagery and related to vaguely identified linear features like streams, depressions and vegetation changes on the ground. The prevailing trend of faults/fractures in the study area is roughly

southeast. These major faults/fractures and the extensive deposition control the drainage pattern of the surface water in the area. These faults/fractures, where present, rarely reach the surface, but are instead covered by the

RECHARGE AND DISCHARGE.

The hydrogeology of an area is determined by the nature of the parent rock, structural he central and western part of

the location drains into the Weiwei River. The entire area belongs to the Turkwell catchment area and the water of both rivers finally flows into Lake Turkana if it has not

erennially flowing rivers in the study region are the above mentioned rivers. At the moment there are no discharge data available, but it is obvious that the discharge will be


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much less in the dry season than in the rainy season. In the latter period the rivers may flood in years with a high amount of rainfall while in a dry year the Kerio River might not even reach Turkana District but may stop flowing above ground before that. Specially in the dry season, when the concentration of dissolved minerals is relatively high the water of the Kerio River may be rather polluted due

5.2 Recharge

There are two possible ways through which aquifers in this area may be recharged.

• Direct replenishment at the surface: this may be by way of percolation of rainwater through the overlying sandy soils and fracture/faults

• Indirect recharge: there is a obvious indirect recharge from the Motong, Mtelo and chachai hills through faults and fractures that connect to the aquifers in the study area.

5.3 Discharge

There are two ways through which groundwater is discharged in the area. The first one is through abstraction from the scattered boreholes. Secondly, groundwater in the area may also flow through fractures and faults to the areas of lower elevation. For this to happen however, the fractures and faults have to be extensively interconnected to allow for movement of water.

6.0 EXISTING BOREHOLES AND RECHARGE

Borehole data within a locality is useful in estimating the depth of a new borehole, expected water quality and yield. Only one borehole was found within the vicinity of the project area. The data is given in Table 2. Table 2: Chepkolol Borehole Data B/H No

Borehole owner Distance/ Bearing

Total Depth

Water Struck level

Water Rest level

tested Yield M3/h

Pumping water level

C- Chepkolol BH 7km NW 140 60 10 5 125

7.0 AQUIFER PROPERTIES

7.1 Borehole specific capacities (S) Transmissivity (T) Coefficient

The borehole specific capacities have been calculated based on the formula; S=Q/s (Driscoll, 9860);

Where Q is the yield during the pump test and s is the draw down i.e. PWL-WRL Transitivity on the other hand is calculated using the formula T= 0.183 Q/s. however this formula is applicable where well test data is available in long scale. Logan’s formula T=1.22Q/s is the best for estimating Transmissivity. The area does not have aquifer test and it is difficult to ascertain specific yields, storage coefficients of existing boreholes in the project area.


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From Driscoll 1986 the following summary of specific yield ranges for earth materials: Specific capacity = tested yield / (pumping level – water rest level) Transmissivity = 1.22 x specific capacity x 24hrs

Table: 3 Specific capacities and Transmissivity of existing boreholes

Borehole name. Specific capacity(m2/hr) Transmissivity (m²/day) Chepkolol BH 0.04348 1.2731

7.2 Hydraulic Conductivity (K) and Ground Water Flux

Locations laboratory investigations and isotope methods are very expensive methods and are the best for determining hydraulic conductivity and ground water flux correctly. The results are confined to few location and they depend on the scale of the investigation method. Rock sample measurements in the laboratory vary from well test results. Ministry of water and irrigation data is also not very reliable. Hydraulic conductivity is calculated using the formula K=T/D where K is the hydraulic conductivity, T is the transmissivity and D is aquifer thickness. D is assumed to be 30m. In the ministry of water and irrigation data the start of the aquifer is the one recorded and most of the time, the thickness is not given. Due to this a lot of assumptions will be made in order to calculate the Hydraulic conductivity. Darcy formula is used to calculate ground water flux. It is given as Q=T .I.W, where T is the transmissivity of the borehole, I is the gradient and W is the width. From the above formula I is the hydrostatic head. Where I=0.0375 and the width (W) is considered as 1000 meters. Hydraulic conductivity is calculated using the formula K=T/D Thus Ground water flux = Transmissivity x 37.5 The calculated hydraulic conductivities and ground water fluxes of the existing boreholes are presented in the table below.

Table 4: Hydraulic conductivity and ground water flux

Borehole Name Hydraulic conductivity Ground water flux

m3/day

Chepkolol BH 1.6 48


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7.3 Assessment of Availability of Ground Water

Regarding assessment of available ground water, the following conclusions can be made:

• Assuming an abstraction of about 20m3/ day for all the boreholes in the area, then the abstraction per day can be estimated to be about 100m3 / day

• The available ground water can be calculated as the available through-flow (ground water flux) less the amount of water abstracted per day.

7.4 Analysis of Reserve and Ground Water Level Evolution

An adequate estimate of the availability of ground water in storage beneath an area requires determination of the ground water basin boundaries, both vertical and horizontal, and of aquifer dimensions and characteristics. Such an analysis requires careful and accurate determination of the aquifer characteristics, GIS techniques to indicate the extent of the aquifer in question and accurate pump test to determine the capacity of the aquifer(s) In addition, recharge and discharge must be fairly quantified.

8.0 GEOPHYSICS

Several geophysical methods are available to assist in the assessment of geological sub surface conditions. In the present survey the resistivity method also known as the (geo electrical method) has been used. Four (4) Vertical electrical soundings (VES) were carried out to probe the cautions at such anomalous zones within the sub-surface and to confirm the existence of deep ground water. The techniques are described below.

8.1 Basic principles of the resistivity methods

The electrical properties of the upper parts of the earth’s crust are dependent upon the lithology. Porosity and the degree of pore space saturation and the salinity of the pore water. Saturated rocks have lower resistivities than unsaturated and dry rocks. The higher the porosity of the saturated rocks the lower its Resistivity. The higher the salinity of the saturating fluids, the lower the resistivity. The presence of clays and conductive minerals also reduce the resistivity of the rock.

8.2 Vertical Electrical Soundings (VES)

Vertical Electrical Soundings were carried out to probe the electrical properties and depth to sub surface layered formations below the site of measurement at the most anomalous zones. When carrying out a resistivity sounding electric currents is led to the ground by means of two electrodes and the potential field generated by the current measured. The separation between the electrodes is step – wise increased (in what is known as schlumberger array) observed resistivity values are plotted in log-log paper and the graph obtained depicts resistivity variation against depth. This graph can be interpreted with aid of a computer and the actual resistivity lying of the sub soil is obtained. The depths and resistivity values provide the hydro geologist with information on the geological layering and thus the occurrence of ground water.


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8.3 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 ‘ƒ’ the resistance ‘R’ is proportional to the length ‘L’ of the material being measured and inversely proportional to its cross-section area ‘A’

i.e. R= ƒL or ƒ= RA A L

In this procedure, a series of stations is established and careful depth soundings are taken by evaluating the Resistivity values at different electrode spacing, an understanding of the sub-surface materials can be developed. This method is useful for estimating the depth to water bearing strata or estimating the thickness of selected formations. In order to help probe the subsurface rock conditions capable of groundwater storage, the schlumberger configuration method was used. One Horizontal profiling and Six (5) vertical electrical Resistivity soundings designated as ORWA VES I-V were carried as shown below:-


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9.0 RESULTS AND DISCUSSION

9.1 Geophysical Field Data For the Orwa survey area 5 sites named as VES I to VES V were conducted with VES V being an existing borehole which served as a control. The location of the four survey sites and the control borehole are presented in Table 5. The VES data is presented in table 6

Table 5: The VES sites on Orwa Study

SITE Name VES No. Coordinates (UTM)

Longitude Latitude Altitude(m) 1 Orwa I 035027’ 19.78’’ 01032’ 52.7’’ 918

2 Orwa II 035029’ 19.78’’ 01039’ 33.8’’ 923

3 Orwa III 035029’ 5.33’’ 01039’ 31.8’’ 903

4 Orwa IV 035028’ 57.8’’ 01039’ 30.5’’ 960

5 Control point V 035027’ 19.78’’ 01032’ 52.07’’ 962

Figure 5: Layout of the borehole VES points


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Table 6: VES DATA Stn.No.

ORWA VES I

ORWA VES II ORWA VES IV ORWA VES III CALIBRATION DATA

AB/2(m)

∂V/I (Ωm)

Rho(Ohm-m)

∂V/I(Ωm) Rho (Ohm-m)

∂V/I(Ωm) Rho(Ohm-m)

Rho (Ohm-m)

∂V/I(Ωm)

Rho (Ohm-m)

∂V/I(Ωm)

1.6 4.39 32 4.69 34 4.70 34 1.871 36 287.00 2.0 1.967 23 2.46 29 1.863 22 2.42 29 250.00 2.5 0.661 12 1.422 27 1.193 22 1.037 20 215.00 3.2 0.319 10 0.694 22 0.778 24 0.555 17 136.00 4.0 0.275 14 0.459 23 0.487 24 0.330 16 91.00 5.0 0.1794 14 0.312 24 0.328 26 0.222 17 82.00 6.3 0.1131 14 0.221 27 0.1873 23 0.145 18 82.00 8.0 0.0668 13 0.1521 30 0.1133 23 0.0156 17 81.00 10 0.0421 13 0.0646 20 0.074 23 0.0575 18 87.00 13 0.0212 11 0.0378 20 0.0439 23 0.0382 20 75.00 16 0.01566 13 0.0230 18 0.0274 22 0.275 22 69.00 20 0.00673 8 0.009 11 0.01622 20 0.0206 26 72.00 25 0.00434 9 0.0133 11 0.01052 21 0.01592 31 67.00 32 0.1296 19 0.0971 14 0.00527 17 0.1766 26 17.00 40 0.0594 14 0.0649 15 0.0292 7 0.1143 27 20.00 50 0.0263 10 0.0328 13 0.01415 5 0.0657 25 17.00 63 0.00965 6 0.0142 11 0.00687 4 0.0379 23 80 0.00428 4 0.00991 10 0.00530 5 0.0225 22 100 Aborted 0.00641 10 0.00330 5 0.01495 23 130 0.00532 14 0.00385 10 0.00982 26 160 0.00382 15 0.00101 40

9.2 Results, Interpretations and Discussion

Vertical electrical soundings (VES) provide quantitative depth-resistivity information for a particular site. VES sites were selected at representative points, and at locations of particular interest for groundwater resources development. The measurements were executed in an expanding Schlumberger array, with electrode spread of AB/2=160 to 250 m. This separation gives fairly reliable interpretations down to a depth of 80 to 120 m, but only approximate solutions for resistivity layering at deeper levels. Depth indications beyond this level are only indicative, and do not give the precise position of the measured contact zone. The locations of the geophysical soundings (those carried out within the plot) are shown in figure 2 Apparent resistivity curves were interpreted using the "Schlumberger" program (Hemker, 1989). The main aim of the measurements was to determine the depth to the fresh basement formation rocks, the degree of fracturing at depth, which should be directly related to the Transmissivity layer and thus the potential yield. As a general rule, it can be assumed that in this case the sounding with the lowest basal resistivity’s in the expected water bearing range represent the most favorable drilling site. The Hydrogeological Interpretation of ORWA VES’s is represented in following table


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Table 7: VES Interpretations

VES No.

Depth Apparent Resistivity (Ohm-m)

Expected formation Acquiferous

I 1.6 32 Dry Sands No

3.2 10 Lithic material(Dry) No

10 13 Highly weathered gneissic material Moist, No

20 8 differentiated) pelitic-gneisses material No

50 10 Fresh Basement (Dry) No, impervious


II 2.5 27 Dry Sands No

10 20 Lithic material(Dry) No


50 13 Fractured basement wet with clays Probably and brackish


III 1.6 36 Dry Sands No





V 1.6 34 Dry Sands No

5 26 Lithic material(Dry) No

20 21 Highly weathered gneissic material Perched aquifer, Yes

50 5 Fractured basement wet with clays Yes

100 5 Fractured basement wet with clays Yes

130 10 Highly weathered gneissic material Perched aquifer, Yes



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Figure 6: Orwa VES I geophysical graph

Figure 7: Orwa VES II geophysical graph


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Figure 8: Orwa VES III geophysical graph

Figure 9: Orwa VES IV geophysical graph


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Figure 10: Control VES along Chepkolol Borehole.

The analysis of the VES sites is presented in Table 8 below

Table 8: VES sites ratings

SITE Name VES No.

Coordinates (UTM) WSL (m)

Max. Depth (m)

Prospective Yield(m3/hr) Longitude Latitude Altitude(m

) 1 Orwa I 035027’ 19.78’’ 01032’ 52.7’’ 918 25 130 Negative

2 Orwa II 035029’ 19.78’’ 01039’ 33.8’’ 923 25-80 150 Fair

3 Orwa III 035029’ 5.33’’ 01039’ 31.8’’ 903 20-80 130 Fair to Good

4 Orwa IV 035028’ 57.8’’ 01039’ 30.5’’ 960 20-80 150 Good

9.3 Impacts of Proposed Drilling Activity.

In project’s study area, the rock formations are Metamorphic basement in nature. Basement aquifers are localized, therefore drilling activity within the study area shall have no impact on the aquifers, water quality, and the abstractors and neither shall there be a likelihood of coalescing cones of depression. It shall have no negative implications for other ground water users.


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9.4 Impacts on Local Aquifer’s Quantity And Quality

The sustainability of water quality depends on the level of abstraction and recharge rate. If ground water is abstracted at a rate greater than its natural replenishment rate, then the water table lowers and the project will not be sustainable. Based on the yields of the boreholes in the area, the proposed abstraction of 20m3/day based on a 10 hour pumping regime is not expected to have any major impact on the aquifers, as the aquifer is expected to be quit productive. The water quality will mainly depend on the host rock and construction design. Overall, the expected impacts resulting from the borehole to the environment and their mitigation measures will be adequately addressed by the Environmental Impact Assessment Study allready conducted

9.5 Impacts on Existing Boreholes In The Area:

It is noteworthy that the borehole examined in this study area is more than 7km away from the proposed drill site. Therefore we do not expect any negative impacts on any other existing borehole within the vicinity.


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10 CONCLUSIONS/RECOMMENDATIONS

10.1 General

Insufficient water-quality data have hampered conclusive analysis of the geological influence on groundwater in some parts of the area. High fluoride contents and total dissolved solids around the West pokot area, as compared to other areas, indicate an anomaly that has not yet been explained. While the metamorphic rocks are relatively poor in terms of groundwater quantity and quality, they have a useful function as barriers to water that percolates through the relatively impermeable overlying metamorphic rocks. Where the contact is exposed along the tongues of the volcanic flows, springs have developed within the North rift. These springs have been the main sources of water supply some areas, hence the significant positive influence of the metamorphic rocks on groundwater storage.

10.2 Conclusion.

Based on the collected and analyzed data, the hydro geological prevailing conditions it can be concluded as follows:-

1 There are good prospects of striking Groundwater within the investigated site. 2 Water from this borehole is expected to be of fair quality; and slightly brackish 3 Information from the existing boreholes suggests that the locality has moderate to

poor ground water potential. 4 The yield of a borehole drilled in the general area is expected to vary between 1

and 5 m3/hr and sometimes dry!. 5 Water occurrence is within the fractured politic-gneisses of the basement contact

zone

10.3 Recommendations:

Based on the above, it is recommended that:- 1 The study recommends that a borehole be drilled at the site designated as VES IV

ORWA, to an approximate depth of 150-metres below ground level: this will be

sufficient for a sustainable yield of approximately 3m3/hr. It is however expected that

if drilling proceeds to bottom with good construction and borehole design, more than

4 m3/hr can reasonably be attained.

2 It should be lined with appropriate casings and screens.

3 It should be protected from possible sources of contamination by grouting a certain

length of the borehole from the ground surface.

4 The borehole should be properly gravel packed to enhance yield.

5 The drilling and test pumping should be supervised by water office.

6 Upon completion, the borehole should be fitted with an airline/ piezometre and a

master meter to facilitate monitoring of static water level and groundwater

abstractions respectively.

7 A two (2) litres water sample of this water is to be collected in a clean container and

be taken to any competent water testing authority for a full chemical, physical and

bacteriological analysis.


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8 It is a legal requirement, stipulated in the water act 2002,that the client applies for a

ground water permit from the Water Resources Management Authority (WRMA) to

sink a borehole. For this purpose, three signed copies of the present report must be

submitted to the authority by the consultant for examination and approval.

10.4 Further Recommendations:

1 The site is known to the chairman, committee of Orwa water Project and the World Vision, ORWA IPA WASH Engineer

2 The site is accessible by a drilling rig as it is plain and road infrastructure leading to the site is motorable.

3 To achieve and maintain a high yield, and maximize the efficiency of the borehole, the importance of proper design and construction methods cannot be overemphasized.

4 The water quality of the proposed borehole is expected to be palatable, though slightly brackish.


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11.0 APPENDIX

DRILLING METHODS/ TECHNIQUE Drilling should be carried out with an appropriate tool-either percussion or rotary machine. The latter are considerably faster. Geological rock samples should be collected at 2- metres intervals. Struck and Rest water levels and if possible, estimate of the yield of individual aquifer encountered should also be noted. Borehole Design The design of the well should ensure that screens are placed against the optimum aquifer zones. An experienced works drilling consultant/hydro geologist should make the final design, and should make the main decision on the screen setting. Casings and Screens The well should be cased and screened with good quality screens, considering the depth of the borehole, it is recommended to use steel casing and screens of 153/6” diameter. Slots should be maximum 1mm in size. We do not encourage the use of torch-cut steel well casing as screens. In general, its use will;

• Reduce well efficiency (which leads to lower yield). • Increase pumping costs through greater draw down; • Increase maintenance costs and eventually • Reduction of the potential effective life of the well. Gravel Pack The use of gravel pack is recommended within the aquifer zones, because the aquifer could contain sands or silts which are finer than the screen slots size. An 8” (203mm) diameter borehole screened at 6” (153mm) will leave an annular space of approximately 1”, which should be sufficient. Should the slot size chosen to be too large, the well will pump sand thus damaging the pumping plant and leading to gradual siltation of the well. The grain size of the gravel pack should be having an average of 2-4mm. Borehole Construction Once the design has been agreed, constructions can proceed. In installing screens and casing, centralizers at 6 metres interval should be used to ensure centrality within the borehole. This is particularly important to insert the artificial gravel pack all around the screen. If installed, gravel packed sections should be sealed off top and bottom with clay (2m). The remaining annular space should be backfilled with an insert material and the top five meters grouted with cement to ensure that no surface water at the well head can enter the well and thus prevent contamination. Borehole Development Development aims at;


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• Repairing the damage done to the aquifer during the course of drilling by removing clays and other additives from the borehole walls.

• Secondly, it alters the physical characteristics of the aquifer around the screen and removes fine particles. We do not advocate the use of over pumping as means of development since it only increases permeability in zones, which are already permeable. Instead, we would recommend the use of air or water jetting, or the use of the mechanical plunger, which physically agitates the gravel pack and adjacent aquifer material. This is an extremely efficient method of developing and cleaning wells. Well development is an expensive element in the completion of a well, but is usually justified in longer well life, greater efficiencies, lower operational and maintenance costs and a more constant yield. Within this frame the pump should be installed at least 2m above the screen. Borehole Testing After development and preliminary tests, a long duration well test should be carried out on all newly completed wells. This gives an indication of the quality of drilling, design and development. It also yields information on aquifer parameters which are vital to the hydro geologist. A well test consists of pumping a well from measured start level (water rest level (WRL) at a known or measured yield, and simultaneously recording the discharge rate and the resulting draw downs as a function of time. Once a dynamic water level (D.W.L) is reached, the rate of flow to the well is equal to the rate of pumping. Towards the end of the test a water sample of 2 litres should be collected for chemical analysis. The duration of the test should be 24 hours; followed by recovery test until the initial W.R.L has been reached (during which the rate of recovery to WRL is recorded. The results of the test will enable the hydro geologist to calculate the following;

• Optimum pumping rate, • Installation depth, • Draw down for a given discharge rate. • Pump size

WATER QUALITY Classifications of Ground Water Quality According to WHO (1984) water for human consumption should have a maximum TDS of 1000mg/1itre, see Table 5.1.

Table A1: WHO Water Quality Classification TYPE OF WATER TDS (MG) Fresh < 100 Brackish 1,000 – 10,000 Saline 10,000 – 100,000 Brine >100,000


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A Guideline for Evaluating Water Quality The guidelines given in Table 5.2 are used in evaluating the quality of groundwater. Table 5.2: Water Quality Guidelines PARAMETERS THRESH HOLD (Mg/1) LIMIT (Mg/1) TDA 2500 5000 CALCIUM 500 1000 MAGNESIUM 250 500 SODIUM 1000 2000 BICARBONATE 500 1500 CHLORIDE 1500 3000 FLUORIDE 1 6 NITRATE 200 400 SULPHATE 500 1000 PH 6.0 – 8.5 5.6 – 9.0 Water from the proposed boreholes should be analyzed to ascertain its chemical, bacteriological suitability before it is made available for domestic use. Water Quality Protection In order to protect the quality of the water, the borehole should be located as far as possible from all sources of danger e.g. septic tanks, pit latrines, polluted water bodies e.t.c.


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Schematic Design for Borehole Completion

NB: Not to scale

Groundlevel

Cement grout

Inert backfill

Bentonite seal

2-4 mm Gravel pack

Bottom cap

Groundlevel

Concrete slabWell cover

Plain casing

Sanitary casing

Screens

Schematic Design for Borehole completion

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