FINAL MANGELETE WATER PROJECT _Repaired_

Borehole Site Investigations World Vision-Mtito-Andei IPA

1Hydrogeological Survey Report Mang’elete Community Water ProjectUtu Area, Mang’elete, Mtito IPA

HYDROGEOLOGICAL SURVEY REPORT

FOR

BOREHOLE SITE SURVEY

AT

UTU AREA, KAMBU DIVISION

OF

KIBWEZI DISTRICT

DECEMBER 2010

CLIENT CONSULTANTSTHE CHAIRMANMANG’ELETE WATER PROJECT Justus T. Ituli /Kithome®

P.O BOX ….. Registered Hydro-geologists

NGWATA P.O. Box 51259 – 00200NAIROBI.

Date: 15/12/2010



SUMMARY

Background

The consultants were commissioned by the Mang’elete Water Project/ World Vision

Mtito-Andei IPA to undertake investigations for suitable borehole site within UTU village,

Mang’elete Location of Kambu Division 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 26th November and 27th 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 Mang’elete Water project in Kibwezi 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 advice on the requirement of an EIA ahead of the actual drilling



Project area

The Project Area is located at the southern tip of Kibwezi District and is bounded

by latitudes 2°5'S and 3°5'S and longitudes 37°20'E and 38°35'E (Fig. 2).The

physiography is characterised by flat lowland, with an altitude varying from 300m

to 900m above mean sea level (amsl). It has been described by Pulfrey (1960) and

Ojany (1966) as the low foreland plateau. It borders Cenozoic lava highlands,

represented by the Chyulu Range (2000m amsl), to the west and an

intermediate lava plateau, the Yatta Plateau (900m

amsl), to the far east of the study area.

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

Conventionally, the semi-humid highland environment of Chyullu ranges about 3km away from

the study site appears to offer reasonable groundwater recharge potential to the investigated

area. The relatively high rainfall, undulating topography and deep weathering are all

comparatively favorable hydrological/hydrogeological characteristics. The presence of

pronounced relief in some places results in relatively high hydraulic gradients. As a result,

groundwater may be quickly drained from areas of high relief to low-lying areas where it

remains longer (available) for abstraction.

The hydrogeology of an area is determined by the nature of the parent rock, structural

features, weathering processes and precipitation patterns. Lava flows rarely possess significant

primary pore space; instead, groundwater occurs within secondary features, such as fissure

zones, fractures, sedimentary beds, lithological contacts and Old Land Surfaces (OLS). The

weathered zones or “Old Land Surfaces” (OLS) characterize periods of erosion between

volcanic eruptions and subsequent lava flows. These potential aquifers comprise soils,

weathered rocks and water-lain material of volcanic origin. The thickness of the OLS is

variable, but the water-producing zones, which are usually sandy or gravelly, rarely exceed a

few meters. Unlike lava flows, pyroclastic deposits and especially volcanic sediments (e.g.



those embedded in the Chyullu Basalts) are generally porous: the cavities between the mineral

grains are usually open and interconnected. Consequently, they can contain and transmit

water. Whereas sedimentary deposits of pure, unconsolidated sands are highly transmissive,

the permeability rapidly decreases in the presence of clays, even if their portion is very small.

Heavy clays, marked by porosity as high as 50%, are generally impermeable. Tuff layers and

ashes generally possess unfavourable hydraulic characteristics, similar to those of clays.

Below this volcanic carpet, the metamorphic Basements rocks are generally hard and compact,

and posses no primary porosity. 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 hydrogeological 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 six(6) community areas identified previously by

Mang’elete Water Project/World Vision Mtito-Andei IPA as areas that require intervention due

high water demand. In total Six (6)no.VES soundings and One(1) km of profiling was

executed. In the course of the fieldwork, rains caused some slight delays but no significant

amount of rain derailed the project execution. A Garmin e-trex GPS Satellite Navigator

and a Magellan Thales Mapper(Data logger) with GPS and Terrasync software was

used to obtain accurate co-ordinates of the measured/surveyed points and log the identified

VESs as well as collecting secondary hydrogeological 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 quantatitive interpretations were undertaken in the office using Interpex-1D and

Nile software 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 I (MIWA I), VESII(KWA NDIKI), VES III(KWA

PAUL),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 Tables 5.1. 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.

Investigated SitesSITE Name VES No. Coordinates (UTM) WSL

(m)Max.Depth(m)

ProspectiveYield(m3/hr)

1 KWA MIW’A I 37M 385938 9692967 40,80 160 8-122 KWA NDIKI II 37M 390438 9691183 40,80 130 6-103 KWA PAUL III 37M 391035 9694759 65 100 >84 KWA MIWA 2 IV 37M 394333 9695953 50,80 180 6-85 KWA MICHAEL V 37M 390282 9699376 63,120 120 4-66 KWA KAKULI VI 37M 388710 9696023 50,90 130 >67 CONTROL VES VII 37M 387871 9694264 40,60 130 6

TABLE OF CONTENTS



PROJECT SUMMARY SHEET ........................................................................................................ 71.0 GENERAL INFORMATION ........................................................................................... 133.1 Climate........................................................................................................................... 143.2 Physiography ............................................................................................................... 15

4.0 GEOLOGY AND HYDROGEOLOGY ................................................................................. 154.1 GEOLOGY ........................................................................................................................ 16

4.2 HYDROGEOLOGY AND GROUND WATER OCCURRENCE. ..................................... 185.0 EXISTING BOREHOLES AND RECHARGE ................................................................. 24

AQUIFER PROPERTIES .............................................................................................................. 245.1 Borehole specific capacities (S) Transmissivity (T) Coefficient ................................ 245.2 Hydraulic Conductivity (K) and Ground Water Flux ................................................. 256.4 Electrical Resistivity Method .................................................................................. 276.6 6.2 Results and Discussion ......................................................................................... 30

9.0 CONCLUSIONS/RECOMMENDATIONS .......................................................................... 369.1 Conclusions. .................................................................................................................. 369.2 Recommendations: ...................................................................................................... 379.3 Further Recommendations: ............................................................................................ 37

DRILLING METHODS/ TECHNIQUE ......................................................................................... 39Borehole Design...................................................................................................................... 39Casings and Screens .............................................................................................................. 39Gravel Pack .............................................................................................................................. 39Borehole Construction............................................................................................................ 39Borehole Development........................................................................................................... 40Borehole Testing ..................................................................................................................... 40TYPE OF WATER ..................................................................................................................... 41PARAMETERS .......................................................................................................................... 41Water Quality Protection ....................................................................................................... 41Appendicesi. VES Data and Curvesii. Map Extract from Chyullu-Mtito Map Sheet 193/3

iii. Drilling Procedures

Encls. (Provided by 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 MANGELETE WATER PROJECTP.O. BOXNGWATA

Project Borehole at UTU area

L.R. No. (Documents encls)

Locality KAMBU

District KIBWEZI

Selected BoreholeSite

Map Sheet 183/3

Coordinates (GPS) 2°5'S and 3°5'S& 37°20'E and 38°35'E

Elevation (GPS) 2000 m asl

Projected Water Demand 20m3/day

Main Purpose of Water Use Domestic and Minor Irrigation

Investigating Geologists/Engineers Justus T. Ituli / Robert Musyimi /CharlesN. Kithome



ABBREVIATIONS AND GLOSSARY OF TERMS

ABBREVIATIONS

agl above ground levelamsl above mean sea levelbdl below datum levelbgl below ground levelE EastEC electrical conductivity (µS/cm)h hydraulic headhr hourID inner diameterl litrem metreN NorthPWL pumped water levelQ discharge (m3/hr)Q/s specific capacity (yield-drawdown ratio; in m3/hr/m)ROP rate of penetration (min/m)s drawdown (m)sec secondSWL static water levelT transmissivity (m2/day)W WestAWB Athi Water BoardWRL water rest level (also: “static water level”)WSL water struck levelµS/cm micro-Siemens per centimetre: Unit for electrical conductivity°C degrees Celsius: Unit for temperature" InchGLOSSARY OF TERMSAquifer 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 Confined aquifers are those in which the piezometric level is higher than theelevation at which the aquifer was encountered. Static water levels are at ahigher level than the top of the formation.

Development In borehole engineering, this is the general term for procedures applied torepair the damage done to the formation during drilling. Often the boreholewalls are partially clogged by an impermeable “wall cake”, consisting of finedebris crushed during drilling, and clays from the penetrated formations. Welldevelopment removes these clayey cakes, and increases the porosity andpermeability of the materials around the intake portion of the well. As a result,a higher sustainable yield can be achieved.



Drawdown The distance between the static water level and the pumped water level. Theterm residual drawdown is used for the same distance during recovery ofthe well.

Evaporation Loss of water from a land area through transpiration from vegetation andevaporation from the surface.

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

Heterogeneous Not uniform in structure or composition.

Hydraulic Head Energy contained in a water mass, produced by elevation, pressure, or velocity.

Hydrogeological Those factors that deal with subsurface waters and related geological aspectsof surface waters.

Infiltration Process of water entering the soil through the ground surface.

Joint Fractures along which no significant displacement has taken place.

Lacustrine Referring to generally fine-grained, lake deposits.

Obsidian Compact, volcanic glass.

Percolation Process of water seeping through the unsaturated zone, generally from asurface source to the saturated zone.

Perched aquifer Accumulation of groundwater on top of a layer of low conductivity, underlain byunsaturated sediments or rocks.

Permeability The capacity of a porous medium for transmitting fluid.

Phenocrysts The larger, visible crystals in a porphyritic rock.

Phonolite Compact and fine textured volcanic rock, belonging to the trachyte-group(together with trachytes and latite). Defined by a high portion of feldspar (40-90%) and feldspatoidic minerals (10-60%: analcite, nepheline, leucite, etc.),and very low to negligible quartz content (0-2%). Incorporated dark colouredminerals (0-40%) most commonly include hornblende, olivine, melanite andacmite. The structure is porphyritic with common phenocrysts of sanidine(orthoclase, or Potassium-feldspar) and nepheline.

Piezometric level An imaginary water table, representing the total head in a confined aquifer, andis defined by the level to which water would rise in a well.

Porphyritic Containing large, visible crystals or phenocrysts in a finer groundmass.

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.

Pyroclastic rocks Group of rocks consisting of volcanic dust, ashes, lapilli, and coarse fragmentsor boulders of lava (volcanic bombs), which have been thrown out of theeruptive centre in molten condition, and deposited by gravity. Hardened massesof dust, ashes and lapilli are known as tuff, while coarse, consolidatedpyroclastic debris is referred to as agglomerate.

Recharge General term applied to the passage of water from surface or subsurfacesources (e.g. rivers, rainfall, lateral groundwater flow) to the aquifer zones.

Recovery Return to static water level following abstraction of water.

Specific capacity Ratio of pumping rate and drawdown (m3/hr/m); a measure for the wellperformance.

Static water level The level of water in a well that is not being affected by abstraction ofgroundwater.

Trachyte Compact and fine textured volcanic rock, belonging to the trachyte-group(together with phonolite and latite). Often marked by laminar flow structures.Defined by a very high portion of feldspar (40-90%) and a relatively low portionof feldspatoidic minerals (0-10%) and quartz (0-20%). Incorporated darkcoloured minerals (0-40%) most commonly include pyroxene, hornblende,olivine, apatite and biotite. Porphyritic structure with common phenocrysts ofplagioclase (Sodium-Calcium-Aluminium feldspar), sanidine, pyroxene andhornblende.

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

Unconfined Referring to an aquifer situation whereby the water table is exposed to theatmosphere through openings in the overlying materials (as opposed to>confined conditions).

Yield Volume of water discharged from a well.









1.0 GENERAL INFORMATIONThe Chairman, Mang’elete Water Project assisted by World Vision Kenya requested theconsultants to undertake a detailed hydrogeological survey for a borehole siteinvestigations in Utu area, Kambu town of Kibwezi District. Subsequently,hydrogeophysical surveys were undertaken within the community’s designated hotspotswithin different farms to identify the most suitable site for sinking the proposedborehole.

1.1 LocationThe Project Area is located at the southern tip of Kibwezi District and isbounded by latitudes 2°5'S and 3°5'S and longitudes 37°20'E and 38°35'E (Fig.1).The physiography is characterised by flat lowland, with an altitude varyingfrom 300m to 900m above mean sea level (amsl). It has been described byPulfrey (1960) and Ojany (1966) as the low foreland plateau. It bordersCenozoic lava highlands, represented by the Chyulu Range (2000m amsl),to the west and an intermediate lava plateau, the Yatta Plateau (900mamsl), to the east.

The drainage pattern is dominated by consequent seasonal streams that drainthe Chyulu Range and flow in an easterly direction to join the Athi River. Mostof the rivers are seasonal except the Kibwezi and Athi Rivers, which areperennial (Fig. 2) .The rainfall pattern, is bimodal. The short rains occur inNovember - December and the long rains in March - May. The mean annualrainfall is 614mm and the potential evaporation is 2000mm. Thus, evaporationreduces the effectiveness of the already little rainfall quite significantly. Therainfall is characterized by poor reliability, which results in frequent crop failure.

The proposed borehole site is located in UTU village,hinterland of Kambu DivisonKibwezi District some about 15km west of Kambu township town and 25km west Mtito-Andei town. The map indicating the approximate location of the selected boreholedrilling site is attached here with.



2.0 WATER SUPPLY AND SITUATION

2.1 Sources of water

Existing water sources in the study area include several hand-dug wells,and pipedwater infrastructure from existing Mang’elete water project borehole. However, all ofthese sources are not adequate and the Mang’elete water committee assisted by WorldVision Kenya deemed it wise to explore and exploit supplementary groundwater sourceto boost their existing supply. The objective of this investigation was therefore toestablish the optimum location of a borehole which will be near and more reliable to theclient’s envisaged needs.

2.2 Population and Water Demand

The current water demand for the investigated community areas is not known due tolack of proper or scanty demographic data. It is however reported that the mass influxof the Chyullu returnees is very high and this is evidenced by the heavy bush clearing inthe area which is being carried out by people who are returning to settle. The majorwater supply sources in most places however are surface water ponds and ephemeralrivers 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.

According to Sphere Standards however, the longest distance to a water point shouldbe 500 meters while a single hand-pump should serve 500 people. This means that dueto the scattered patterns of settlement in the area, the coverage is far from completeand 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 hoursuffices the community needs. Water from the proposed borehole is to be used fordomestic and micro- irrigation purposes by the target community.

3.0 CLIMATE, PHYSIOGRAPHY AND LAND USE

3.1 ClimateThe project site is located on a typical arid and semi – arid land (ASAL ZONE). It iswithin ecological zones 4 & 5. It receives rains within two seasons of the year i.e.between March & May and November & December rainfall seasons. Mean annualtemperatures range from 27-300C & 18-270C maxima and minima respectively.There is high vapor-transpiration and percolation rates thus during rains water is quicklylost to the atmosphere and to the ground respectively. The mean annual rainfall is 500mm.

The groundwater recharge is through infiltration and subsequent percolation of part ofthe mean annual rainfall as well as regional lateral replishments from areas of higherelevation of the project area.



3.2 PhysiographyThe topography is undulating with basement and granite intrusive rock outcrops inmany places. The general physiography of the area is attributed to prolonged period ofweathering, deposition and volcanic intrusions. There area numerous dome-shaped hillswhich expose the country rock. These outcropping features are dominantly basement innature and include gneiss, paragneisses as well as undifferentiated basement rocks.3.3 Land Use Physical Development

The study area and its environs is characterized by open grasslands punctuated byshrublands but some man-made forests do exist in places. These forests predominantlycomprise indigeneous species and are quite extensive in some places. In some placeswhere shallow groundwater is present, large Khat(Miraa) trees occur but further westtowards Chyullu hills, the vegetation is dominated by ‘thickets’. These consist ofheterogeneous dense trees and bushes, which form almost impenetrable thickets at theirpeak development.

Apart from the thickets, other vegetation types predominant in the region are dependenton the topography and soil types. In the ‘black cotton’ covered areas, tree-less grass coverwith occasional trees dominates the vegetation. The areas adjacent to the black cottongrasslands are covered by different species of acacia while the hill-tops are covered bybushes and woodlands. The seasonally wet areas have more typical woodland vegetation.

3.4 SoilsThe soils of the Study area range widely in depth, colour, drainage, structure, chemicaland physical properties. However, most soils have a sandy texture in the subsoil. Ingeneral, the parent material and physiography largely determine the pattern of thesoils. The variations in the present-day climate are in general too small to appear in themorphology or chemistry of the soils in study area, particularly in the plains. TheMtito-Andei and Chyulu hills form the exception. Here soils are found which differclearly from those dominating the lowlands. This can be attributed mainly to a muchhigher rainfall on the hills. However, there are several major soil regions, which aredistinguishable from each other. These include (i) mountain, hill, low ridge andscarps; (ii) foot slopes, piedmont plain and plateau; (iii) uplands; (iv) erosional plains;(v) sedimentary plains; (vi) floodplains, (vii) alluvial valleys and bottom- lands; and(viii) volcanic plains and lava flows.



4.0 GEOLOGY AND HYDROGEOLOGY

4.1 GEOLOGICAL SETTINGThe oldest rock outcrops in the Chyulu/Mtito-Andei Area consist ofPrecambrian metamorphic rocks of the Mozambique Belt. The mainoutcropping rock types are schists and gneisses; granitoid gneiss is associatedwith the main hills in the area, such as Mbuinzau and Kikwao (Fig. 2). To the east,the Precambrian rocks are overlain by Quaternary volcanic rocks, which werederived from the eruptions of the Chyulu Range. The main rock types arebasalts (Saggerson 1963, Temperley 1955, Searle 1954).The strike of themetamorphic rocks is mainly NNW-SSE. The foliation dips range from 35°to 60° and varies considerably over short distances; their general pattern isindicated in Figure 2. Although most of the faults in the volcanic andmetamorphic rocks tend to follow the strike of the metamorphic rocks, thereare some faults that do not follow that pattern. A good example is shown bythe faults east of Kiboko and south of Makindu.

(a) Regional GeologyThe geological succession underlying the project area consists of a carpet of Chyullubasalts and associated ash agglomerates and alluvial volcanic debris. These are in turnunderlain by weathered biotite gneisses of the basement system derived from theMozambique belt series

(b) Structural GeologyThe geology of the Kibwezi district and, especially, of the Chyullu-Mtito-Kambu areais described as complex, ranging from the Precambrian (over 600 million years old)basement system of crystalline rocks and metamorphose sediments, to the Durumasandstone series of Permian (225-270 million years), Triassic (180-225 millionyears) and Carboniferous (270-350 million years) age. Geologically, the rocks of theTsavo area can be described in two large groups: (i) theArchaean-Precambrian (oldest known Cambrian) rock; and (ii) the Pleistocene (2-3million years) to recent basaltic lava flows.

The Archaean-Precambrian rock includes the basement system, which has beenaffected by metamorphism, granitisation and deep shearing. The Chyulu volcanic hillsare of Pleistocene to recent age. About 600 volcanic cones have been counted fromaerial photographs of the Chyullu area (Saggerson 1962; Sanders 1963; Walsh 1963;Parkinson 1974).

The geomorphology is dominated by the occurrence of an extensive plain of botherosional and sedimentary origin. A number of erosional surfaces can bedistinguished, but only the latest, the Nyika level of the end of the Tertiary age, ispresent over a large area and not much is dissected yet. Of the older surface, only



remnants can be found at the base of the Chyulu hill and Yatta plateau (Ojani1976). The Yatta plateau consists of a protective cap of Miocene phonolite, only about10 m thick, overlying gneiss of the basement system. The erosional plains aredeveloped on a variety of rock types, such as the basement system rock and theDuruma sandstone. The sedimentary plains are developed on the Pleistocene ‘baydeposit’ of an unconsolidated clayey and saline nature on the eastern side. Thegradual east-west gradient rises from the sea level in the east to over 1,000 m in thewest. The slope is gentle in the east, becoming steeper in the west. The generally flatrelief is interrupted by (i) a basement massif which constitutes the Taita Hills-Dabida,which comprises about 48 hills, of which the highest are Vuria (2,228 m) and Sagalla(1,450 m); (ii) the Machakos highlands in the west; and (iii) by ‘Inselbergs’17 such asEndau, Ngulia, Kasigau and Kilibasi. They consist mostly of a quartzitic type ofbasement system rock, which makes them resistant to weathering and erosion.Where the basement system consists of crystalline limestone, low elongated ridgesare often formed.

(c.)Geology of the study Site

The local geology has been delineated based on the few visible outcrops and geologicallog of boreholes that have been drilled in the area. As observed during the fieldwork,the investigated area around the compound is entirely underlain by rocks of the TertiaryVolcanics and Pleistocene Pyroclastics composed of basaltic cones, cinders and ash. Thevolcanic material tectonically sits on the basement regolith at depth.

Figure 4Geology of the study area



4.2 HYDROGEOLOGY AND GROUND WATER OCCURRENCE.

(a) HydrogeologyThe hydrogeology of an area is determined by the nature of the parent rock, structuralfeatures, weathering processes and precipitation patterns. Lava flows rarely possesssignificant primary porespace; instead, groundwater occurs within secondary features,such as fissure zones, fractures, sedimentary beds, lithological contacts and Old LandSurfaces (OLS).

The weathered zones or “Old Land Surfaces” (OLS) characterize periods of erosionbetween volcanic eruptions and subsequent lava flows. These potential aquiferscomprise soils, weathered rocks and water-lain material of volcanic origin. Thethickness of the OLS is variable, but the water-producing zones, which are usuallysandy or gravelly, rarely exceed a few meters.

Unlike lava flows, pyroclastic deposits and especially volcanic sediments (e.g. thoseembedded in the Chyullu Basalts) are generally porous: the cavities between themineral grains are usually open and interconnected. Consequently, they can containand transmit water.

Below the volcanic carpet, the metamorphic Basements rocks are generally hard andcompact, and posses no primary porosity. Groundwater occurrence in the Basement rocksis 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, andsignificant amounts of groundwater can only be expected in fractures (cracks, joints,fissures, and faults).

Water exists in the aquifers under two different physical conditions namely confined andunconfined aquifers. Confined groundwater is isolated from the atmosphere at the pointof discharge by impermeable geological formations and is generally subject to pressureshigher than the atmospheric pressure.

(b) Groundwater Occurrence.The hydrogeology of the Chyulu Area is characterized by two mainaquifers, namely Quaternary volcanic rocks and Precambrian metamorphicrocks. The Tertiary volcanic rocks that form the Yatta Plateau are too thin andnarrow to form any significant aquifer.



(i) Groundwater in Volcanic RockS

The Quaternary volcanic rocks are characterized by basalts with columnarjointing. As a result, very little run-off occurs from the Chyulu Ranges; mostof the water infiltrates along the joints and reappears as springs wherethe contact between the underlying metamorphic rocks and the overlyingvolcanic rocks is exposed at the surface. Examples of these springs are Mzima,Umani, Chae, Manone, Makindu and Kiboko. The discharges of some of thesprings, as estimated by Temperley (1955), are shown in Table 1.

Table 1 Discharge of some of the springs in the Chyulu Area

Attempts have been made to correlate the occurrence of springs withgeological factors, such as faults, strike direction, foliation dip, andmetamorphic/volcanic rock contacts. The only observed correlation is with thecontact between the metamorphic and the volcanic rocks along specific volcanictongues. At the tips of the tongues, rivers have developed, namely theTsavo, Kibwezi, Makindu and Kiboko rivers (Fig 2). Temperley (1955) and Mailu(1983) have associated the occurrences of the springs along the tongueswith pre-volcanic river channels. The proposition is supported by thefact that the springs do not occur everywhere at the metamorphic/volcanic rockcontact. Temperley (1955) has associated the magnitude of discharge with the sizeof the pre-volcanic catchment. The chemical quality of the water has beenanalyzed. Classification according to Gorrel (1958) has been referred to for theanalysis of total dissolved solids (TDS), as shown in Table 2 below.

Table 2 Classification of water according to Gorrel (1958)

Quality TDS (mg/1)

Fresh < 1000Brackish 1000 - 10,000

Saline 10,000 - 100,000

Brine > 100,000

Name Average Discharge(m3/s )Kiboko 0.12Makindu 0.01Umani 1.05Chae 2.10Mzima 6.00



All the springs yield water that is classified as fresh; Umani has the lowest TDSvalue of 315 mg/l. According to WHO (1984), fluoride levels above 1.5 mg/1may cause fluorosis. According to chemical analysis by Mailu (1983) andTemperley (1955), Umani and Mzima springs are safe for human consumption, butChae, Makindu and Kiboko springs are unsuitable. Makindu has the highestvalue of 3.6 mg/1 fluoride. According to WHO (1984), sulphate concentrationsabove 400 mg/l may cause laxative effects in human beings. Records availablefor Kiboko, Mzima, Makindu and Umani springs show that only Kiboko springshave sulphate levels above 400 mg/l (Mailu 1983 and Saggerson 1963).

(ii) Groundwater in Metamorphic RocksIt has been observed that most of the boreholes drilled in the volcanicfields have penetrated a thin veneer of basalts (maximum depth of 9 metres)before entering underlying metamorphic rocks. Some examples of theseboreholes are located west of Mtito Andei, around Kibwezi and southwest ofMakindu. The depths of water-bearing horizons have tended to beconsiderably below the base of the volcanic rocks; thus, the water-bearinghorizons are not associated with the latter. The only such boreholes withchemical analyses are C 2779 and C 2944, west of Mtito Andei Town. Theirfluoride and sulphate levels are within acceptable limits but the water is brackish.Aquifer depths for boreholes drilled in metamorphic rocks have beenanalyzed, and the results are shown in Table 3.Table 3 Borehole depths

Table 4 Borehole yields in Chyulu Area

Table 3 shows that none of the water-bearing zones exceeds 100 m depth andthat most are shallower than 50 m. The deeper aquifer horizons can occur nearto the shallow ones, without any particular pattern. No lithological orstructural correlation can be observed. Table 4 shows that no boreholes haveyields greater than 15m3/hr. Boreholes that yield 10-15m3/hr are mainly

Depth (m) No. of boreholes

< 50 10

50 - 100 6

> 100 Nil

Yield (m3/hr) No. of boreholes< 5 65-10 410 -15 5> 15 nil



associated with volcanic tongues, as evidenced by boreholes in Mtito Andei andMakindu. Boreholes with yields less than 5 m3/hr are associated withmetamorphic rocks away from the volcanic tongues.Table 5 TDS in water from boreholes

Table 5 indicates that most of the boreholes have brackish water (>1000mg/1). No clear pattern of distribution can be identified in relation tolithology or structure

Table 6 Fluoride levels in water from boreholes in Chyulu Area

F- (mg/1) No. of boreholes

< 1.5 3> 1.5 2

It will be observed from Table 6 that most of the water samples have fluoridelevels less than 1.5 mg/l. spatial distribution analysis has indicated that lowlfuoride levels are common in the Mtito Andei area and high levels are common inthe Kibwezi area.

Table7 Sulphate levels from borehole samples in Chyulu Area

Table 7 shows that most of the borehole samples have sulphate levelsabove 400 mg/l. The spatial distribution analysis shows that boreholesaround Mtito-Andei have sulphate contents of less than 400 mg/l, whilethose around Kibwezi and Kiboko have concentrations greater than 400 mg/l.

TDS (mg/1)No. of boreholes

< 1000 11000 -5000 6> 5000 2

SO4 (mg/1) No. of boreholes

< 400 2

>400 4



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

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

2. Indirect recharge: there is a obvious indirect recharge from the Chyullu hillsthrough faults and fractures that connect to the aquifers in the study area.

Fig.5 Hydrogeology of the study area and proposed borehole site

The study area andthe proposed borehole site

















4.4 DISCHARGEThere are two ways through which groundwater is discharged in the area. The first oneis through abstraction from the numerous boreholes. Groundwater in the area may alsoflow through fractures and faults to the areas of lower elevation. For this to happenhowever, the fractures and faults have to be extensively interconnected to allow formovement of water.

4.4.1 Potential EvapotraspirationPotential evapotranspiration is defined as the evapotranspiration that would occur if soilmoisture were not limited. The free water evaporation usually gives an indication of thepotential evaporation from a vegetated soil surface. The potential evapotranspiartion isbetween 75% of the free water evaporation in vegetated highlands and 80% or more inhot and dry lowland areas according to TAMS(ref 9). The rate of evaporation is relatedto elevation in Meters and is given by the following formula:-

E = 2775-0.4838*EL ….Where Eo is evaporation in metersHowever, what is very important is the combination of evaporation, which is herereferred to as evapotranspiration and is given by the following relationship:-

ETo = 2329.9 – 0.0323*EL

Where ETo is Evapotranspiarion is 2329-0.03235*60 = 2327mm

The actual value as indicated in the water master plan is around 600mm.

4.4.2 SOIL MOISTURE STORAGE

When rainfall, runoff and evapotranspiration data are known for particular catchmentarea, the amount of infiltrated water can be calculated as follows:-INF = P – Q – EtaINF = Amount of infiltrated water (mm/year)P = Mean Annual rainfall (mm/year)Q = Mean annual runoff (mm/year)Eta = Mean annual evapotranspiration(mm/year)

However, not all the infiltrated water will percolate to the permanent groundwater.Part of the infiltrated water will remain in the subsurface as the moisture that may laterbe evaporated. The water percolates to the deeper groundwater when the soilmoisture has reached a saturation level.In the literature, several valves are used for maximum soil storage. Woodhead andKer, Priestman and Ass use amounts from 100mm – 200mm depending on the gradeand depth of weathering and climatologically conditions of the area.



5.0 EXISTING BOREHOLES AND RECHARGE

Borehole data within a locality is useful in estimating the depth of a new borehole,expected water quality and yield. The following boreholes are found within the vicinityof the project area. The data is given in Table 2.1.

Table 8: Borehole Data from Boreholes within Close Vicinity

B/HNoC-

Borehole owner Distance/Bearing

TotalDepth

WaterStrucklevel

WaterRestlevel

testedYieldM3/h

Pumpingwaterlevel

C-2779 Ivingoni 2Km SW 120 60 30 8 110C-2944 Mang’elete 2.5km SE 110 63 42 15 85

Iiani 1.5km NW 120 80 36 10 100Komboyoo 7.5km NW 130 80 15 6 120Utithi 7km NW 140 60 10 5 125

AQUIFER PROPERTIES

5.1 Borehole specific capacities (S) Transmissivity (T) CoefficientThe 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-WRLTransitivity on the other hand is calculated using the formula T= 0.183 Q/s. howeverthis 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 nothave aquifer test and it is difficult to ascertain specific yields, storage coefficients ofexisting boreholes in the project area.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: 9 Specific capacities and Transmissivity of existing boreholes

Borehole no. Specificcapacity(m2/hr)

Transmissivity(m²/day)

Ivingoni 0.114286 3.34630Mang’elete 0.25862 7.57339Iiani 0.15625 4.575Komboyoo 0.05714 1.6731Utithi 0.04348 1.2731

The Specific capacity ranges from 0.04348 to 0.25862m²/hr while the transmissivityrange from 1.2731 to 7.57339 m²/day.

5.2 Hydraulic Conductivity (K) and Ground Water FluxLocations laboratory investigations and isotope methods are very expensive methodsand 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 theinvestigation method. Rock sample measurements in the laboratory vary from well testresults. 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 hydraulicconductivity, 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 recordedand most of the time, the thickness is not given. Due to this a lot of assumptions will bemade 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 Tis 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) isconsidered 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 existingboreholes are presented in the table below.



Table:10 Hydraulic conductivity and ground water flux

Boreholes number Hydraulic conductivity Ground water flux

m3/day

Ivingoni 4.2 126

Mang’elete 9.5 284

Iiani 5.7 172

Komboyoo 2.1 63

Utithi 1.6 48

The hydraulic conductivity ranges from 1.6 to 284 m/day and ground water flux rangesfrom 48 to 284 m3/day.

5.4 Assessment of Availability of Ground Water

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

• 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.

5.5 Analysis of Reserve and Ground Water Level Evolution

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

6.0 GEOPHYSICS

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

6.1 Basic principles of the resistivity methods



The electrical properties of the upper parts of the earths crust are dependant upon thelithology. Porosity and the degree of pore space saturation and the salinity of the porewater.Saturated rocks have lower resistivities than unsaturated and dry rocks. The higher theporosity of the saturated rocks the lower its Resistivity. The higher the salinity of thesaturating fluids, the lower the resistivity. The presence of clays and conductiveminerals also reduce the resistivity of the rock.

6.2 Vertical Electrical Soundings (VES)

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

6.4 Electrical Resistivity MethodThis is a major geophysical tool used in groundwater exploration efforts. Resistivity,the inverse of electrical conductivity is the resistance of the geologic medium to currentflow when a potential [voltage] difference is applied for a given material with acharacteristic Resistivity ‘ƒ’ the resistance ‘R’ is proportional to the length ‘L’ of thematerial being measured and inversely proportional to its cross-section area ‘A’

i.e. R= ƒL or ƒ= RAA L

In this procedure, a series of stations is established and careful depth soundings aretaken by evaluating the Resistivity values at different electrode spacing, anunderstanding of the sub-surface materials can be developed.This method is useful for estimating the depth to water bearing strata or estimating thethickness of selected formations. In order to help probe the subsurface rock conditionscapable of groundwater storage, the schlumberger configuration method was used.One Horizontal profiling and Six (6) vertical electrical Resistivity soundings designatedas VES I(KWA MIW’A),VES II(KWA NDIKI),VES III(KWA PAUL),VES IV(KWAMIWA 2),VES V(KWA MICHAEL),VES VI(KWA KAKULI),VES VII(CONTROLVES),were carried as shown below:-



Table 11

6.5 Geophysical Field Data

Table 12.Tabulated below is the analysis of the field geophysical data;STATIONNo.

R-064/10:VESI(KWA MIW’A)

R-065/10:VESII(KWA NDIKI)

R-066/10:VESIII(KWA PAUL)

R-067/10:VESIV(KWA MIW’A 2)

AB/2(m)

∂V/I(Ωm)

Rho(Ohm-m)

∂V/I(Ωm)

Rho(Ohm-m)

∂V/I(Ωm)

Rho(Ohm-m)

∂V/I(Ωm)

Rho(Ohm-m)

1.6 7.27 53 6.79 49 33.3 242 3.70 272.0 5.71 67 5.19 61 23.8 281 2.63 312.5 4.06 73 4.29 81 18.79 353 1.989 373.2 4.45 100 3.36 106 13.72 431 1.518 484.0 2.31 136 2.73 135 10.61 525 1.136 565.0 2.19 170 2.24 174 7.63 594 0.911 716.3 1.1693 210 1.804 226 5.50 682 0.703 878.0 1.228 246 1.320 264 7.63 756 0.506 10110 0.897 281 0.926 290 5.50 867 0.381 11913 0.624 331 0.558 296 3.78 908 0.291 15416 0.264 374 0.362 291 2.77 803 0.230 18520 0.466 420 0.219 276 1.713 804 0.1783 22525 0.333 433 0.1387 272 1.000 729 0.1187 23332 3.35 372 1.914 278 0.638 670 1.978 28740 1.066 252 0.903 213 0.372 401 1.143 27050 0.536 202 0.522 197 0.729 275 0.683 25863 0.265 161 0.227 138 0.334 203 0.326 19880 0.166 155 0.124 123 0.1637 162 0.1727 171100 0.1291 201 0.0912 142 0.0671 105 0.1193 186130 0.0924 244 0.0738 195 0.0452 119 0.0769 203160 0.075 301 0.0513 206 0.0333 134 0.0592 237200 0.0603 378 0.038 238 0.0329 206 0.0448 281250 0.0480 470 0.032 314 0.0254 249 0.0998 365

SITE Name VESNo.

Coordinates (UTM) WSL(m)

Max.Depth(m)

ProspectiveYield(m3/hr)

RANKING

1 KWA MIW’A I 37M 385938 9692967 40,80 150 8-12 12 KWA NDIKI II 37M 390438 9691183 40,80 130 6-10 33 KWA PAUL III 37M 391035 9694759 55,100 120 >8 24 KWA MIWA 2 IV 37M 394333 9695953 50,80 150 6-8 45 KWA MICHAEL V 37M 390282 9699376 63,120 120 >6 56 KWA KAKULI VI 37M 388710 9696023 50,90 130 ≥6 67 CONTROL VES VII 37M 387871 9694264 40,60 130 6



Table 13 VES DATA

STATIONNo.

R-068/10:VESV(KWAMICHAEL)

R-069/10:VESVI(KWAKAKULI)

R-070/10:VES(CONTROL-WELL)

AB/2(m)

∂V/I(Ωm)

Rho(Ohm-m)

∂V/I(Ωm)

Rho(Ohm-m)

∂V/I(Ωm)

Rho(Ohm-m)

1.6 6.78 49 19.92 145 6.79 492.0 4.92 58 12.69 150 5.19 612.5 3.83 72 9.03 170 4.29 813.2 2.95 92 5.78 182 3.36 1064.0 2.24 111 4.01 199 2.73 1355.0 1.722 134 2.94 229 2.24 1746.3 1.281 159 2.08 258 1.804 2268.0 0.879 176 1.449 290 1.320 26410 0.619 194 0.991 310 0.926 29013 0.402 213 0.624 331 0.558 29616 0.278 223 0.421 338 0.362 29120 0.1718 217 0.279 352 0.219 27625 0.1185 232 0.1762 345 0.1387 27232 0.0733 236 1.876 272 1.914 27840 0.974 230 0.921 217 0.903 21350 0.551 208 0.460 173 0.522 19763 0.304 185 0.300 182 0.227 13880 0.224 222 0.211 209 0.124 123100 0.1767 275 0.1611 251 0.0912 142130 0.1349 356 0.1129 298 0.0738 195160 0.0513 206200 0.038 238250 0.032 314



6.6 6.2 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).

Figure 4 shows Horizontalprofile conducted within the designated points to reveal the respective anomalies

VES 3 VES 4VES 5/6















VES 3 VES 4VES 5/6















VES 3 VES 4VES 5/6



The main aim of the measurements was to determine the depth to the fresh basementformation rocks, the degree of fracturing at depth, which should be directly related tothe Transmissivity layer and thus the potential yield. As a general rule, it can beassumed that in this case the sounding with the lowest basal resistivities in theexpected water bearing range represent the most favourable drilling site.

Table : Table 14:Hydrogeological Interpretation of VES I MIWA-1.

Table 14: VES I (MIWA). AB=250m; sounding data: consistent. Physiographic zone: Relativelylow ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by pyroclastic layer (50 Ωm) grades into weathered basaltic pyroclasticmaterial (300Ωm). Below this material lies an highly undifferentiated basaltic formationof (3000Ωm) and further highly fractures to (25 Ωm) which happen to be the Mainaquifer targeted in this study is believed to be the contact zone between the fracturedbasalts/basement. The underlying layer with resistivity of (7000Ωm) is composed offresh basement at depth

Table 15: Table 2:Hydrogeological Interpretation of VES II (KWA NDIKI-1.

Depth(m)

Resistivity(Ohm)

Interpretation Aquiferous?

0.0 – 1.8 15 Dry phanasols No1.8 – 4.0 2500 Fresh Pyroclastic tuffs(Dry) No4.0 – 11 600 slightly weathered pyroclastic material No11 - 24 100 Compact (undifferentiated) basalts No24-150 35 Fractured volcanic material basalts/basement(wet) Yes>150 2000 Fresh Basement (Dry) No, Impervious

Table 15: VES II (KWA NDIKI). AB=250m; sounding data: consistent. Physiographic zone:Relatively low ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by fresh pyroclastic layer (2500 Ωm) grading into slightly weatheredbasaltic pyroclastic material (600Ωm). Below this material lies a differentiated basaltic

Depth(m)

Resistivity(Ohm)


0.0 – 1.8 10 Dry phanasols No1.8 – 8.0 50 Very loose Pyroclastic tuffs(Dry) No8.0 – 11 300 Slightly weathered pyroclastic material No11 - 25 3000 Compact (undifferentiated) basalts No25-160 20 Fractured volcanic material basalts(wet)/basement Yes>100 7000 Fresh Basement (Dry) No, impervious



formation of (100Ωm) and further fractures to (30 Ωm) which happen to be the Mainaquifer targeted in this study. The underlying layer with resistivity of (2000Ωm) iscomposed of fresh basement.Table 16:Hydrogeological Interpretation of VES III(KWA PAUL).

Depth(m)

Resistivity(Ohm)


0.0 – 0.8 70 Dry phanasols No0.8 – 4.0 2000 Fresh Pyroclastic tuffs(Dry) No40 – 11 500 slightly weathered pyroclastic material No11 - 34 300 undifferentiated basalts No23-100 40 Fractured volcanic material basalts(wet) Yes>100 7000 Fresh Basement (Dry) No, impervious

Table 16: VES III (KWA PAUL). AB=250m; sounding data: consistent. Physiographic zone: Relativelylow ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by fresh pyroclastic layer (2000Ωm) gradeing into slightly weatheredbasaltic pyroclastic material (500Ωm). Below this material lies a further differentiatedbasaltic formation of (300Ωm) and further fractures to (4o Ωm) which happen to be theMain aquifer targeted in this study. The underlying layer with resistivity of (7000Ωm) iscomposed of fresh basalts.Table 17:Hydrogeological Interpretation of VES IV( KWA MIWA-II).

Table 17: VES IV(KWA MIW’A II). AB=250m; sounding data: consistent. Physiographic zone:Relatively low ground, with Phanasols.The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by fresh pyroclastic layer (135 Ωm) grades into highly weathered basalticpyroclastic material (3Ωm). Below this material lies an undifferentiated basalticformation of (430Ωm) and further fractures to (49 Ωm) which happen to be the Mainaquifer targeted in this study. The underlying layer with resistivity of (600Ωm) iscomposed of fresh basalts.

Depth(m)

Resistivity(Ohm)


0.0 – 0.5 15 Dry phanasols No0.5– 6.0 130 Loose Pyroclastic material (Dry) No6.0 – 9.0 650 slightly weathered pyroclastic material No9.0 - 27 1300 Compact (undifferentiated) basalts No27-65 40 Fractured volcanic material basalts(wet) Yes>65 4000 Fresh Basement (Dry) No, impervious



Table 18:Hydrogeological Interpretation of VES V( KWA MICHAEL).

Depth(m)

Resistivity(Ohm)


0.0 – 0.9 18 Dry phanasols No0.9 – 1.0 200 Loose Pyroclastic tuffs(Dry) No1.0 – 5.0 600 weathered pyroclastic material No5.0 - 24 350 Differentiated basalts No24-100 100 Fractured volcanic material basalts(wet) Poor>100 5000 Fresh Basement (Dry) No, impervious

Table 2: VES V (KWA MICHAEL). AB=250m; sounding data: consistent. Physiographic zone: Relativelylow ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by loose pyroclastic layer (200-Ωm) transforming into sloose weatheredbasaltic pyroclastic material (200Ωm). Below this material lies an slightlyundifferentiated basaltic formation of (600Ωm) and fslightly fractures to (100 Ωm)which happen to be a probable aquifer which is not reliable.. The underlying layer withresistivity of (5000Ωm) is composed of fresh basement.Table 19:Hydrogeological Interpretation of VES VI(KWA KAKULI).

Table 2: VES VI (KWA KAKULI). AB=250m; sounding data: consistent. Physiographic zone: Relativelylow ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by loose pyroclastic layer (170 Ωm) grades into weathered basalticpyroclastic material (180Ωm). Below this material lies an undifferentiated basalticformation of (900Ωm) and further fractures to (50 Ωm) which happen to be the Mainaquifer targeted in this study and is believed to be the contact zone between thefractured basalts/basement. The underlying layer with resistivity of (5000Ωm) iscomposed of fresh basement.

Depth(m)

Resistivity(Ohm)


0.0 – 2.0 40 Dry phanasols No2.0 – 8.0 170 Loose Pyroclastic tuffs(Dry) No9.0 – 11 180 Highly weathered pyroclastic material No11 - 25 900 Compact (undifferentiated) basalts No25-100 50 Fractured volcanic material basalts(wet) Yes>100 5000 Fresh Basement (Dry) No, impervious



Table 20: Hydrogeological Interpretation of VES #( CONTROL VES-EXISTING WELL).

Depth(m)

Resistivity(Ohm)


0.0 – 1.8 15 Dry phanasols No1.8 – 8.0 50 Fresh Pyroclastic tuffs(Dry) No8.0 – 11 300 Highly weathered pyroclastic material(wet) Probably11 - 23 2000 Compact (undifferentiated) basalts No23-130 30 Fractured volcanic material basalts(wet) Yes>130 4000 Fresh Basement (Dry) No, impervious

Table 20: VES (CONTROL DATA-EXISTING WELL). AB=250m; sounding data: consistent.Physiographic zone: Relatively low ground, with Phanasols.

The geo-electric model consists of a thin sheet of dry Phanasols as the top mantle. Thisis underlain by very loose pyroclastic layer (50 Ωm) grading into weathered basalticpyroclastic material (300Ωm). Below this material lies highly undifferentiated basalticformation of (2000Ωm) and further fractures to (30 Ωm) which happen to be the Mainaquifer targeted in this study. The underlying layer with resistivity of (4000Ωm) iscomposed of fresh regolith.

7.0 GROUND WATER QUALITY

Generally ground water chemistry from volcanic rocks varies from place to place due tothe chemical constituents of various lavas. Water quality from the proposed borehole isexpected to be good though the fluoride level will be higher than the recommendedlevel by WHO. Some of the factors which determine the degree of mineralization ofground water are as follow:-

7.1 Evaporation and Transpiration

Direct evaporation by the heat of the sun and preferential up take of certain mineralsions by plants can in certain environments, lead to hardness of water and salinity.

7.2 Dissolution of evaporates

The process of evapotranspiration may in arid conditions lead to the precipitation ofsalts in the unsaturated zone. These salts may then be carried down to the groundwater stone during periods of rain; thus leading to high ion concentrations in space andtime.7.3 Dissolution Host rock

Given relatively long residence times and fairly ambient temperatures in ground watersystems, progressive salinity or mineralization of ground water can be expected via thesolution of various constituents of the host rock. This will vary according to localstructures (which may speed the passage of water through an aquifer by means offaults etc & so limit retention time) local climate and so on.



Considering the above factors the quality of water in our project area is expected tovary from one bore hole to the other but generally the water has a high fluorideexceeding the WHO limit.

8.0 Impacts of Proposed Drilling Activity.

In client’s study area, the rock formations are volcanic with basement at depth innature. Basement aquifers are localized, therefore drilling activity within the study areashall have no impact on the aquifers, water quality, and the abstractors and neithershall there be a likelihood of coalescing cones of depression. It shall have no negativeimplications for other ground water users.

8.1 Impacts On Local Aquifer’s Quantity And Quality:The sustainability of water quality depends on the level of abstraction and rechargerate. 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 yieldsof the boreholes in the area, the proposed abstraction of 20m3/day based on a 10 hourpumping regime is not expected to have any major impact on the aquifers, as theaquifer 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 theirmitigation measures will be adequately addressed by the Environmental ImpactAssessment Study to be conducted

8.2 Impacts On Existing Boreholes In The Area:

It is noteworthy that all the boreholes examined in this study area are more than 2kmaway from the proposed drill site. Therefore we do not expect any negative impacts onany other existing borehole within the vicinity.



9.0 CONCLUSIONS/RECOMMENDATIONS9.1 General

Studies and analyses have shown that all boreholes drilled withinthe volcanic fields abstract their water from metamorphic rock ratherthan volcanic rock aquifers. It is concluded, therefore, that all boreholesconsidered in this study have derived their water from water-bearinghorizons within metamorphic rocks. Lithology and structural characteristicsof the rocks do not appear to have a significant influence on thedepths of water-bearing zones, borehole yields or water quality. However,the contact between the metamorphic rocks and the overlying volcanicrocks(Old surface Lands) along the tongues of volcanic flows hasproved promising for high yields for some boreholes.

Insufficient water-quality data have hampered conclusive analysis ofthe geological influence on groundwater in some parts of the area.High fluoride contents and total dissolved solids around the Kibwezi area,as compared to other areas, indicate an anomaly that has not yet beenexplained. While the metamorphic rocks are relatively poor in terms ofgroundwater quantity and quality, they have a useful function asbarriers to water that percolates through the relatively permeableoverlying volcanic rocks. Where the contact is exposed along thetongues of the volcanic flows, springs have developed. These springshave been the main sources of water supply in the Chyulu Area, hencethe significant positive influence of the metamorphic rocks ongroundwater storage.

9.2 Conclusion.Based on the collected and analyzed data, the hydro geological prevailingconditions it can be concluded as follows:- There are good prospects of striking Groundwater within the investigated

site. Water from this borehole is expected to be of good quality; Information from the existing boreholes suggests that the locality has high

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

between 8 and 12 cubic metres of water per hour. Water occurrence is within the basaltic-basement contact zone



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

MIWA I, to an approximate depth of 160metres below ground level: this will besufficient for a sustainable yield of approximately 12m3/hr. It is howeverexpected that if drilling proceeds to bottom with good construction and boreholedesign, more than 15 m3/hr can reasonably be attained.

It should be lined with appropriate casings and screens. It should be protected from possible sources of contamination by grouting a

certain length of the borehole from the ground surface. The borehole should be properly gravel packed to enhance yield.• The drilling and test pumping should be supervised by water office.• Upon completion, the borehole should be fitted with an airline/ piezometre and a

master meter to facilitate monitoring of static water level and groundwaterabstractions respectively.

• A two (2) litres water sample of this water is to be collected in a cleancontainer and be taken to any competent water testing authority for a fullchemical, physical and bacteriological analysis.

• It is a legal requirement, stipulated in the water act 2002,that the client appliesfor a ground water permit from the Water Resources Management Authority(WRMA) to sink a borehole. For this purpose, three signed copies of the presentreport must be submitted to the authority for examination.

9.3 Further Recommendations:

The site is known to the the chairman, committee of Mang’elete water Projectand the World Vision, Mtito Andei IPA MWA Engineer

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

To achieve and maintain a high yield, and maximize the efficiency of theborehole, the importance of proper design and construction methods cannot beoveremphasized.

The water quality of the proposed borehole is expected to be palatable.



ReferencesBaker, B. H. (1954). Geology of the Southern Machakos District. Report No. 27. Nairobi,Geological Survey of Kenya.

Braunn, H. M. H. (1978). "Climate." In D. O. Michieka, J. J. Vlee Shouwer and B. J. A. Van DerPouw (Eds.), Soils of the Kwale - Mombasa - Lunga Lunga Area. Report No. R3 Kenya Soil Survey,Nairobi. pp.11-22.

Braunn, H. M. H. (1977). Average monthly rainfall as a percentage of the annual rainfall in Kenya andTanzania, with particular reference to the Kenya coast, Misc. paper No. M14, Kenya Soil Survey,Nairobi. pp. 1-5.

Central Bureau of Statistics. (1981 & 1986). Republic of Kenya. Ministry of Economic Planningand Development.

Ferguson, A., E. Absalom, W. Kogi, and D. Omambia. (1985). Kibwezi Integrated Survey, AMREF.April 1985. pp. 46-67.

Gachimbi, L. N. (1990). Land degradation and its control in the Kibwezi area, Kenya (Thesis).University of Nairobi, 1990. 180p.

GoK. (1993). Makueni District Development Plan for the period 1994-1996. Govt. printer NRB.

Mbinda, B. M. (1992). Agronomic and Socio-Economic factors affecting Cotton Production in MakueniLocation, Machakos District. A Dessertation submitted as part of requirements for Bsc. inGeography. University of Nairobi.

Mbithi, P. M. and C. Barnes. (1975). The spontaneous settlement in Kenya. East Africa LiteratureBureau. pp. 128-167.

Michieka, D. O. and B. J. A. Van Der Pouw. (1977). Soils and vegetation of the Kiboko rangeresearch station. A semi detailed report No. 53.

Saggerson, E. P. (1963). Geology of Simba Kibwezi report No. 58 Geology Survey of Kenya,Nairobi.

Tiffen, M. (1991). Environmental change and dryland management in Machakos District, Kenya 1930- 1990. Population Profile: Working paper number 54. (ODI)

Touber, L. (1983). Soils and vegetation of Amboseli- Kibwezi area. Kenya soil survey report No. R6 Nairobi In Pouw et al. ed. 1983. pp. 29-138.



APPENDIX

DRILLING METHODS/ TECHNIQUE

Drilling should be carried out with an appropriate tool-either percussion or rotarymachine. The latter are considerably faster. Geological rock samples should be collectedat 2- metres intervals. Struck and Rest water levels and if possible, estimate of theyield of individual aquifer encountered should also be noted.

Borehole DesignThe design of the well should ensure that screens are placed against the optimumaquifer zones. An experienced works drilling consultant/hydro geologist should makethe final design, and should make the main decision on the screen setting.

Casings and ScreensThe well should be cased and screened with good quality screens, considering thedepth 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, itsuse 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 PackThe use of gravel pack is recommended within the aquifer zones, because the aquifercould 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 ofapproximately 1”, which should be sufficient. Should the slot size chosen to be toolarge, the well will pump sand thus damaging the pumping plant and leading to gradualsiltation of the well. The grain size of the gravel pack should be having an average of2-4mm.

Borehole ConstructionOnce the design has been agreed, constructions can proceed. In installing screens andcasing, centralizers at 6 metres interval should be used to ensure centrality within theborehole. 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 withclay (2m). The remaining annular space should be backfilled with an insert materialand the top five meters grouted with cement to ensure that no surface water at thewell head can enter the well and thus prevent contamination.

Borehole DevelopmentDevelopment aims at;

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

• Secondly, it alters the physical characteristics of the aquifer around the screenand removes fine particles.

We do not advocate the use of over pumping as means of development since it onlyincreases permeability in zones, which are already permeable. Instead, we wouldrecommend the use of air or water jetting, or the use of the mechanical plunger, whichphysically agitates the gravel pack and adjacent aquifer material. This is an extremelyefficient method of developing and cleaning wells.

Well development is an expensive element in the completion of a well, but is usuallyjustified in longer well life, greater efficiencies, lower operational and maintenance costsand a more constant yield. Within this frame the pump should be installed at least 2mabove the screen.

Borehole TestingAfter development and preliminary tests, a long duration well test should be carried outon all newly completed wells. This gives an indication of the quality of drilling, designand development. It also yields information on aquifer parameters which are vital tothe 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 thedischarge rate and the resulting draw downs as a function of time. Once a dynamicwater level (D.W.L) is reached, the rate of flow to the well is equal to the rate ofpumping. Towards the end of the test a water sample of 2 litres should be collected forchemical analysis.

The duration of the test should be 24 hours; followed by recovery test until the initialW.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 TDSof 1000mg/1itre, see Table 5.1.

Table 5.1: WHO Water Quality Classification

TYPE OF WATERTDS (MG)

Fresh < 100Brackish 1,000 – 10,000Saline 10,000 – 100,000Brine >100,000

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

PARAMETERSTHRESH HOLD (Mg/1) LIMIT (Mg/1)

TDA 2500 5000CALCIUM 500 1000MAGNESIUM 250 500SODIUM 1000 2000BICARBONATE 500 1500CHLORIDE 1500 3000FLUORIDE 1 6NITRATE 200 400SULPHATE 500 1000PH 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 ProtectionIn order to protect the quality of the water, the borehole should be located as far aspossible from all sources of danger e.g. septic tanks, pit latrines, polluted water bodiese.t.c.



Schematic Design forBorehole 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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Figure 5 SELECTED PROPOSED BOREHOLE SITE

Proposed UtuCommunity BH site









