Download GEOLOGICAL CONTROLS ON GROUNDWATER DISTRIBUTION IN

GEOLOGICAL CONTROLS ON GROUNDWATER
DISTRIBUTION IN NAIROBI COUNTY. KENYA
BY
KIPCHUMBA J LOICE
I13/2369/2007
A Dissertation submitted to the Department of Geology in partial fulfillment of the
requirements for the Award of Bachelor of Science Degree in Project in Geology
University of Nairobi.
JUNE 2011
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DECLARATION AND APPROVAL
This report is my original work and has not been presented for a Degree in any other University.
Signed
Date
KIPCHUMBA J LOICE
I13/2369/2007
This report has been submitted for examination with approval by the University Supervisors;
Signed
Date
DR. C.M GICHABA
Signed
Date
DR. D. OLAGO
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DEDICATION
I dedicate this work to Aileen Jeptoo, Edwin, My Parents, Brothers and Sisters.
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ABSTRACT
Research works have revealed that the yields of boreholes in Nairobi area are heterogeneous.
This variation is greatly due to location of Nairobi area on the Eastern flank of the Kenya Rift
valley by which affected it during its development. The volcanism which occurred led to the
volcanic lithology dominating the area, while the tectonic activities led to numerous faults,
fractures and joints in the area. Indeed the aquifers provide a valuable base flow supplying water
to rivers during periods of no rainfall. Borehole drilling in Nairobi area is increasing rapidly due
to increase in the demand of water supply. This increased demand is attributed by population
increase, industrialization, agriculture and also contamination of surface waters thereby posing a
challenge to groundwater exploration. The review hence carries a critical analysis and library
synthesis on the direct effect of both lithology and structures delineating the major controlling
factors in the groundwater distribution in the area. Furthermore discriminate the productive
aquifers in the study area and how they are classified in relation to geology.
Though there have been continued increases in the demand of water supply in Nairobi area,
already the groundwater in the study area have shown that it has been subjected to
overexploitation. This has been clearly noted in some of the boreholes with greater depth but
show low yields and also continued water level fluctuations around Langata area, to minimize
this it is advisable that safe yields be determined by test pumping the boreholes for a satisfactory
period of time and monitoring the water levels of the neighboring boreholes, however enhancing
artificial recharge that can replenish these aquifers. However, a more detailed hydrogeological
study be conducted to define the long-term sustainable abstraction rate and measures needed to
maintain the related recharge rates.
Several lithologies have been identified which form very good aquifers without any influence of
structures in them like the unconsolidated sandy sediments, while others are aquifers only
because they are structurally affected like the fractured and weathered volcanic. Aquifers struck
at the Old Land Surface and the fractured or weathered volcanics are highly productive, hence is
advisable that borehole drilling depths verily concentrate on these aquiferous lithologies and also
exploit the less exploited lithologies like the weathered Kandizi phonolites encountered at greater
depths in the area.
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TABLE OF CONTENTS
DECLARATION AND APPROVAL ................................................................................................... ii
DEDICATION...................................................................................................................................... iii
ABSTRACT.......................................................................................................................................... iv
TABLE OF CONTENTS ...................................................................................................................... v
LIST OF FIGURES............................................................................................................................. vii
LIST OF TABLES............................................................................................................................... vii
CHAPTER ONE.................................................................................................................................... 1
1.0 INTRODUCTION ........................................................................................................................... 1
1.1 STATEMENT OF PROBLEM ...................................................................................................... 5
1.2 AIM AND OBJECTIVES .............................................................................................................. 5
1.2.1 AIM ............................................................................................................................................ 5
1.2.2 SPECIFIC OBJECTIVES ........................................................................................................... 5
1.3 RESEARCH QUESTIONS ............................................................................................................ 5
1.4 JUSTIFICATION .......................................................................................................................... 6
1.5 METHODOLOGY ........................................................................................................................ 6
CHAPTER TWO................................................................................................................................... 7
2.0 GEOLOGY AND HYDROGEOLOGY .......................................................................................... 7
2.1 GEOLOGY.................................................................................................................................... 7
2.2 HYDROGEOLOGY ...................................................................................................................... 8
CHAPTER THREE............................................................................................................................. 10
3.0RESULTS AND DISCUSSION ...................................................................................................... 10
3.1 Lithology and structures ............................................................................................................... 10
3.2 Aquifer discrimination and productivity ....................................................................................... 16
3.3 Abstraction and groundwater distribution ..................................................................................... 17
CHAPTER FOUR ............................................................................................................................... 19
4.0CONCLUSION AND RECCOMMENDATIONS ......................................................................... 19
4.1 CONCLUSION ........................................................................................................................... 19
4.2 RECCOMENDATION ................................................................................................................ 20
CHAPTER FIVE ................................................................................................................................. 21
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5.0 FERERENCES .............................................................................................................................. 21
ACKNOWLEDGEMENTS................................................................................................................. 24
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LIST OF FIGURES
Figure 1. 1 .Map showing the extent of early lava flows in the Nairobi area (after Saggerson,
1991 as cited in Wilson and Steve 1998) ..................................................................................... 2
Figure 1. 2 .Map of Kenya showing the area of study (Kenya Advisor.com) ................................4
Figure 2.1Outline of the major volcanic formations in the Nairobi area (Stephen and Albert
2005) ..........................................................................................................................................7
Figure 3. 1.Illustration of various formations bearing aquifer layers with depth (not to scale) .... 12
Figure 3. 2.Map showing location of the boreholes in Nairobi area. Scale 1:125000. (Saggerson
1991) ........................................................................................................................................ 13
Figure 3. 3.Hydrogeological cross- section covering the selected boreholes along cross sectional
line A-B (Scale: X-axis; 1cm Rep 0.02m, Y-axis; 1cm Rep 50m). ............................................. 14
Figure 3. 4.Geological map of Nairobi area showing the borehole locations in the study area.
Scale 1:125,000 (Saggerson 1991 )............................................................................................ 15
Figure 3. 5.Groundwater level hydrograph for water well in Nairobi area (Stern and Albert 2005)
................................................................................................................................................. 17
LIST OF TABLES
Table 1. 1.Aquifer characteristics in the Nairobi area ................................................................ 10
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CHAPTER ONE
1.0 INTRODUCTION
Water is a life support resource without a substitute and its availability is unevenly distributed.
Most of the earth’s fresh water is found not only in lakes and rivers, but also in underground
aquifers. Indeed these aquifers provide a valuable base flow supplying water to rivers during
periods of no rainfall. The increased demand for water supply in Nairobi area is attributed by
population increase, industrialization, agriculture and also contamination of surface waters
thereby posing a challenge to groundwater exploration. The Nairobi area covers part of the
Eastern flank of the Great Rift Valley and its geological history has been dominated by
widespread volcanic activity of Cainozoic age overlying a foundation of folded Precambrian
schist and gneisses of the Mozambique belt (Saggerson 1991). Volcanicity is generally
associated with tectonic movements’ attendant upon rifting. Deep soils and gravels of quaternary
age cover all formations. The major volcanic formations which underlie the area as pointed out
by Saggerson 1991 are mainly Ol Esayeati volcanics which mantle the older Ngong volcanics to
the West and East including phonolites which display a complex stratigraphic relation intruded
from Ol Esayeati hills (fig 1.1). The Ngong volcanic on the other hand consist mainly of a series
of stratified alkaline lavas and agglomerates.
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Figure 1. 1 .Map showing the extent of early lava flows in the Nairobi area (after Saggerson,
1991 as cited in Wilson and Steve 1998)
Due to increased demand of water in Nairobi area, groundwater exploitation is on the rise which
is very evident from the existing borehole records (yield records), groundwater in Nairobi area is
varied with some boreholes very successful while others having very little yields. This is directly
connected to the geology of the area, both structurally and lithologically as outlined in the next
chapter.
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STUDY AREA
The study area covers the Nairobi city centre (fig 1.2).It lies within the approximate latitudes
1008’S to 1023’S and longitudes 36044’ E to 37000’E in the geological map of the Nairobi area
degree sheet number 51 North- east quarter (Directorate of Overseas Surveys Sheet No. 148)
after (Saggerson 1991).The area is approximately 832.5km2 and lies at an altitude range of about
1493-1836m above sea level and it forms the eastern flank of the eastern branch of the Great Rift
valley. The land in this area rapidly changes in topography from the gently sloping and finally to
the flat plain that is characteristic of the down thrown horst less than a kilometer away to the
west (Rift valley). The current physiography is as a result of the past tectonic activities of the rift
valley.
The climate of the study area can be described as sub humid to semi arid with the precipitation
increasing from the plains at Athi River towards the Kikuyu escarpment. Nairobi city centre
shows an average rainfall of 906 mm per year (Saggerson 1991). Temperature is greatly
modified by variations in the altitude of the area, the temperatures show that months of OctoberMarch as the hottest months while June-August are the coldest months of the year. A mean
annual temperature of the area is approximately 19.40C (Ahmed 2001).
The area is generally drained by many streams which are perennial as well as other large streams
and rivers. The Kikuyu highlands form a parallel drainage pattern running in a smooth easterly
general direction. The main drainage is consequent upon the regional slope of the volcanic rocks
towards the east. The streams are frequently fed by springs that issue from the forest belt. The
principal occurrences being those at Kikuyu which feed the headwaters of the Nairobi River
(Saggerson 1991).
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Study area
Figure 1. 2 .Map of Kenya showing the area of study (Kenya Advisor.com)
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1.1 STATEMENT OF PROBLEM
Preliminary examination of borehole data for the Nairobi area obtained from the Ministry of
Water Resources and Irrigation showed variation in borehole yields and water rest levels. This
could be attributed not only to sub surface lithological units and structures especially fault but
also due to increase in abstraction rates in the area which have resulted to overexploitation of
some of the aquifers in the study area. Hence a library research work was conducted to delineate
the structures, lithological units and the abstraction rates in the area and how they have affected
the groundwater distribution in the study area.
1.2 AIM AND OBJECTIVES
1.2.1 AIM
 To investigate various borehole yields from different locations in the area of study and
account its attributes in relation to lithology and structural influence, hence determine the
key factors to such yields.
1.2.2 SPECIFIC OBJECTIVES
 To analyze borehole data from different locations in relation to lithology and structures
hence characterize their role in the groundwater distribution in the study area.
 To discriminate the aquifers and delineate the highly productive ones in the study area.
 To establish the relationship between the abstraction and groundwater distributions in the
study area.
1.3 RESEARCH QUESTIONS
 How does various lithological units and structural setting influence the groundwater
distribution and its potential in the study area?
 Which aquifers show high productivity in the area and how are they classified in relation
to geology?
 What are the abstraction estimates of aquifers in the study area and how does it relate to
groundwater distribution and its potential resources in the area?
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1.4 JUSTIFICATION
The drilling of boreholes in Nairobi area started in the 1930s and the number of water wells
increased from less than 10 in 1940 to almost 2000 in 1997 and further increased to 2250 in
2001((Stephen and Albert 2005). This has been highly due to the rapid increase in population
and industrialization in the area hence an increased demand for groundwater. The review is
hence important as it aids in having a proper understanding on groundwater flow and distribution
which is essential in borehole citing. Furthermore, it highlights the possible high productive
aquifers in the Nairobi area.
As much as the groundwater is increasingly on high demand, already the resource has been
subjected to over exploitation in the study area. Hence the review is this vital since it aids in
delineating the possible remediation to the already overexploited areas in the study area.
1.5 METHODOLOGY
Since it is a library research work, intense library research and desk study was carried out to
delineate the various lithological units and geology of the area. This was mainly obtained from
existing geological reports, journals and also geological maps and the cross section of the study
area which was obtained from the department.
The borehole records obtained from the ministry of water was fital for this assessment because it
is from it that the hydrogeological cross section was constructed from and hence helped to
delineate the number of aquifers in the area and also to obtain the recharge direction and
presence of hydraulic barriers in the study area. From this, the appropriate assessment could be
made and arrive at a conclusion.
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CHAPTER TWO
2.0 GEOLOGY AND HYDROGEOLOGY
2.1 GEOLOGY
Groundwater distribution in Nairobi area has been a subject of several research works. This is
due to increased demands for water supply in the area as a result of population increase,
industrialization, agriculture and also deterioration of the quality of surface water. The geological
history of the Nairobi area is dominated by volcanic activities according to Schackleton 1945 as
cited by Saggerson 1991. The youngest Tertiary rocks are Limuru trachytes and the Kerichwa
valley tuffs which are subsequently underlain by Ol esayeiti volcanic phonolites followed by
Ngong volcanic. The volcanic comprise of Ol Donyo Narok agglomerates, Nairobi trachytes,
Nairobi phonolites, Kandizi phonolites, Mbagathi phonolitic trachytes which are underlain by
Athi tuffs and lake beds with chert band (fig 2.1). It is assumed that phonolites exist underlying
the Nairobi trachytes. These tertiary volcanic rocks overly folded Precambrian basement system
rocks of the Mozambique Belt occurring at a greater depth (Mulwa et al 2005). Cooling of the
differing lithologies influences the distribution of groundwater the research study.
Figure 2.1Outline of the major volcanic formations in the Nairobi area (Stephen and Albert
2005)
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Faults in the recharge area and minor in the study area are noted to greatly influence borehole
yields since the productivity of boreholes in the study area are site specific (they vary greatly
from one borehole site to the other), in that those sited in the faulted regions have higher yields
as compared to the ones sited in unfaulted or less faulted areas (Mulwa 2001). This is because
faults acts as drainage channels and when filled with weathered material they form good
aquifers. They also act as dams or barriers for groundwater flow and also drain water to very low
areas making it unavailable for groundwater development (Freeze and Cherry 1990).
Clarke et al 1990 noted that groundwater flows along faults and formations contacts towards
regions of lower elevation. They thus concluded that boreholes that have been sunk within this
horst have exceptionally high amounts of yields as compared to the boreholes sunk elsewhere.
Fractures and joints in the area increase secondary porosity in the rocks, forming good aquifers
(Sharp1993), hence hydrofracturing in the study area is a good groundwater remediation method
(Nebel and Wright 2000).According to Baker 1985, high or low aquifer yields tapped through
boreholes cannot be specifically associated with a particular aquifer formation since porosity and
permeability in volcanic formations is influenced by fracturing and secondary weathering, this
was also noted by (Kulkarn and Deolankar 1993) and (Gavearts 1964) who pointed out that high
yields in volcanic formations were due to presence of vesicles and amygdales, bedding planes
also influences groundwater flow and distribution (Lapcevic et al 1993).
2.2 HYDROGEOLOGY
The Nairobi groundwater basin extends from the zone of North-South rift faulting west of the
city towards the Athi river flood plain east of the city centre. Volcanic activity has controlled the
geomorphologic evolution ,the rocks on the basin mainly comprising a succession of volcanic
lavas and ashes(tuffs) whose thickness reaches some 400m underneath the city itself and which
eastwards gradually merge into to the Tertiary deposits of the Athi floodplain (Stephen and
Albert 2005). The volcanic rocks show a wide range of porosity and permeability and have
developed aquifer units separated by lower permeability strata. The aquifers mainly comprise the
Kerichwa Valley Series and Upper Athi Series (transmissivity of 5-50 m2/d and low
storativity).Kerichwa valley tuffs are important in respect, especially in Karen where it is
characterized by the adequate shallow supply. In the city centre, water draining eastwards from
hill area accumulates on the low- lying ground between Parklands in the North and Nairobi
South Estate, forming a perched water table above Nairobi phonolotes (Saggerson 1991).The
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extension of this multi-layered aquifer system is fairly well known from the many boreholes that
have been drilled to depths of 100-350m.
Since the area is far to the east of the Rift valley, it has no extensive network of faults. Most of
the groundwater recharge occurs far on the slopes of the rift zones, west of the city where the
volcanic rocks are incised by numerous streams related to fault lines and weathered zones of the
previous land surface. Recharge also takes place through infiltration of waste water, water mains
leakage and excess rainfall (Stephen and Albert 2005). The total leakage from the water
distribution system is estimated to be 180 mi/d, but it is difficult to say how much of it reaches
groundwater. There is some evidence that part of the infiltration in Nairobi area is intercepted by
localized perched aquifer and discharged locally to springs and streams (Stephen and Albert
2005).Groundwater flow is principally directed in East to South east from the main recharge area
towards the Athi floodplains where most of the groundwater formerly discharged as springs or
seepages into local streams and depression. Most of the water wells in this area intercept most of
this groundwater flow.
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CHAPTER THREE
3.0RESULTS AND DISCUSSION
3.1 Lithology and structures
To determine the influence of lithology and geologic structures on the groundwater distribution
in Nairobi area, borehole data which included borehole ID, geographical coordinates, elevation,
total depth, WRL, WSL, drawdown and the yields that were collected from the ministry of water
was very vital(Table 1.1). Groundwater yields in Nairobi area is quite heterogeneous even within
a very short distance, a factor attributed to geological evolution of the area and adjoining rift
valley.
Table 1. 1.Aquifer characteristics in the Nairobi area
Elevation
BOREHOLE
ID
(m)
OWNER
TOTAL
DEPTH
WSL
(m
(m
WRL(m bgl)
YIELD
SPECIFIC
3
bgl)
capacity(m3/d
(m /hr)
bgl)
KEN-CHRISTIAN
1670
233
ay)
146,
178,
114
5.04
11.4
C11086
HOMES
C 10734
CBK
1620
220
54, 90, 146
6.2
8.1
30.384
FARMERS
1570
139
46, 130
30.5
30
64.296
1685
204
40, 144
110
18
9.048
1650
150
35, 82
84
7.68
3.888
C 10857
204
CHOICE LTD
TWIGA
C 10301
CHEMICALS LTD
BOELCKE
C 10060
CHRISTIAN
C9765
NAIROBI HILTON
1660
200
132, 168
122
9
17.28
MUSLIM
1720
235
208, 232
143
6
4.296
WORLD
C 14054
LEAGUE
C 10080
JANI P.K
1680
162
25, 101, 131
131
5.82
8.342
C 10072
UTALII COLLEGE
1600
218
100, 158
96.4
11.6
2.928
1820
177
109,
86.5
0.84
2.3712
77.5
7.92
10.272
85.6
13.8
24.353
C 9062
DOD BOSCO
C 9079
MUTEGI JIM
1592
162
128
GLEENSONS
1620
190
104,
C 9002
INTER. LTD
120,150
134,
152
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SABATIA
C 10919
262
244, 254
127
5.46
1.555
1720
252
135,
114
4.2
2.755
INVESTMENT
CONSOLATA
C 10053
1790
SISTERS
165,
228
KENYA RE- INSR.
1660
200
32, 167
123
12
21.12
C 9771
PLAZA
C 14221
SHAH SURESH
1730
250
13, 82
60
14.4
5.616
C 9764
POLISH EMBASSY
1760
181
131, 179
86.4
0.6
0.271
EMBASSY
OF
1723
250
12, 228
28
5.6
24.437
LITTLE SIS OF ST.
1600
120
76
63
6.6
11.82
C 10431
JAPAN
C 9733
FRANCIS
C 10077
BARCLAYS BANK
1615
200
15, 154
86.5
6.06
8.218
C10076
SHRETTA Y.A
1650
180
108, 156
109
8.4
11.390
GETRUDES
1640
181
125, 175
112
5.87
8.004
1820
200
102
97
5.28
14.735
C 9006
GARDEN C SHOP
PEREZO,
F.R,
C 10694
V.C.D.
C 10696
UNICEF
1790
315
90, 94, 108
113
6
1.706
MATER MISERICO
1662
170
4, 164
87
5.64
4.8
C 10122
HOSP
From the data of the existing boreholes above, the boreholes exploit aquifers between 4-46m
depth, 54-120m, 128-178m, and above 200m respectively. When correlated with the geologic
logs of the boreholes, the aquifers located between 54-76m and 179m depth correspond to
fractured aquifers located within fractured and weathered volcanic rocks while those struck at
128-178m deep occurs in coarse sediments and red gravel which forms the main aquifer as
shown in the schematic figure 3.1 below.
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4-46m:Weathered hard tuff
54-120m:Fractured weathered loose rock
128-178m: Sediments and red gravel
179m:Fractured volcanics
Figure 3. 1.Illustration of various formations bearing aquifer layers with depth (not to scale)
The fractured volcanics are noted to form good aquifers from the data above which was also
noted by Saggerson 1991 as notable zone of groundwater supply in the study area. Fractures in
this rock formations increase the flow and transmissivity of groundwater (Sander 1997) which
increases with increased degree of fracturing.
In borehole C 9079, C 10734, C 10857and C 10301, water is struck in coarse sediments and
gravel which form the main aquifer in the study area. These formations are probably sedimentary
and volcanic in origin and were deposited during the intervening non- eruptive phases. They
characterize periods of erosion between volcanic eruptions and subsequent lava flows and are
potential aquifers (Nyambok 2001). They mostly comprise soils, weathered rocks and water-lain
erosion material of volcanic origin. Their thickness is variable but the water producing zones,
which are usually sandy or gravelly, rarely exceed a few meters (Gavaerts 1970).
For the analysis of the groundwater level, a hydrogeological cross section was drawn from
representative boreholes along the cross section (fig 3.2). The water rest level was reduced to
datum level to form a piezometric level. The cross section (fig 3.3) basically helps to show the
direction of groundwater flow in the area and also presence of hydraulic gradients that deviates
the groundwater flow resulting to varied borehole yields.
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Ololua Trachytes-Tvt3
Nairobi Trachytes-Tvt2
Kiambu Trachytes-Tvt1
Middle upper Kerichwa valley tuffs- Tvtf2
Lower Kerichwa valley tuffs- Tvtf1
Nairobi phonolites-Tvp3
Kandizi phonolites-Tvp2
MbagathiPhonoliticTrachytes-Tvtp
Athi Tuffs and Lake Beds with chert band-T
Kapiti phonolites-Tvp1
Figure 3. 2.Map showing location of the boreholes in Nairobi area. Scale 1:125000. (Saggerson
1991)
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Figure 3. 3.Hydrogeological cross- section covering the selected boreholes along cross sectional
line A-B (Scale: X-axis; 1cm Rep 0.02m, Y-axis; 1cm Rep 50m).
The cross section shows that groundwater flows from elevated areas to the low lying discharge
areas in a SW-NE and NE-SE directions. Faulting is an outstanding phenomenon in the area, as a
result, flow occurs both laterally. However the piezometric level is not uniform hence means that
most of the boreholes don’t share the same aquifer.
A characteristic feature of aquifers for boreholes located along or close to faults which deviate
the lateral flow of groundwater is that all of them have yields in excess of 9m3/hr (C 10072, C
9002,C 10734) (table 3.1)which can be considered to be a reasonable good yield. The fault
therefore acts as aquifers and also as conduits through which groundwater flows. Fig 3.4 below
shows the general borehole locations in the Nairobi area.
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Ololua Trachytes-Tvt3
Nairobi Trachytes-Tvt2
Kiambu Trachytes-Tvt1
Middle upper Kerichwa valley tuffs- Tvtf2
Lower Kerichwa valley tuffs- Tvtf1
Nairobi phonolites-Tvp3
Kandizi phonolites-Tvp2
Mbagathi PhonoliticTrachytes-Tvtp
Athi Tuffs and Lake Beds with chert band-T
Kapiti phonolites-Tvp1
Figure 3. 4.Geological map of Nairobi area showing the borehole locations in the study area.
Scale 1:125,000 (Saggerson 1991 ).
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The regions with yields >10m3/hr (C 10857, C 14221) are those which are immediately recharged
by faults and rain water infiltration into the ground i.e they are on the recharge region. For
boreholes located along or close to faults, their mean aquifer yield is >7m3/hr. The boreholes are
quite deep with an average total depth of 200 m blg. However their respective aquifers were
struck at relatively shallow depths. Faults therefore in the area therefore act as excellent aquifers.
Boreholes C 10696 and C10919 are quite deep but show very little yield, this can be due to their
location in the discharge zone and also as noted by Mulwa 2001 may be due to a large number
of boreholes in Karen area which might have resulted into overexploitation of groundwater
hence low yields.
3.2 Aquifer discrimination and productivity
From table 3.1, most of the aquifers in the area are considered to be confined. This is because
geologically an aquifer qualifies to be confined when the boreholes water rest level (WRL) lies
above the water strike level (WSL) which is true in most of the boreholes in this study area. The
boreholes have tested yields varying between 0.6-30m3/hr. The wide range in yield is accounted
for by differences in geological conditions i.e. both lithological and structural factors. It should
be noted that the tested yield of a borehole is in most cases the maximum discharge sustained
during a 24 hour constant discharge test.
However the boreholes indicate that various water levels were struck within a depth range of 4254 m bgl. Generally, the first (perched) aquifer is locally found approximately between 20-100
m blg in Nairobi area and its environs. This water- bearing zone occurs within the Kerichwa
valley tuffs and in an Old land Surface between the tuffs and the underlying lava flows.
The first aquifer (perched aquifer) is encountered between depths of approximately 4-54 m, the
second between 76-120 m bgl while the third is between 120 and 250 m bgl.
A comparison of the pump tested yields in the sediments and gravel aquifers (128-158m) with
the aquifer in fractured rocks (54-76m and 179m) can be drawn. The aquifers in the sediments
show high yields > 9m3/hr whereas those in the fractured rocks show <9m3/hr. This can be
attributed to unconsolidated nature of the gravel which has high tramsmissivity of water hence
facilitating movement of groundwater.
Yields are noted to increase with depth in the 120-250m aquifer, which is mainly constituted by
the Nairobi trachytes and Kandizi phonolites. This is strongly attributed to the increased degree
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of weathering and fracturing (Toll et al 2004). Although water is likely struck at shallower
levels, it is the deepest of the water bearing zones that should be targeted.
However, the specific capacities of the boreholes in the area is heterogeneous, with those in the
recharge region showing high capacities (C 10857).Boreholes C10734, C9002 shows specific
capacities of >20m3/day hence forms the high potential areas for borehole citing in the study
area.
3.3 Abstraction and groundwater distribution
According to Stephen and Albert 2005, groundwater abstraction in Nairobi county has been
increasing throughout the years. For instance in 1980 the total abstraction from 1400 boreholes
was put at 12 Mm3/a, this was based on the initial pumping rate during the first test reduced by
an annual utilization rate (‘UAR factor’) which takes account of (a) lower unit pumping rates
compared to the initial test, (b) different pumping schedules typical of the given use and (c)
periods out of production. By 2000 the total abstraction is estimated (on the basis of an inventory
of 175 ‘representative water wells’) to have increased to 32 Mm3/a from 2000 boreholes. In a
recent World Bank report, a ‘UAR factor’ of 0.2 is used to allow for more intensive pumping
during a period of increased water shortage, and applying this to the 2250 operating boreholes a
total abstraction of 31 Mm3/a (85 Ml/d) for the 2002 abstraction. This has hugely contributed to
water level fluctuations evident by monitored borehole in Langata area (fig 3.6), though its
influence in the yields of the boreholes was not stated.
Figure 3. 5.Groundwater level hydrograph for water well in Nairobi area (Stern and Albert
2005)
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Comparison of data from MWI (Kenya’s Ministry of Water and Irrigation) for new water wells
of different construction date during the period 1950-98 also indicates a substantial lowering of
the groundwater level in the upper aquifer units (above 100 m bgl),but no significant change in
the deeper aquifer units, but iso-piezometric maps from 1974 and 1997 suggest little change in
groundwater levels on a regional scale (Stephen and Albert 2005).It is hence clear that
abstraction rates in Nairobi area highly controlled groundwater distributions in Nairobi county.
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CHAPTER FOUR
4.0CONCLUSION AND RECCOMMENDATIONS
4.1 CONCLUSION
On the basis of the investigative data obtained by examining the borehole logs in the borehole
completion reports of various boreholes, the groundwater distribution in Nairobi area is highly
and strongly influenced or controlled by both lithology and geological structures. Lithological
variations coupled with varied degree of weathering and fracturing attributes a high degree of
inhomogeinity in hydrogeological characteristics of different aquifers.
Lithologically, groundwater distribution in Nairobi area is clearly illustrated by formation of
individual aquifers within unconsolidated sand sediments. These lithologies are noted to form
excellent aquifers since they have high degree of transmissivity. Old Land Surface layers and
bedding planes are also noted to positively influence groundwater availability in the area.
Structurally, groundwater distribution in Nairobi area depends primarily on structures. Since
Nairobi area is composed of volcanic rocks, they are mostly considered as impervious in nature.
Through the tectonic activities during the formation of the Rift Valley, faults and joints were
generated in these rocks which greatly controlled the distribution of groundwater. As noted in the
results, high yields say >5m3/hr is achieved from boreholes located in highly fractured and
fissured zones, this is because fractured rock formations are noted to be excellent aquifers.
Fractures increase the permeability of crystalline formations, hence increasing their permeability
and are thus good aquifers. Weathered layers of rocks are also potential aquifers. In addition,
many of the contacts between the different flows are water bearing. This means that the yield of
a borehole increases with depth. Generally, the aquifers are confined as noted from the water rest
levels.
Abstraction rates in Nairobi area have been increasing throughout the years mainly due to
increased number of boreholes and wells drilled in this area. This has also contributed to some
extend in groundwater distribution through fluctuations of water levels noted in the boreholes
within the study area.
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4.2 RECCOMENDATION
The high yielding/excellent aquifers are located in highly fractured volcanics and Old Land
Surfaces. Thus it is recommended that such lithologies be concentrated especially during
borehole site investigation. It is advisable to penetrate the Kandizi phonolites at depth to ensure
that the proposed borehole will provide sustainable water supply without interfering with the
nearby boreholes.
From the geological map showing the boreholes locations in the study area, it can be deduced
that the groundwater around Westlands and parklands areas are likely to be subjected to
overexploitation due to intense boreholes in the area. To avoid this, it is hence recommended that
safe yields be determined by test pumping the boreholes for a satisfactory period of time and
monitoring the water levels of the neighboring boreholes.
Not all of the groundwater in the area comes from the recharge source in the hills but rather from
infiltrations of surface runoffs during rainy season, to date exact amounts that infiltrates and
reaches the groundwater/water table is not clearly known. Hence it is recommended that such
important prerequisite as recharge rates should be determined since it helps in quantifying the
amount of water that infiltrates and form the groundwater enabling a proper use of groundwater
without overexploiting.
Having worked with the maximum abstraction rate of upto 85 Ml/d. The aquifers have shown
evidence of overexploitation through the fluctuation of water levels and relatively low yields in
quite deep boreholes in the area. It is hence recommended that a more detailed hydrogeological
study be conducted to define the long-term sustainable abstraction rate and measures needed to
maintain the related recharge rates.
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CHAPTER FIVE
5.0 FERERENCES
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http://www.kenya-advisor.com/Kenya-map.html. Accessed on 10/3/11 at 10:50 am.
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Lapcevic, P.A., T.M. and Novakowski, K.S., 1993. “The interpretation of pumping tests
conducted in vertically fractured rock using models developed for porous media”. In. proc
NGWA Focus East Conference, Sept 27-29, pp 839-849.
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Mulwa, J.K., 2001. Geological and structural set-up of Kiserian-Matathia area and its influence
on groundwater distribution and flow. Msc. Thesis (unpublished). University of Nairobi.
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Saggerson, E.P., 1991. Geological map of Nairobi Area (Directorate of Overseas Survey Sheet
no. 148): scale 1:125,000. Ministry of Natural Resources, Mines and Geological department,
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probabilistic approach. Hydrogeology journal, 5 (3), Pp. 32-43.
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of Greater Nairobi.GW,MATE Case profile collection. The World Bank, Washington D.C.,
U.S.A. No.13, 6 pp.
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Toll, M., Dietrich, P. and Sauter, M., 2004. Intergrated approach to the investigating of
shallow unconsolidated aquifers in lower Jordan Valley (Jordan), 5th International Symposium
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4, pp 225-228.
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ACKNOWLEDGEMENTS
I owe sincere appreciation to the course tutors Dr. C.M., Gichaba and Dr.D. Olago for their
guidance and support throughout the project preparation. I also thank both the teaching and
support staff of the Department of geology for their unfailing support throughout my period of
study.
Sincere gratitude’s is greatly due to the Ministry of Water and Irrigation for provision of
borehole records which helped me compile my project report. I also wish to thank my colleague
for their encouragement and discussions.
However I’m greatly indebted to my family for their support and prayers and ever ensured that I
had a good learning environment.
Lastly and the greatest of all, I thank my Almighty Lord God for the life and good health that I
have enjoyed throughout the study period and in my whole life.
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