PROJECT TITLE: MAPPING
POTENTIAL
GROUNDWATER AQUIFERS
IN NAIROBI COUNTY
Author: Mugo Dixon Mugai.
F19/1469/2010.
Supervisor: Dr. Ing. F. N
Karanja.
Outline
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Introduction
Problem Statement
Objectives
Methodology
Results and Discussions
Conclusions
Recommendations
INTRODUCTION
An aquifer is an underground layer of water-bearing
rock. Water-bearing rocks are permeable, meaning
that they have openings that liquids and gases can
pass through
This illustration shows the two most common types of aquifers, confined
aquifers and unconfined aquifers. An unconfined aquifer can receive water
directly from the surface, while a confined aquifer is trapped between two
layers of rock.
The Role of Geoinformation in
Aquifer Siting
The main existing groundwater exploration methods is
are the use geophysical and geo-electrical techniques
which are relatively expensive and time consuming.
The use of a GIS in the modeling of potential
groundwater zones is not a new concept. There are a
number of works where groundwater potential aquifers
have been estimated using geospatial technologies such
as; in Kenya Kyalo (2013) used Remote Sensing and GIS
to identify and delineate groundwater potential zones in
Lake Chala Basin. Karanja (2011) has carried out a
project on Effective Planning and Management of water
resources through Borehole Profiling using GIS.
Problem Statement
Water resources are of critical importance to society
because these resources sustain our livelihood and
the ecosystems on which we depend.
The increasing urban population is causing strain on
the amenities and resources in most urban centers,
water being one of the most strained resources.
A research done in 2005 on Nairobi and funded by the
World Bank, shows that the surface water from the Athi
Basin which is the main source of water for the city is not
sufficient to satisfy the industrial enterprises, commercial
users as well as the domestic users in Nairobi county. As
a result groundwater beneath the county is being
pumped by private operators to supplement surface
water supply.
Thus, optimal exploitation of underground water would
greatly contribute to the water supply system of a city
like Nairobi and ease the water shortage problems in
many parts of the county.
However, underground water is a delicate and
scarce resource. Over abstraction of underground
water leads to depletion of the specific aquifer
from which the water is being drown from.
Therefore, it is highly significant to map
groundwater aquifers for the optimal groundwater
exploitation, monitoring, management and
conservation of the aquifers for sustainable
development.
OBJECTIVES
Overall Objective
The main objective of this project was to map
potential groundwater aquifers to facilitate
the exploitation, monitoring, management
and conservation of the aquifers using,
Nairobi County as a case study.
Specific Objectives
• Identifying variables for aquifer siting.
• Developing a GIS aquifer database for
Nairobi County
• Modeling potential aquifer sites.
• Application of the mapped potential
aquifer sites to support decision
making.
Methodology
Area of Study.
The area of study is
Nairobi County.
Nairobi located at 1°16’S,
36°48’E and occupies an
area of approximately
696.1 sq. km . It has a
population of about 3
million persons(2009
Census).
The maximum elevation is
1924 m and the minimum
elevation is 1452 m.
Overview of the Methodology.
ANCILLARY DATA.
Lithology data, Soil
data, Contours, land
use/Land cover data,
Landform data
GIS Processing,
Clipping, Editing
attribute tables, ,
DEM generation(GIS
database creation)
Derived Factor
Maps
Lithology map, Soils map, Land
use map, Land cover map,
Drainage density map, Slope map.
Overlaying of the thematic
layers (Weighted Overlay
analysis)
Informed Decision
Making.
Potential
Groundwater Aquifer
Map.
Data Collection.
DATA
SOURCE
CHARACTERISTICS
Soils
Kenya National Soils Format: Shape file Feature Class
FUNCTION
Create a soils thematic map
and Terrain Database
Land use/Cover
Department
of Format: Shape file Feature Class
Geology, UoN
Land
Oakar Services
Create
a
land
use/cover
thematic map
Format: Shape file Feature Class
cover(Rivers)
Evaluate the drainage density
factor map
Topo sheets
Oakar Services
Format: JFIF
Base information
Contours
Oakar Services
Format: Shape file Feature Class
Generate a drainage density
10 m interval
Boundaries
Oakar Services
Format: Shape file Feature Class
and slope maps
Clip national datasets to area
of interest
Boreholes
WaRMA
Format: Shape file Feature Class
Validate the yield in potential
groundwater aquifers zones
Lithology
Landforms
Kenya National Soils Format: Shape file Feature Class
Create a lithology thematic
and Terrain Database
map
Kenya National Soils Format: Shape file Feature Class
Create a thematic map for
and Terrain Database
landforms
Modeling Potential Sites for
Underground Water Aquifers
Weight Factors Determination
The weights have been taken considering the works
carried out by researchers such as Krishnamurthy et
al 1996, Saraf &Chowdhury, 1998.
The multi influencing factors for groundwater
potential zones namely lithology, slope, landuse/land-cover, lithology, landforms, drainage and
soils were examined and assigned an appropriate
weight and are shown in table 1.1.
This method uses the principle of interrelationship
between the various factor layers influencing
occurrence of underground water.
The effect of each influencing factor may contribute to delineate the
groundwater
potential
zones.
Moreover,
these
factors
are
interdependent. The effect of each major and minor factor is
assigned a weightage of 1.0 and 0.5, respectively. The cumulative
score of both major and minor effects are considered for calculating
the relative rates (Table1.1). This relative rate is further, used to
calculate the proposed weight factor of each influencing factor map
layer. The proposed weight factor for each influencing factor is
calculated using the formula shown in Equation 1.1.
Table 1.1
FACTOR
Major
Minor
Relative Rates Proposed Weight of
Effect(A)
Effect(B)
Soils
1
0
1
(1/12.5)×100= 8
Lithology
1+1
0.5
2.5
(2.5/12.5)×100= 20
Land
1+1
0.5+0.5
3
(3/12.5)×100= 24
Landform
1
0.5+0.5
2
(2/12.5)×100= 16
Drainage
1+1
0.5
2.5
(2.5/12.5)×100=20
Slope
1
0.5
1.5
(1.5/12.5)×100= 12
Each Factor
Use/Land
cover
TOTAL
∑12.5
∑ 100
LITHOLOGY.
LAND USE/LAND
COVER.
SLOPE
DRAINAGE.
LANDFORM.
SOIL
Minor Effect
Figure 1.2 Major-Minor Flowchart
Major Effect
Major-minor Equation
Validation of the Model Results
The potential groundwater zones generated
by the created model were verified with the
yield data of the existing boreholes in the
study area and found that it was in agreement
with the boreholes yield data.
RESULTS AND DISCUSSIONS
This section presents and discusses the results
that were acquired after the data acquisition,
manipulation, processing and analysis that
were carried out during the implementation
of this project
In reflection to the set objectives the following
results were obtained:
Results
· A geodatabase for the study area that contained
various datasets that were used for analysis
· Creation of a model to generate potential
groundwater aquifer maps
· A potential groundwater aquifer map for the study
area
Factor Maps Created:
Lithology Factor Map
Lithology Map
Drainage Density Factor Map
Drainage Density
Slope Factor Map
Slope Map
M
Landforms Factor Map
Landform Map
Land Cover/Use Factor Map
Land Cover/ Use
Map
The Potential Groundwater Aquifers Map
Potential Groundwater
Aquifers
Quantification of Area Covered by the
Modelled Aquifer Zones
The attribute query calculated the area of each potential aquifer zone and the
bar graph on figure 4.8 was generated from the results of the query
Zone Categories
Score
Category
0
Very Poor
1
Poor
2
Good
3
Moderate
4
Excellent
Discussion of the Results
This project has focused on empirical models in which
map weighting is controlled subjectively (knowledge
driven).
The weighted overlay approach which, was used in this
study, overlays several raster layers using a common
measurement scale and weights each according to its
importance.
Most of the study area is covered by the moderate zone
followed by the poor potential zone. The good zone
comes third in terms of the area covered. The excellent
zone then follows and finally, the very poor zone covers
the least area as shown by the bar graph in figure 4.7.
CONCLUSIONS
The objectives of the study were met since the thematic
maps for the various parameters influencing the
occurrence of groundwater have been developed.
The Geospatial Technology techniques used were
adequate to generate and map potential groundwater
aquifers for the study area.
The project has achieved the preparation of a database
for future updates in the study area.
The project has also succeeded in creating a model that
can generate and map potential groundwater aquifers for
any area of interest.
Recommendations
The author feels that the study was limited in
terms of the temporal accuracy and
completeness of some of the datasets and
hence, makes the following recommendations
meant at improving the accuracy of the
groundwater potential map for the study area:
Land use mapping
The land use map used in the study was rather
general given the abrupt changes in land use that are
occurring within the area of study. The land use map
was also incomplete.
Use of the SPOT satellite images is recommended
which, are more accurate compared to the land use
shape file used.
Recharge zones
A similar study may be carried out to establish
recharge zones in the area of study. This will
help in addressing the problem of lowering
water table levels
Addition of other thematic layers
• Addition of other data e.g. recharge, soil suction,
pore pressures, temperature changes, annual
rainfall as thematic GIS layers, determination of the
relevant criteria/ constraint factors, and assigning
weightage to data layers based on accepted
engineering principles to reflect their
characteristics and relative importance.
Screening and Analysis of the
Developed Map
There is also need for further screening and analysis
of the developed map by experienced hydrologists
and geologists.
The more accurate map may then be incorporated in
the planning mechanisms e.g. use by local
authorities for zoning by-laws to protect the high
potential groundwater zones and the recharge zones
if they are further generated.
Q&A