GROUNDWATER POTENTIAL OF THE THIKA AREA, KENYA SGL 413:PROJECT IN GEOLOGY Presented by: Muraguri Peter Njuguna Reg No. L13/21429/2008
Contents Study area Objectives Geology of the study area Hydrogeology
Objectives To investigate the geology, hydrogeology and the aquifer characteristics, groundwater flow and aspects of quality in Thika. To investigate sustainability of the aquifer with respect to abstraction, pollution and also consider possible influence to quantity and quality of the ground water To identify suitable zones for drilling boreholes for water supply, the depths at which water will be struck, the expected yields and quality. To make identify good ground water management practices in the area.
Map of Kenya showing the study area Key Thika
STUDY AREA Thika town and its environ bounded by coordinates 37 0 00 00 to 37 0 06 50 E and 1 0 0 00 to 1 0 05 45 S (Survey of Kenya topographic sheet, No. 149/1, Thika) Covers an area of about 110 km 2 The elevation of Thika is approximately 1540m above sea level.
Topographical Map of study area indicating some boreholes C 2174 C 1326 C 2098 C 4019 C 11659 C 2233 C 13217 C 2747 C 2754 C 11190 C 4690 C 13217 C 12148 C 4910 C 18154 C 11432 C 11077 C 12310
Introduction..contd In Thika area, the main groundwater resources occur as aquifers found at the contact zones between the Tertiary volcanic sediments and the Basement rocks or fracture zones. The groundwater potential and chemical quality may vary but usually range between medium to average. The recharge rate also varies across the area, but is unlikely to be a limiting factor in a decision as to whether to utilize groundwater.
Geology of the study area
Geology of the study area The area can be described by the combination of soils covered by Tertiary volcanics, (represented by the upper Athi series composing of the sediments, tuffs and basalts) underlain by the argillaceous lower Athi series,the Kapiti phonolites and then the metamorphic rocks.the volcanic rock thickness increases northwest wards and thins out eastwards. Stratigraphy of Thika Upper Athi Series Middle Athi Series Lower Athi Series Kapiti phonolite series Basement rocks The basement rocks consist mainly of gneisses and schists
Hydrogeology Ground water systems can be defined as rock framework components of the earth s crust that contains saturated ground water. Saturated groundwater systems usually stretch out below the ground water table and can be either be near the earth s surface or lower Surface water systems are the surface water runoff flows to a system of lakes, streams and rivers and these are mainly controlled by geology and slope of an area.
Discussion 1 Characteristics of aquifer and ground water flow From the study, the WRL and WSL indicated that the area has some confined aquifers. WRL (static water level) are at higher elevations compared to WSL thus the aquifers contain water at higher pressures. Power required to pump water from a well is dependent on WRL. Thus confined aquifers require less power to pump water. Most of the boreholes drilled in the area have depths of more than 100m.
Hydro-geological Profile Hydrogeological Profile between BH 2098-12310 Horizontal Distance (M) 1600 Borehole No. Distance (m) Elevation above sea level (M) 1550 1500 1450 1400 1350 1300 1250 Ground Surface WSL 1 WSL 2 WRL WSL 3 2098 2233 4000 2033 13272 3000 13272 11659 1000 1200 2098 4019 2742 12148 12310 Borehole Numbers
Hydrogeological Profile between BH 2098-11659 Horizontal Distance (m) 1600 Borehole No. Distance (m) Elevation above sea level (m) 1550 1500 1450 1400 1350 1300 1250 Ground Surface WSL 1 WSL 2 WSL 3 WRL 2098-4019 1000 4049-2742 1800 2742 12148 6000 12148 12310 2500 1200 2098 2233 13272 11659 Borehole Numbers
Transmissivity Transmissivity Quantities 400 350 Transmissivity (M sq/day) 300 250 200 150 100 Transmissivity Quantities 50 0 Transmissivity Quantities 4690 11432 3986 2743 11190 13468 0.5339 0.976 18.154 14.41 268.4 365.82 Borehole No.
2. Sustainability of aquifer in relation to abstraction, pollution and influence to quantitiy and quality of the grounwater Since there is a limit to which water can sustainable be abstracted from an aquifer, there is need to determine the optimum discharge that each aquifer in the area can give. Over abstraction from a well may affect other wells within the area. The environment in which the aquifer is found may affect the aquifer especially if there are uncontrolled human activities that may contribute to pollution of the ground water. Areas that are prone to pollution may contribute to groundwater pollution.
3. Zoning of ground water potential This can be achieved by developing a ground water potential map by delineating the into zones in terms of the transmissivity calculated from the observed boreholes To determine the exact condition of a an area, geophysical investigations are necessary e.g Electrical Resistivity Method. Recharge in the area most likely takes place at the upper areas in the west or by leakage from other aquifers since the confining aquifers are mostly impermeable
Conclusion There is a potential for ground water in the area occurring mostly in contact zones and weathered rocks which define the aquifers. The aquifers found in the area are confined since the water levels rose after the aquifers discharged into the wells. It can also be concluded that the area under study has variations in the water potential as can be seen from the results of the transmissivity. Some of the wells show relatively high potential while others are moderate and others show low potential of the surrounding aquifer.
Recommendations There is need to establish the optimum ground water output in the area so that over abstraction can be avoided so that some areas may not be deprived of the water Establish the viability of artificial recharge to the aquifers in order to have reliable information if that possibility becomes an option in the future 1. There is need to determine the extent of threat to ground water pollution so that mitigation measures can be put in place to deal with the situation
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