Groundwater model calibration of Ngong area in the Upper- Athi basin using optimal Parameter estimation

Abstract

Ngong area is a high rainfall zone lying on the eastern arm of the Gregory Rift and underlain by volcanic lava flows that erupted from Ngong and 01 Doinyo volcanoes. It is an important recharge area for the Upper Athi Basin Aquifer. This is because of the high incidence of fractures that strike in a north - south direction. Ngong area forms one of the fastest growing areas with groundwater forming a major component for potable water. To sustainably manage the resource, it is necessary to simulate a flow model in order to make the right decisions. A ground water flow model was calibrated for the area using the inverse method of parameter estimation. Borehole lithological logs, geological reports, topographical maps, hydrogeological and hydrological information were used to develop the conceptual model of the area. The main aquifers consist of the weathered and fractured basalts and trachytes below which lie two layers of the Upper Athi Series aquifer. A semi-confining unit consisting of the Mbagathi trachyte separates the two layers of Upper Athi Series. ,.: Fixing those parameters that were mostly insensitive to th"e observations reduced high parameter correlation in the estimation process. The estimates of these parameters were obtained in the early stages of parameterization. These included all the vertical hydraulic conductivities. Results indicate the lower aquifer to have -a higher horizontal hydraulic conductivity (8.8E-06 mlhr) than the two upper aquifers. Vertical leakance from the first layer to the second layer was high at 2.8E-04 m/hr probably due to the high occurrence of fractures in the upper layer but low from the second layer to the third. The high leakance in the first layer is explained by the high incidence of fractures in the layer while the low leakance in the second layer is due to the semi-confining unit of Mbagathi Trachyte. The model indicates a net aquifer recharge of 4.5E-06 mlhr (39.7 mm annually) in this main recharge zone of the upper Athi basin. The flow direction was found to be from the western to easterly direction