The impact of abstraction on the quality and quantity of groundwater in Kampala: A case of Kawempe Division

Abstract

Studying the impact of abstraction of groundwater on its quantity and quality in the deep aquifers will contribute to the mitigation of pollution in water resources in urban areas. Assessment of groundwater quality and quantity data from NGWDB in the period of 2007-2011 was made based on borehole DWD23386. For a period of 2007-2011, historical data on water quality of the borehole was assessed for changes with respect to abstraction. More historical data for boreholes on water quality from UNBS was also assessed. In April 2011, water quality tests on groundwater samples of three boreholes, one spring and one hand dug well which is near borehole DWD23386 were done. The impact of abstraction on the quantity of groundwater was simulated in a model. Using Modflow software, a model to predict the impact of abstraction on groundwater quantity was developed and used to simulate future abstraction scenarios.The recharge model of 5.1% was calibrated with an NSE 0.7 indicating the model’s suitability for use. The model was validated and overestimated the hydraulic head by an error of 7.4%. The value of 26mm/year (50% of 52mm/year) is comparable with the recharge models of Albert (25.6mm/year,Victoria 27.5mm/year and Kyoga 36.5mm/year); for sustainable abstraction.The minimum and maximum hydraulic head drop resulting from abstraction was 1.5% and 5.8% respectively in the high and low recharge period of 2010-2011. The results show that groundwater quantity from rain fed recharge has not been significantly reduced by abstraction at borehole DWD23386, but likely to reduce with long periods of droughts. The sustainable abstraction rate which does not stress the aquifer for the recharge rate (5.1%) in this model is 158 m3 /day.Analysis of water quality data shows that in the low recharge month of April 2011, the nitrate concetration of 36.1mg/l in the boreholes data from UNBS is below the maximum accepted value for nitrates but very high for a deep borehole. In June 2011( a month of high recharge) the nitrate concetration of 63.2mg/l in the borehole data from UNBS is above the maximum accepted value for nitrates. The high concetration of nitrates (36.1-63.2mg/l) in the groundwater abstracted from the deep boreholes of UNBS shows that groundwater in the deep aquifer is deteriorating with time. The presence of high nitrate concentration in the deep aquifer of the borehole highlights the vulnerability of the deep aquifer to contamination of which stressful abstraction over time might be one of the causes. This study informs the wider community about the susceptibility of the deep aquifer to pollution. Key words: Deep aquifer, groundwater abstraction, stressful abstraction, dry season, pollution.