Assessment of Groundwater Vulnerability to Pollution Using a Modified DRASTIC Model in the Stony Athi Sub-catchment, Kajiado County, Kenya

Abstract

Groundwater is increasingly becoming vulnerable to pollution in response to an increase in anthropogenic activities and changes in climatic trends. Anthropogenic activities cause land use and land cover changes, which may result to considerable threats to groundwater quality. Climate change has the potential to affect groundwater quality through reduced recharge, increased evapotranspiration and abstraction rates. Assessment of groundwater vulnerability to pollution is therefore necessary so as to delineate zones that are more susceptible to degradation for appropriate planning and management. The objective of this study was to assess the groundwater vulnerability to pollution in the semi-arid Stony Athi sub-catchment of Kajiado County, Kenya. DRASTIC, an overlay and index model, was used to prepare a vulnerability index map on a Geographic Information System platform. The DRASTIC model determines the vulnerability index by taking into account seven parameters that influence water transfer from the surface to the groundwater zone, namely, depth to water, net recharge, aquifer media, soil media, aquifer transmissivity, impact of vadose zone and hydraulic conductivity. The model was modified to incorporate a land use and land cover parameter. The Modified DRASTIC Vulnerability Index was calculated as the sum of the product of ratings and weights assigned to each parameter on a scale of 1 to 10 and 1 to 5, respectively. Groundwater vulnerability to pollution was categorized based on the United States Environment Protection Agency classification of low, moderate, high and very high vulnerability. Results indicated that 4% of the study area can be classified as having a high vulnerability, 9% moderate vulnerability while 87% has a low vulnerability. Groundwater nitrate concentration measured in selected boreholes within the study area indicated a positive correlation with the calculated vulnerability index.

Keywords: Groundwater pollution vulnerability; DRASTIC; Geographic Information System; Stony Athi sub-catchment

DOI: 10.7176/JEES/14-4-05

Publication date: June 30th 2024