Groundwater Vulnerability Assessment as A Strategy for Protecting Against Groundwater Pollution: A Case Study of Mid River Njoro Catchment, Kenya

Abstract

Groundwater pollution is becoming a major concern worldwide. The impact of polluted groundwater resources is three-fold: artificial water scarcity, human health problem and an impediment to economic development. Mid River Njoro catchment has experienced numerous human settlements and intensive agricultural activities over the years, threatening the quality of water from wells and boreholes. This study assessed the groundwater quality, the land-use types and the vulnerability of groundwater resources to surface pollution within Mid River Njoro catchment, Kenya, using a modified DRASTIC model in a GIS environment. Groundwater samples were collected from boreholes and analyzed for pH, temperature, electrical conductivity, dissolved oxygen, nitrate, ammonia, and total phosphorus to calculate the Groundwater Quality Index (GQI). The land-use map was prepared from a high-resolution Google earth satellite imagery of 2015. The vulnerability zones developed using parameters such as depth to water table, net recharge, aquifer media, topography, impact of vadose zones, hydraulic conductivity and land use types. Validation nitrate values were compared to the calculated DRASTIC index to assess the efficacy of the modified DRASTIC model. From the results, the GQI range from 68.38 to 70.92, suggesting fairly good groundwater quality. The major land-use types identified include agricultural land, built-up areas, forests and agroforestry areas. The agricultural land dominated the study area, followed by built-up areas, forests and agroforestry areas. The study area is characterized by three vulnerability zones: very low (6.1%), low (87.4%) and moderate (6.5%). The validation results obtained from Pearson’s correlation (0.49) and chi-square values (5.01), revealed a positive relationship between the modified DRASTIC index and nitrate values. The area with moderate vulnerability overlapped built-up areas and those with very low vulnerability had deeper water table depths and forest covers. The land-use types impacted negatively on the groundwater quality and vulnerability of the study area. This study reveals the importance and potential application of vulnerability assessment in protecting and controlling groundwater pollution. To mitigate further contamination in the moderate vulnerability zones, the study recommends protective and recovery measures to be put in place.