Assessment of the Potential for Managed Groundwater Recharge as a Mitigation to Saline Water Intrusion - Case Study of Coastal Aquifers, Kwale County, Kenya

Abstract

Aquifers on the south coast of Kenya have been identified as a potential seawater intrusion risk due to the growing water demand. The research aims to investigate the extent of seawater intrusion (SWI) in the coastal aquifers of Kwale and to determine possible managed recharge zones. To determine the extent of SWI, borehole data was spatially analysed using ArcGIS 10.4 software by IDW. Satellite images of the study area acquired from Sentinel 1 and 2 missions were analysed using ArcGIS, google earth engine and QGIS Orfeo Toolbox to determine possible managed recharge zones. Vertical Electrical Sounding (VES) data were analysed and interpreted using IPI2win software and MS Excel to determine the position of the transition zone between saltwater and freshwater. Precipitation, temperature and relative humidity data were analysed using the R-statistical tool to determine their trends in the past decade (2011-2020), with precipitation being the dependent variable. Also, the data was used to justify the use of stormwater for managed groundwater recharge, and the effects of climate change as a growing threat of SWI on the Kenyan coast. The findings from geophysical data analysis and the borehole data confirm that the area suffers from SWI. The transition zone between fresh and saline water is 37-60mbgl at about 4.4kms from the shoreline and 70-200mbgl at about 7.5km from the shore. The borehole data indicated that boreholes within areas bordering the coastline have higher EC values than those inland. Lineament occurrence, geology, hydrogeological characteristics and land use of areas within Shimba, Mwabila and Matuga have favourable characteristics for managed groundwater recharge, hence the potential recharge zones. Regression analysis of climatic factors obtained a good overall fit model output. Minimum temperature and humidity positively correlated with coefficient values (64.21, p = 0.00) and (7.86, p = 0.04), respectively. Minimum temperature revealed a negative correlation with coefficient values of (-43.73, p = 0.01). Conversely, the coefficient estimate for evaporation is close to zero (-0.05, p = 0.93), indicating a weak relationship between evaporation and precipitation. From this analysis, one can recommend stormwater to be used for managed recharge due to favourable weather conditions. If practically done, a managed recharge can be a better solution for SWI on the Kenyan coast.