Characterization of the Hydrogeology of the Lodwar Alluvial Aquifer System, Turkana County, Kenya

Abstract

Drylands account for more than a third of the world's land area and are characterized by less than 250 mm of rainfall per year. In these regions, groundwater is a strategic resource and plays a key role in economic development, especially in sub-Saharan Africa, where it is responsible for improving livelihoods. Lodwar town depends primarily on a poorly studied groundwater system for its municipal water supplies. The aim of this research was to establish the sustainability of this aquifer system through a comprehensive study of its hydrogeological characteristics, sensitivity to climate variability, and the influence of natural and anthropogenic processes, all of which are currently unknown yet critical for its sustainable management. The methods used were: detailed geological mapping and rock analysis (petrography and X-ray fluorescence); remote sensing (digital elevation models and vegetation cover maps) and drone mapping of Lodwar town for stream lineament analysis; evaluation of borehole drilling datasets including yields, static water levels, water rest levels, drawdowns, transmissivities, and borehole depth; geophysical surveys involving vertical electrical soundings for evaluation of the hydrogeological characteristics; aquifer hydrogeochemistry of surface (river, scoop holes, water pans) and groundwater, where field measurements included pH, Temp, and EC using hand-held Combo Tester HI98129 while, turbidity, total hardness, alkalinity, Ca2+, Mg2+, Na, K+, Fe2+, Mn2+, Cl-, F-, HCO3-, SO42- and CO32- NO3-, NO2- were measured at the Water Resources Central Laboratory based standard analytical procedures. Furthermore, stable isotopic analyses of oxygen-18, deuterium and tritium in water samples was done at Elemtex Lab, United Kingdom to establish the rainfall-surface water-groundwater interactions, groundwater age and recharge sources. The multifaceted dataset was analysed using descriptive and inferential statistics, including principal component analysis (PCA), hierarchical cluster analysis (HCA), the software PHREEQC for analysis of the water chemistry data, and all together, to develop the first conceptual aquifer model for this system. The findings of this research revealed that Lodwar and its surroundings are underpinned by three different and interconnected freshwater (<1000μS/cm) aquifers (shallow alluvial aquifer (SAA), the intermediate aquifer (IA) and the deep aquifer (DA)) that are collectively referred to as the Lodwar Alluvial Aquifer System (LAAS). The fourth, the Turkana Grit Shallow Aquifer (TGSA), is highly saline with electrical conductivity > 5000μS/cm and fluoride values between 2.20 to 18.74 mg/L. The dominant water types are: Ca-HCO3 (SAA and Turkwel river), Na-HCO3 (IA), Ca-HCO3 (Napuu Bh) and Na-HCO3 (DA) and NaCl (TGSA). The petrographical, geochemical, isotopic and inferential statistical analyses indicate that rock-water interaction, Turkwel river recharge, and oxidation reactions control the SAA chemistry, while dissolution and evaporation are key factors affecting TGSA. The dominant processes in the IA include dissolution, ion exchange, and dilution. Elevated concentrations of NO3- and SO42- in the wet as compared to the dry seasons, but still within WHO recommended limits, tritium values ranging from 1.10 to 2.24 in the SAA, IA and DA, and the isotopic values of surface water and groundwater, reflect strong links to modern rainfall and the Turkwel river, indicating that the LAAS is highly susceptibility to climate variability and pollution. The decreasing d-excess values from the SAA (2.18‰) to the intermediate aquifer (-6.81‰) and TGSA (-8.14‰) indicate that they are interlinked and isotope fractionation occurs during the lateral groundwater flow away from the Turkwel River. The study has attributed recharge of the LAAS to diffuse recharge by the Turkwel River and from the surface water of the Kawalase River during the wet season, as well as direct infiltration during rainfall events. This study provides comprehensive approaches for investigating the groundwater resources in data-scarce regions for their sustainable use and management.