Groundwater recharge processes in the Nasia sub-catchment of the White Volta Basin: Analysis of porewater characteristics in the unsaturated zone

Abstract

Vertical infiltration of precipitation has been examined in this study for the purpose of evaluating groundwater recharge processes in parts of the Nasia sub-catchment of the White Volta Basin. As recharge is an essential component in the detailed assessment of groundwater resources potential in a basin, evaluating its processes is vital in determining the spatial and temporal variability of the resource. Stable isotope data of precipitation, groundwater, surface water and porewater in the area suggest that the local precipitation is largely enriched compared to global meteoric water. This is consistent with the prevailing local conditions in the region and ties in with observations in other parts of the sub-region. The groundwater and porewater data indicate that prior to, and in the process of infiltration and final percolation into the saturated zone, rainwater undergoes evaporative enrichment such that the finally recharged water plots along an evaporation line with a much shallower gradient and intercept compared to the global meteoric water line and the local meteoric water line. The isotope data further suggest that through the shallow unsaturated zone, a significant fraction of the initial precipitation would have been evaporated by a depth of 3.0m. Evaporation rates in the range of 38–49% have been estimated for the depth range of 0–3.0m based on the porewater stable isotope data. Details of the procedures and implications of high evaporation rates within such shallower depths are presented and discussed. Groundwater recharge rates estimated from the chloride mass balance technique report values in the range of 73.26mm/yr (390Mm3/yr)–109.89mm/yr (585.27Mm3/yr), with an average of 94mm/yr (500.6Mm3/yr). These translate into 6.6–10.9% of annual precipitation. Based on the current population trends and per capita water demand of 50L per capita per day, this study finds that the estimated recharge rates exceed the demand 59 times. This suggests significant promise for developing groundwater resources for climate-proof livelihood support projects in the terrain. However, much more detailed hydrogeological assessments are required to constrain the effects of lateral and vertical lithological variations on the availability of the recharged groundwater for immediate abstraction and use. The study also observes that since evaporation is one of the main dynamic processes influencing the fraction of the initial precipitation that finally reaches the saturated zone, projected increases in temperature during the next decade may increase evaporation rates of infiltrating rainwater leading to reduced groundwater rates.