Estimation of Groundwater Recharge in Gushiegu and Surrounding Areas

Abstract

Gushiegu and surrounding areas are lowland terrain underlain by the Voltaian System which consists of medium grains sandstones, micaceous sandstones, interbedded mudstones, siltstones and shales. Lack of perennial surface water has led to most of the communities in the area relying solely on groundwater as source of water for domestic and commercial use. Over abstraction of groundwater can cause significant drawdown of the hydraulic heads leading to significant ecological consequences. In other to avoid these consequences, proper management of groundwater resource in the area is encouraged. This begins with fairly accurate estimation of groundwater recharge; identification of recharge areas and the source of groundwater recharge. This study was therefore carried out to estimate the groundwater recharge and its spatial variations in the study area using Chloride Mass Balance (CMB) and Water Table Fluctuation (WTF) techniques. The CMB method suggests recharge in a range 13.9 mm/y to 218 mm/y with an average of 89 mm/yr which translates into about 1.3% to 21.8 % of the annual precipitation in the area, with an average of 8.9%. The low recharge rate may be attributed to the geological characteristic of the unsaturated zone material. Inverse Distance Weighting (IDW) was applied to predict the spatial variation of the estimated groundwater recharge. The same interpolation technique was applied to isotopic composition of the groundwater to predict the spatial variation using Surfer 9. The predicted map of the estimated groundwater recharge suggests that the southwestern and northeastern part of the study area is most recharged. The highest recharge rates are associated with areas where open wells encourage significant amount of groundwater recharge from precipitation in the area. The WTF technique applied to two monitoring wells also suggest a cumulative average groundwater recharge of 71.4 mm/yr and 81.0 mm/yr which translate into 7.14 % and 8.1 % of the annual precipitation. These figures agree with the values obtained by the CMB technique. Isotopic data of precipitation and groundwater were used to infer the origin and the possible relationship between groundwater and surface water in the partially metamorphosed sedimentary aquifer system in the Gushiegu area. Though, the data does not significantly establish potent relation between groundwater and surface water. The study suggests that groundwater in the area is of meteoric origin. However, the data suggests significant enrichment of the heavier isotopes in groundwater relative to rainwater in the region. The high level of evaporative enrichment of the groundwater was observed because the unsaturated zone in the study area has been noted for significant clay fractions which retard the vertical infiltration and percolation of precipitation. This leading to significant proportion of the annual rainfall lost through evapotranspiration and runoff. The low EC (68.4 – 1114 μS/cm) values lend credence to the suggestion that the groundwater is of meteoric origin.