Hydrogeological And Hydrochemical Characterization of Crystalline Basement Rocks in The Upper District Region; The Talensi District

ABSTRACT

Groundwater remains the single most important source of water in Talensi district for various purposes. Notwithstanding, little research on the quality and sustainability of the resource in the district exist. Hence a comprehensive quality assessment and a quantification of groundwater resources in the shallow unconfined aquifers of the Talensi district has been conducted using conventional graphical methods, mass balance and multivariate statistical techniques coupled with steady state numerical groundwater modelling. The study sought to determine the main controls of groundwater chemistry and its suitability for various uses, as well as the impacts of population growth and climate change on groundwater resources in the district. Based on a vigorous water quality index (WQI) technique modified for the district and an interpolation technique using ordinary kriging developed from a well-fitted exponential semivariogram for the estimated WQIs, the groundwater quality has been spatially classified as generally ‘good’ to ‘excellent’ for domestic purposes. Generally, the quality of the groundwater for domestic usage deteriorates as one moves towards the north of the district, whereas waters in the east and west present the best quality. Classifications based on the United States Salinity Laboratory (USSL), Wilcox and Doneen diagrams suggests groundwater from the unconfined aquifers of the district is of excellent quality for irrigation purposes. Three main flow regimes have been identified with Q-mode cluster analysis, in which mixed cation water types have been revealed; where areas designated as recharge zones are dominated by Na+K–Mg–HCO3 fresh water types characterised by low mineralisation and pH, which evolves into Mg–Na+K–HCO3 fresh water type with corresponding increased mineralisation of the groundwater. The calibrated numerical groundwater model reveals an apparent dominant northeast–southwest flow pattern influenced mainly by the hydraulic conductivity field of the district which has been iii estimated to range between 0.001 and 58 m/day, with local flow systems which are controlled mainly by local variations in the topography of the district. The flow patterns as revealed in this study is in conformity with the spatial orientation of the regional structural grain, which suggest groundwater flow in the district is mainly controlled by discrete entities oriented in northeastsouthwest direction. Furthermore, the calibrated model and chloride mass balance technique estimated average groundwater recharge to be 19.60 mm/year and 20.29 mm/year respectively which represents 2.00% and 2.07% of the annual precipitation in the district, and has a direct positive correlation with elevation. The low recharge rate is backed by stable isotope (δ²H and δ¹⁸O) analysis which revealed that precipitation, surface water bodies and groundwater in the district undergo high evaporation rate, which is consistent with the weather conditions that characterise the district. Notwithstanding the low recharge rates, this study reveals the aquifer is partly recharged by the White Volta river and holds good groundwater fortunes and promise for potential commercial groundwater development; as it can sustain increased abstractions by more than 100% of the current rate given that the current recharge rate is maintained. However, for the aquifer to sustain commercial abstractions beyond 100% of the current rate under conditions of reduced vertical recharge by more than 40% of the current rate, deliberate efforts would have to be made to enhance artificial vertical recharge to augment the natural recharge.