Hydrogeochemical evolution and contamination of groundwater in the Albertine Graben, Uganda

Abstract

Hydrochemistry of the rift margin environments in the Albertine Graben (Uganda) was used to understand the influence of both geology (Albertine Rift, Karuma, and Bunyoro Groups) and potential chemical contaminants derived from petroleum development on groundwater resources. Petroleum development is taking place in a highly populated region where groundwater is the main source of domestic water supply whose origin of potential contaminants is uncertain. Surface and ground water major chemistry and heavy metals for wet (n = 62) and dry (n = 63) seasons, and archived/historical data (n = 124) and stable (δ¹⁸O and δ²H) isotope data (n = 41) and archived/historical (n = 384) were analysed. WHO (2012) guideline values exceedance included physico-chemical (pH, EC, TDS), major chemical parameters (Ca²⁺, Na⁺, K⁺, and SO ²⁻₄ ) and heavy metals (Pb, Fe, As, Mn, and Hg), ranging from 2 to 100%, during both wet and dry seasons. Key processes influencing groundwater chemistry are weathering of felsic granites and reverse cation exchange in both wet and dry seasons. Groundwater is predominantly earth alkaline in both Bunyoro and Karuma Groups and alkalis in the Albertine Rift, with bicarbonate as the major anion. Multivariate statistical analyses attribute the presence of heavy metals (Fe, Hg, Cd, Cu, Zn, and Mn) to anthropogenic sources associated with petroleum development. Geogenic sources are suggested for all the major cations and anions and some heavy metals (Pb, As, Cr, and Co). Environmental isotope data suggested recharge by direct precipitation to the Albertine Rift, Karuma, and Bunyoro Groups as well as through the rift margin environment.