Potential Hydrogeological Contaminants derived from petroleum activities in the Albertine Graben

Abstract

Current understanding of the hydrogeological characteristics of rift-margin environments is limited and not well reconciled to large- scale conceptual models of rift evolution. In Uganda with substantial dependence on groundwater for rural water supplies, it is unclear how global change including localised development of petroleum impacts the quality and quantity of available groundwater resources. Here, synthesis of hydrogeological and hydrogeochemical properties of the rift margin environments of the Albertine Graben, Uganda is used to understand the influence of geology and petroleum development on groundwater quality. Datasets include surface resistivity surveys and hydraulic tests, supported by evidence from lithology, structures, drainage, and water chemistry from both the current study and archived/historical sources. There are two aquifer environments, (1) transmissive (mean T of 45 m2.d-1), unconsolidated sediments within the trough below the escarpment and, (2) deeply weathered Precambrian crystalline basement rocks of lower transmissivity (mean T of 6 m2.d-1 - 28 m2.d-1) within the Lake Albert basin above the escarpment. WHO (2012) guideline values exceedance included physico-chemical (pH, EC, TDS), and major chemical parameters (Ca, Na, K, and SO4) 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 being 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.