GEOGENIC METHANE AND FLUORIDE IN GROUNDWATER OF THE KAROO BASIN, SOUTH AFRICA

The Karoo Basin in South Africa hosts hydrocarbon-rich shale formations that are under consideration for future unconventional natural gas development. However, Karoo Basin is a critically water-restricted region where groundwater is the primary source of drinking and agricultural water. Prior to shale gas development, we investigated the geochemical and isotopic composition of twenty-two artesian springs and groundwater wells. Based on the geochemistry, we identified three end-members: (1) a shallow calcium-bicarbonate-chloride freshwater; (2) a deep, saline groundwater with a sodium-chloride composition, and (3) an older, deep freshwater with a sodium-bicarbonate-chloride composition. We also observed direct mixing of the deep saline water into shallow groundwater wells used for drinking and agriculture. The geochemical and isotopic signatures in the groundwater, in combination with elevated helium levels, suggest that exogenous fluids are the source of the saline groundwater in springs and shallow boreholes. Elevated methane, boron and fluoride, were found in the saline shallow wells located near dolerite intrusions. These geological features likely provide a pathway fdeep hydrocarbon-rich saline waters to migrate to the surface. This geochemical evaluation identifies natural migration of methane-rich and salt-rich waters ass a source of geogenic (eg. fluoride) contamination to shallow aquifers in the Karoo Basin.