Hydrochemistry, stable isotopes, and radon in waters of the Greater Timbavati catchment, South Africa

Abstract

A baseline study was conducted to better understand the relationship between groundwater, geology, and geography in the Greater Timbavati area, South Africa. Samples of rainwater, surface water, and groundwater were collected across nature reserves and rural communities. Basic water chemistry parameters, nitrates, and ammonium were measured in the field using an Aquaread AP-5000 probe, and water samples were collected for radon determination and stable isotope analysis. Significant differences in chemistry and isotopic signatures between surface water and groundwater were observed. Groundwater exhibited a mean total dissolved solids (TDS) of 1217 mg/l and pH of 7.5, whereas surface water had mean TDS of 332 mg/l and pH of 8.3. Groundwater δ2H and δ18O means were – 20.5 and – 3.8‰, respectively, contrasting with + 12.2 and + 2.3‰ in surface water. Hydrochemistry correlated with bedrock type and a close correlation between land use and nitrate concentrations was evident, particularly in groundwater, exceeding 500 mg/l in areas with dense human and livestock presence, suggesting urban and agricultural contamination. Radon concentrations in groundwater ranged from 188 to 51,400 Bq/m3, showing no clear correlation with underlying geology, suggesting groundwater flow paths across bedrock types. Overall, a strong distinction between groundwater and surface water characteristics suggests limited interaction, caused by deep water tables, especially at the end of the dry season when streams stop flowing, warranting further research in quantifying sustainable levels of groundwater abstraction.