Recharge mechanisms and geochemical processes in a semi-arid sedimentary basin, Eastern Cape, South Africa

Abstract

Stable isotope ( 18O and 2H) and geochemical signatures are used to gain some insight into recharge processes and groundwater salinization mechanisms in a semi-arid rangeland catchment. The 665 km 2 catchment is underlain by fractured lower Beaufort sandstones and mudstones and is intruded by several dolerite dykes. Chloride and isotopic relationships suggest that chloride ions in ground water are of meteoric origin. The hypothesized recharge and salinization mechanism is that large storm events periodically dissolve accumulated surficial meteoric salts and, after a period of evaporative enrichment at or near the soil surface, leach them into the ground water. Geochemical variations imply that spatial differences exist in recharge volumes, in evaporative enrichment or in the extent of leaching of surficial salts. Meteoric sodium chloride contributes greater than 90% of the dissolved sodium, except at low salinities, where cation exchange processes yield additional sodium inputs in exchange for dissolved calcium and magnesium. At high salinities this process reverses and sodium is adsorbed, releasing calcium and magnesium. This process accounts for up to 40% of the dissolved calcium and magnesium. Contributions from geochemical weathering are relatively uniform in spite of highly variable salinities, which implies that residence times are also relatively uniform.