Saline Groundwater Generation From Paleo-Termite Mounds In The Buffels River Valley, South Africa

Saline groundwater in semi-arid to arid areas is typically ascribed to evaporative concentration of salts on or near the surface followed by dissolution of salts during episodic rainfall events and then percolation of saline downwards. This has been previously postulated for large parts of the west coast of South Africa where groundwater electrical conductivity (EC) ranges between 804 ?S/cm and 21 300 ?S/cm. However, due to the spatial variability of groundwater salt concentrations, it is unlikely that simple evaporation is the only process leading to elevated salinity levels in this region. Palaeo-termite mounds, known as heuweltjies (meaning little hills), are common surface features along the west coast of South Africa, covering an estimated 14 to 25% of the land surface. These structures consisting of aerated and nutrient-rich sediments, containing elevated levels of micro- and macro elements, including salts, compared to the surrounding sediments. For this reason, it is postulated that heuweltjies are an important source of groundwater salts. In the Buffels River valley, exchangeable ions from sediments in the centre of the heuweltjies are up to 20 times higher than in the inter-heuweltjie material and are consistent with elevated heuweltjie salts determined via in-situ EM38 scanning. 36Cl/Cl ratios are highly variable ranging between 25.94 x10-15 and 156.19x10-15, indicating that recharge occurs inland. Groundwater 87Sr/86Sr ratios in the same area are extremely elevated, up to 0.78240, suggesting a direct link to the underlying radiogenic granitic gneisses but decrease to the west suggesting interaction or mixing between different water sources. We propose that in this case, the two water sources are older groundwaters mixing with direct recharge that contains elevated salts but lower 36Cl/Cl and 87Sr/86Sr ratios derived from preferential flow paths through the heuweltjies.