Saline groundwater in the Buffels River catchment, Namaqualand, South Africa: A new look at an old problem

Highlights: • Evaporation does not appear to be a significant contributor to groundwater salinisation. • Salts are derived from marine aerosols, ion-exchange reactions and weathering processes. • Salts are concentrated in heuweltjies, palaeo-biophysical features, common in Namaqualand. • Heuweltjies may thus be an important unrecognized source of groundwater salts. • Flushing of heuweltjie salts likely controlled by incidence of high volume precipitation events. Namaqualand, South Africa, is a global biodiversity hotspot but local populations are affected by challenging economic conditions largely because of poor access to water. In this study groundwater types are characterised and sources of salts and salinisation processes are identified using hydrochemistry and δ¹⁸O, δ²H and ⁸⁷Sr/⁸⁶Sr data. Analysis of δ¹⁸O and δ²H data suggests that evaporation does not play a major role in salinisation of the groundwater. However, major ion chemistry and ⁸⁷Sr/⁸⁶Sr ratios indicate that salts present in the groundwater are linked to dry deposition of marine aerosols and ion-exchange reactions in soils in the alluvial aquifer systems. The hydrochemical variability of the groundwater in the basement aquifer system suggests that there are strong local controls linked to weathering processes in individual basement rock types. The region is also notable for the high density of heuweltjies, biophysical features associated with increased nutrient levels, associated with termite activity. Electromagnetic scanning as well as measurement of water-soluble soil electrical conductivity values on and off heuweltjies, show that heuweltjies are saline with salinity increasing with depth. The level of groundwater salinity correlates with the level of heuweltjie salinity. Precipitation records from the last 150 years provide support for the hypothesis that accumulated salts, and in particular, heuweltjie salts are flushed into the groundwater system during sporadic large volume precipitation events. Thus, heuweltjies and hence termite activity, could potentially represent a previously unrecognized contributor to groundwater salinisation across Namaqualand and in other parts of the world.