Downward migration of salts in termite-affected soils: Implications for groundwater salinization
Geoderma
Volume 413, 115747
Abstract. The Buffels River catchment, in the Northern Cape Province of South Africa, is affected by saline groundwater. Large, shallow, circular earth mounds (around 2 m high and 30 – 40 m diameter) that are associated with termite activity are dominant microtopographical features within the catchment. These mounds (known as heuweltjies) have salinity levels an order of a magnitude higher than the surrounding soils but it was unclear whether the salinity was caused by the upward movement of saline groundwater, or if the downward movement of heuweltjie salts contributed to groundwater salinization. Thus, this study aimed to determine i) the net direction of solute migration through the mound profiles, ii) the association of salts with mound features and iii) possible sources of salinity. Detailed analyses were conducted on two excavated heuweltjies to determine the chemical, mineralogical and physical characteristics of the mounds as well as their salt type and concentration profile. Heatmaps and correlation matrices were generated to show the spatial and statistical relationships between the individual ion species within the heuweltjies. Results showed that heuweltjies have elevated soluble ion concentrations compared to the interheuweltjie soils. Ion activity ratios in the heuweltjies are consistent with a marine-aerosol origin of the soluble ions. High concentrations of salts are associated with termite frass and nest chambers, suggesting termite activity plays a role in their concentration in the mounds. Gypsum and calcite are only present in the heuweltjie soils. Halite is undersaturated throughout both heuweltjies but is least undersaturated in the centres of the heuweltjies. Calcite nears saturation towards the surface of the mounds whereas gypsum reaches saturation at the middle to base of the heuweltjies. This downward separation of increasingly soluble minerals suggests a surface source of salinity that is being transported downward through the heuweltjie. Faunal burrows on the surface of the mound crests form local microtopographic lows, and preferential flow pathways through granular soils and termite nests and tunnels were observed. These aid in the transfer of salts to greater depths. Based on these results it is proposed that heuweltjies form an important conduit transferring marine-derived aerosol salts towards the groundwater through the harvesting activities of termites and that the salt concentrations in the heuweltjies are not the result of capillary rise from an elevated water table.
