Characterization of Regolith Salt Storage in a Small Catchment of the Berg River Basin

Nebo Jovanovic, Richard D.H. Bugan;Ashton Maherry;Louise Soltau;Willem P. De Clercq;Martin V. Fey;

Abstract


Dryland salinity recharge from agricultural land has been identified as the main source of salinization in the Berg river basin (Western Cape, South Africa), which represents a challenge to water users and water quality managers. The aim of this study was to characterize and quantify regolith salt storage in a small catchment (~ 20 ha) representative of the saline environment and drier bio-climatic conditions of the mid- and lower reaches of the Berg river basin, where storage and potential discharge of salts are likely to be the greatest. Electromagnetic induction measurements with an EM-38 meter indicated that salinity in the landscape is driven by the topo-sequence, anti-erosion contours act as barriers to water and salt fluxes, highly saline scalds are associated with convex hillslopes, and soil salinity is lower at the end of the rainy winter season compared to that at the dry summer. Resistivity tomography measurements taken with an ABEM Terrameter SAS1000 ground imaging apparatus down to approximately 70 m depth indicated that subsurface drainage features are often associated with salinity and topographic flow pathways, as a result of shale weathering. Analyses of core samples collected during borehole drilling indicated that the regolith contains about 400 tons of salts per hectare, stored mainly as a bulge in the unsaturated vadose zone (between about 5 and 10 m depth), above the water table and below the soil zone, discharging mainly through lateral leaching by groundwater from the phreatic zone. Groundwater quality becomes increasingly saline from the recharge area (0.68 dS m-1) towards the bottom end of the catchment (3.75 dS m-1), and it is a mixed cation-chloride-type. The effects of different climates, hydrogeological characteristics and land uses on the salt discharge into the mid- and lower reaches of the Berg river need to be investigated.

Keywords


Dryland Salinity, Electromagnetic Induction, Groundwater Quality, Regolith, Resistivity Tomography

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