Surface water and groundwater interaction in the Vredefort Dome, South Africa: a stable isotope and multivariate statistical approach

Abstract

The growing importance of groundwater as a freshwater supply in semi-arid areas such as the Vredefort Dome World Heritage Site (VDWHS) demands the judicious management and development of this vital resource. The increased demand for groundwater due to the contamination of surface water, coupled with the lack of information on hydrological interaction and associated water quality implications, present difficulties in establishing management strategies. An integrated study based on hydrochemistry and multivariate statistical techniques supplemented by environmental isotopes delineated discrete areas of surface water and groundwater interaction in a fractured-rock terrain. Surface water loss was observed in sections that exhibited declining groundwater levels, whereas limited baseflow was restricted to zones with stable groundwater levels. The multivariate statistical analysis revealed the combined effect of natural hydrochemical processes and anthropogenic sources as controlling factors of water composition, and highlighted zones of aquifer-river water mixing, where certain areas were found to be additionally polluted by human-derived contaminants. The stable isotope (¹⁸O and ²H) ratios confirm mixing between depleted groundwater and enriched river water, producing a composition that reflected an integration of the isotopic variations. The continuous wastewater discharge into the Vaal River combined with the increased groundwater exploitation may be prompting induced recharge conditions. The results suggest compartmentalization of the groundwater systems, where certain areas within 1 km of the channel were not influenced by river-induced contamination. This indicates that hydrological connectivity is governed by site-specific hydraulic properties. This study shows the usefulness of a multi-method approach by combining environmental isotopes, hydrochemistry, and multivariate statistics to characterize hydrological linkage in semi-arid regions.

Keywords: Environmental isotopes; Hydrochemistry; Semi-arid; Surface water and groundwater interaction; Vredefort Dome World Heritage Site.