Surface Water-Groundwater Interaction Using Tritium and Stable Water Isotopes: A Case Study of Middelburg, South Africa

With an increasing population, development of the country and a changing climate, an increased demand for fresh water, coupled with negatively impacted natural water resources, are observed. One impacted component of the water resource may have an impact on another, due to the interaction between water resource components in the water cycle. All water resource components need to be well-managed and protected to ensure their availability and sustainability. Studies on water quantities, flow dynamics, quality, and contamination are essential in this regard. Isotopes are used as a tool in these studies to define the interconnection between different water resource components. The information gained from isotope studies is valuable in the planning of activities in areas where interacting water resource components may potentially be affected. A study in Middleburg comprised a literature review and field investigations at and around a cemetery, as part of a Water Research Commission project on impacts on the water resource from large scale burials. A seasonal wetland is located downgradient of the cemetery, between the cemetery and a stream that flows past the cemetery. In order to assess possible flow pathways from the cemetery to the stream, monthly monitoring of surface and groundwater quality and level fluctuations was carried out on the stream, as well as existing and newly installed boreholes at the site. The water samples were analysed for inorganic constituents, tritium, and stable water isotopes. The isotope results - revealed the comparative influence of rainfall and shallow groundwater contributions to streamflow, while groundwater provides base-flows as the stream levels recede. The depth to groundwater reduced with increasing rainfall, indicating direct recharge. The difference in concentrations of some inorganic parameters in the stream compared to the groundwater at the cemetery revealed the effect of natural attenuation and the wetland acting as a filter to improve the water quality of the shallow interflow.