Characterizing water quality of rivers and underlying aquifers in the Nuwejaars Catchment, South Africa

Abstract

Non-perennial rivers are the main source of surface water in drylands. These rivers are unpredictable and highly variable in terms of flow regime and water quality. Despite their abundance, and importance in the landscape, non-perennial rivers still remain under- studied. The functioning of non-perennial rivers seems to be strongly dependent on groundwater, especially during dry seasons when pools develop, but the relationship between surface water and groundwater has seldom been established and quantified. Many of these systems are ungauged and hydrological and chemical data are available for very few systems in South Africa. The present study falls under the Non-perennial Rivers Project, and aimed to characterize the water quality of rivers and aquifers in the Nuwejaars Catchment, located in Cape Agulhas, near the southern-most tip of Africa. To achieve this aim, a quantitative approach that provides the ability to assess relationships between variables both spatially and temporally was applied. Field visits were conducted on four occasions during July 2017 and July 2018, to characterize the chemistry of surface water and groundwater during dry and wet seasons. Physical parameters were measured in situ, while water samples were collected from boreholes, piezometers, springs, artesian boreholes and selected points along the river for laboratory analysis of major ions, nutrients and stable isotopes. Descriptive and bivariate statistical methods were used to summarise and evaluate the strength of the relationship between variables, while multivariate statistical methods were applied to group similar samples based on their chemical compositions. Piper diagrams were generated to characterize water type based on the proportions of cations and anions, while correlation and stoichiometric analysis was applied to identify hydrogeochemical processes influencing groundwater chemistry. Stable isotope compositions were presented on a graph, along with the global meteoric water line (GMWL) and local meteoric water line (LMWL) to establish similarities between isotope signatures of groundwater and surface water.