The impact of urban and sub-urban land use on the surface water quality of two South African dams and their tributaries

Abstract

Hartbeespoort Dam, a hypertrophic impoundment, is located approximately 53 km north-west of Johannesburg and 30 km west of Pretoria, while Roodeplaat Dam, also hypertrophic, is situated about 24 km north-east of Pretoria. Both dams are key freshwater reservoirs, primarily used for irrigation, domestic, and industrial purposes, and are also well-known for recreational activities. The Hartbeespoort catchment area includes two perennial rivers: the Crocodile River, whose main tributaries are the Jukskei and Hennops Rivers, and the Magalies River. It also features two non-perennial rivers, the Leeuspruit and Swartspruit, which flow into Hartbeespoort Dam. The Roodeplaat catchment area consists of the Pienaars River, Edendalespruit, and Hartbeesspruit, with its main tributary, the Moreletaspruit, flowing into Roodeplaat Dam. This study aims to identify the potential contamination sources affecting river water quality in both catchments. Sampling sites were selected based on adjacent land uses to assess water quality changes and pinpoint potential contamination sources. Water quality degradation in both catchments is primarily driven by the malfunction of wastewater treatment plants (WWTPs), with urban and agricultural runoff contributing to a lesser extent. These WWTPs are linked to substantially high concentrations of total suspended solids (TSS), ammonium, orthophosphate, and total organic carbon (TOC), as well as elevated microbial counts. They also contribute to reducing conditions, with ammonium constituting the largest proportion of the total nitrogen composition. The Olifantsfontein and Zeekoegat WWTPs are the most concerning plants in the Hartbeespoort and Roodeplaat catchment areas, respectively. Alexandra Township is the most concerning urban area in the Hartbeespoort catchment, while Mamelodi is the most concerning in the Roodeplaat catchment, with both areas associated with elevated TSS concentrations, TOC concentrations, and microbial composition. Agricultural activities impacted the water quality of the Leeuspruit in the Harbeespoort catchment, as well as the Edendalespruit and Harbeesspruit in the Roodeplaat catchment, by increasing TOC concentrations due to the runoff of herbicides or pesticides. In both catchments, agricultural, mining, and industrial areas are linked to oxidised conditions, where nitrate constitutes the largest proportion of the total nitrogen composition.