Reduction of pollution levels in the Chatty River, Gqeberha, South Africa, through sustainable drainage systems: Bethelsdorp sub-catchment case study

Abstract

Chatty River, located in Gqeberha, South Africa, is the largest tributary feeding into the Swartkops Estuary, and a major source of pollution. Its catchment is mainly occupied by low-income residential areas resulting in polluted stormwater runoff from litter and raw sewage discharge. This study employed both experimental and modelling approaches to assess pollution sources and mitigation options. Water quality sampling was conducted across various sub-catchments draining into the Chatty River, and subsequently Swartkops Estuary, to evaluate the physical, nutrient, and microbiological characteristics, revealing eutrophic and hypertrophic conditions and high gastrointestinal health risks to residents. Additionally, hydrological and hydraulic modelling were performed using PCSWMM (Personal Computer Stormwater Management Model) for the Bethelsdorp River sub-catchment. Various scenarios were developed to test the retrofitting of sustainable drainage system (SuDS) interventions for improving water quality.  The model scenarios include: the ‘As-Is’ model representing the current situation; the ‘Pre-Development’ model representing the state before the influence of anthropogenic activities, and various retrofitted SuDS intervention models, including the reduction of pollutant concentration through the rehabilitation of historic wetland areas, a constructed wetland, a retention pond, and various infiltration practices. Rehabilitating the wetlands offered the highest impact in terms of water quality improvement, with a mean pollutant reduction of 30%. However, a combination of all the interventions had the highest pollutant removal when functioning efficiently, of 72% and 80% for dissolved inorganic phosphorus (DIP) and total suspended solids (TSS), respectively. Forming a treatment train was seen as the most effective strategy to adequately improve water quality in the catchment to meet the standards presented by various guidelines.