Impact of effluent discharges on water quality and invertebrates in the Apies River system

Abstract

Large populations in big and developing cities are exerting pressure on wastewater treatment plants. The inadequately treated sewage discharged into the aquatic environments deteriorates the quality of the receiving water resources. The Apies River system has received attention, and several studies have been conducted on the significant sources of pollution, particularly wastewater plants. However, nothing much has been done about the impact of sewage discharges into the Apies River. This study aimed to assess the impacts of effluent discharges and land use on water quality and macroinvertebrate diversity of the Apies River. The study focused on how the effluent discharges affect water quality and macroinvertebrate diversity by comparison to the least affected site. This will therefore add to a significant assessment of the water and ecological quality of the Apies River system. According to literature, the Apies River is one of the most polluted rivers in South Africa; it is characterized by inadequate domestic sewage treatment, power generation, and agricultural and industrial activities. Such activities are known to reduce the diversity of macroinvertebrate assemblages as well as the community structures. According to aquatic health studies, aquatic invertebrates are sensitive to organic pollution, altered physicochemical factors, and habitat transformation; hence, it is appropriate to use them to assess the ecological health of rivers. Four surveys were conducted during the dry and wet seasons at four sites for macroinvertebrate analysis; however, only two surveys were conducted for water and sediment analysis. Macroinvertebrates were collected at all four sites from various biotopes using standard sampling procedures, and ultimately they were counted and idntified. Water and sediments were collected at all sites following the standard method, while physical parameters (temperature, electrical conductivity, pH, TDS, and flow rate) were measured in situ at all selected sites. Sediments and water samples were digested and sent for inductively coupled plasma-mass spectrometry (ICP-MS) analyses for the determination of selected heavy metal concentrations. Chemical analyses for nutrients and selected major ion concentrations were conducted on water samples using ICMS as well. All selected metals were detected in water and sediment samples; however, high concentrations were recorded in sediments. Seasonal variation was recorded during the study. Some metal concentrations were high during the low flow in water and sediments, while others were high during the high flow season. A total of 28 families of macroinvertebrates were collected at the four selected sites during the four surveys. Macroinvertebrates were classified according to their sensitivity. Families classified as highly tolerant were abundant at 3 sites that are impacted by various anthropogenic activities (WWTWs), suggesting that there is a presence of organic enrichment from site 2 further downstream. Site 1 accommodated highly sensitive families and moderately sensitive and highly tolerant families, exhibiting the least levels of organic pollution. Habitat quality was good throughout the study, and all expected biotopes were available. Habitat scores ranged from 55% to 90% at all sites. The macroinvertebrate diversity was high at sites 1 and 3 compared to sites 2 and 4. Sensitive taxa were dominant at site 1, which is the headwater stream, and highly tolerant taxa were dominant at sites 2, 3, and 4. Based on the results obtained from this study, it is evident that the pollution in the Apies River has caused a shift in the macroinvertebrate structure across four sites. Constant monitoring on the river should be implemented going forward.