Bio assessment of water quality using macro-invertebrate communities in the Selati River, Lower Olifants River System

Abstract

Many South African rivers, including the Ga-Selati River have been deteriorating for the past few decades, due to an increase in mining, industrial, agricultural and domestic activities. Around mid-January 2014, the Ga-Selati River was contaminated by mine spills from a nearby phosphate mine, which polluted the river and killed many fish species. This river is a primary source of water for many activities (e.g. mining, industrial, agricultural and domestic activities), and as a result, large numbers of reservoirs, farm dams, and inter-basin transfer schemes have been constructed to increase the reliability of water supply along this river. Contamination of surface waters by agricultural pesticides and fertilizers, as well as by industrial metals, is a cause of increasing public concern. The Ga-Selati River is a major tributary for the Olifants River, among other tributaries (Steelpoort River and Blyde River) and it plays a significant role in the Kruger National Park and other private game reserves in the catchment. This River is also known to supply water of very low quality into the main stem of the Olifants River. The Olifants River System has been described as degraded and is contaminated with metal and chemicals, and is considered to be one of the most threatened river systems in South Africa. The aim of the study was to investigate the ecological state of the Ga-Selati River and the impact of water and sediment quality on the aquatic invertebrate communities, and to propose measures to prevent further degradation of the river ecosystem by human disturbance. The main objectives were to: (i) establish the current physico-chemical composition of the river water and sediment along the entire length of the Ga-Selati River, (ii) to determine if poor water quality at the lower end of the river is due to pollution inputs in the lower reaches, or the result of cumulative pollution inputs along the entire length of the river, (iii) Assess the impact of water and sediment quality on the aquatic macro-invertebrate assemblages in the river. The concentrations of pH, and DO were high at all sites. If there was any sort of pollution in the river, especially downstream by the mining sites, we expected these two variables to be lower. The water variables such as EC, TDS and salinity showed a gradual increase from upstream to downstream. There were also elevated levels of certain metals, such as Mg, Na, Ti, B, Sr, K and Ca showing a pollution gradient. The v

high concentrations of metals in water samples indicate that the Ga-Selati River is heavily impacted downstream by anthropogenic activities such as illegal dumping/littering at site 6 and mining activities at site 7 to site 9. Some of the metal concentration (Na, Mg, K and Ca) in the river were found to be extremely high compared to other rivers in the region. Metal concentrations in sediment samples were very high compared to water samples River. The nutrient concentrations at the Ga-Selati River were high but did not show a pollution gradient. The macro-invertebrate assemblages in the Ga-Selati River were rich in Ephemeroptera, Diptera and Trichoptera. Site 1 and site 2 accounted for most of the sensitive families, reflecting good water quality at these two sites, while site 9, a downstream site recorded the highest number of tolerant families.. The variations in the macro-invertebrate distribution were shown by the differences in the water quality at the various sites by the CCA plot. The effects of main pollution factors such as, EC, TDS, turbidity and nutrients were correlated with the distribution of tolerant taxa.