The use of biological indicators in assessing pollution status in selected rivers in Limpopo Province, South Africa

Abstract

The aim of the study was to assess the extent to which biological indicators and biomarkers can be used to monitor the pollution status of the Sand and Blood Rivers. Physico-chemical parameters were assessed as a primary approach in assessing pollution status of the Sand and Blood Rivers. Temperature, pH, dissolved oxygen (DO), biological oxygen demand (BOD), turbidity, total dissolved solids (TDS), total nitrogen, phosphorus and ammonia were assessed during the rainy and dry seasons. Physico-chemical parameters showed spatial and temporal variation. The hierarchical average linkage cluster analysis grouped the reference sites (S1 and B1) into one group and sites after points of discharge (B2, S2, S3, S4 and S5) into another group. This grouping was due to the pollution status of each site, with the reference sites less polluted than the sites downstream of the wastewater treatment plant effluent points of discharge. The canadian council of ministers of the environment water quality index (CCME WQI) categorized reference sites as good, whilst sites after points of discharge were categorized as poor. This shows that the Sand and Blood Rivers are degrading. Heavy metal contamination in water, sediment and grass from the Sand and Blood Rivers was evaluated during the rainy season and dry season. All assessed heavy metals (Cadmium, Chromium, Copper, Iron, Lead, Manganese, Nickel, and Zinc) were below the detection limit in water, with an exception for Iron and Manganese. All metals assessed were also below the probable effect levels (PEL) according to the CCME. Geo-accumulation Index showed that the Sand and Blood Rivers were not contaminated with heavy metals. The enrichment factor (EF) further showed that only site B2 was enriched with manganese from anthropogenic activities. Heavy metal assessment in Cyperus exaltatus showed that this grass is a poor candidate for phytoremediation. Macroinvertebrates were used as biological indicators to assess pollution status of the Sand and Blood Rivers. Macroinvertebrates were sampled according to South African scoring system (SASS) and identified using aquatic invertebrates of South African rivers field guide. The SASS and average score per taxon (ASPT) categorized all sampling sites as critically modified. The family-level biotic index (FBI) also showed that the Sand and vi Blood Rivers are enriched with organic pollution. The reference sites recorded high macroinvertebrates diversity, compared to the sites after points of discharge. All indices used showed spatial and temporal variation in the water quality of the Sand and Blood Rivers. The relationship between macroinvertebrates and physico-chemical parameters were explored with the use of canonical correspondence analysis (CCA). The CCA triplots showed that the reference sites were associated with pollution sensitive taxa, whilst pollution tolerant taxa were associated with sites after points of discharge. Moreover, sites after points of discharge strongly correlated with phosphorus, nitrogen, ammonia, BOD and TDS. Clarias gariepinus gills and liver histology were used as biomarkers in assessing pollution status in the Sand and Blood Rivers. Gills and liver samples were qualitatively and semi-quantitatively assessed. Fish from the reference sites had less gill lesions than fish from sites downstream. Alterations such as hyperplasia of interlamellar, fusion of secondary lamellae, epithelial lifting and hyperplasia of secondary lamellae were observed in fish collected from downstream. Fish from downstream also showed more liver alterations than fish from the reference sites. Alterations such as melano-macrophage centers (MMC), macrovesicular steatosis, sinusoid congested with kupffer cells, nuclei pleomorphism and vacuolation were identified in liver of fish from sites S4 and S5. This was further confirmed by the high gill and liver indices of fish from downstream. Acetylcholinesterase enzyme (AChE) enzyme in brain and lactate dehydrogenase (LDH) in liver of Clarias gariepinus were used as biomarkers in assessing pollution status of the Sand and Blood Rivers. Both AChE and LDH enzymes were lower on fish from downstream compared to fish from the reference sites. This shows that AChE and LDH enzymes on fish from downstream were inhibited. This was attributed to the use of pesticides on farms surrounding the Sand and Blood Rivers and also the discharge of poorly treated sewage effluent from the Polokwane and Seshego wastewater treatment plants (WWTP’s). The Sand and Blood Rivers are surrounded by farms that utilize pesticides in their practice. This study showed that biological indicators and biomarkers can be used to assess pollution status of the Sand and Blood Rivers. However, biological vii indicators and biomarkers should be used in concurrence with physico-chemical parameters.