Polycyclic aromatic hydrocarbons in the Orange-Senqu River Catchment : Concentrations, patterns, and assessments

Abstract

Persistent and priority organic pollutants have been documented in the Orange-Senqu River Catchment (OSRC). One of the pollutant classes of concern is polycyclic aromatic hydrocarbons (PAHs). PAHs are continuously released and widely distributed into the environment making them ubiquitous, pseudo-persistent contaminants. Sixteen priority congeners have been recognized by the United States Environmental Protection Agency (USEPA), to be monitored and regulated based on their confirmed detrimental effects on wildlife and humans. The objective of this report was to determine the concentrations of the 16 priority PAHs in sediment samples from the OSRC to provide a more detailed view of PAH contamination and risks. The aims were: to review the levels of 16 PAHs in sediment samples from OSRC; determine source appointment and composition profile; conduct an ecological risk assessment; calculate and review sediment toxicity against international sediment quality guidelines; as well as to identify and compare sediment quality guidelines for the protection of benthic life. The concentrations of the PAHs in the matrices were determined by instrumental analysis. Sediment samples were analysed for the 16 PAHs by ÖKOMETRIC, GmbH in Beyreuth Institute of Environmental Research. Soxhlet extraction of sediment samples was followed by clean-up of extracts using silica gel column and aluminum oxide column chromatography. The PAHs were quantified by gas-chromatography coupled to mass spectrometry (GC-MS). The pollutant profile was investigated using percentage congener contributions and potential sources were determined by diagnostic ratios. Multiple approaches for toxicity assessment were completed: (1) overall toxicity, sediment quality, and ecological risk were evaluated by comparing PAH concentrations to international sediment quality guidelines (SQGs), (2) calculating sediment quality indices, (3) determining risk quotients (RQ), and (4) mean sediment quality guideline quotients (m-SQG-q). Specific carcinogenicity and mutagenicity were investigated in terms of benzo(a)pyrene-equivalent assessments. Individual PAH concentrations ranged between 0.5-1001 ng/g dm. Total levels of PAHs in sediment were relatively high (2.4-4969 ng/g dm), but in the same order of magnitude as other studies within South Africa. The mean ring size percentage distribution was 3 > 4 > 2 > 5 > 6. Diagnostic ratios indicate that the PAH content in sediment is likely due to pyrogenic sources of biomass combustion. The sediment toxicity assessment characterised a few samples in which toxicity is feasible. Only phenanthrene and benzo[a]pyrene (considered the most toxic PAH) had concentrations above the probable effects level of the Canadian Council of Ministries of the Environment at Soweto (S63) and at the Vet River (S71). Moderate to high toxicity, and fair to poor sediment quality was expected for benthic organisms based on the international SQGs, sediment indices, and toxic equivalent quotients. Three sites in the Vaal Middle and Vaal Barrage Catchment have high carcinogenicity and mutagenicity levels and are anticipated to present some toxicological risks to sediment-dwelling organisms. Mutagenic levels (1.5-634 ng/g dm) were slightly higher than the carcinogenic levels (0.002-530 g/g) and can lead to some non-cancerous adverse health effects. Ordination methods showed a stronger relationship between PAH contamination and stream systems than for river systems, as well as a site within the Augrabies Falls National Park to be closely

ordinated with poor sediment quality index scores. Univariate, bivariate, multivariate, comparisons with other studies, toxicological guidelines, and diagnostic ratio analyses indicated complimentary and contradictory results. This is most likely due to the complex interactions of source factors, hydrology, and topography with the physical-chemical characteristics of the PAHs. These results emphesis the need for site specific assessments. Overall, the data demonstrate that PAHs remain isolated in hotspots within the Vaal and Upper Orange Catchments. Such PAH levels corrolate to human activity. The Vaal Barrage and Vaal Middle Catchments raised the most concern by frequently exceeding healthy sediment guidelines and assessments. There is concern for the areas around Soweto (S63), Lenasia (S64; S65), and Leeudoringstad (S73).