|Polycyclic Aromatic Hydrocarbons (PAHs) in the aquatic ecosystems of Soweto/Lenasia
Polycyclic aromatic hydrocarbons (PAHs) consist of fused benzene rings and the congeners have varying numbers of benzene rings, usually between two and six. They have a widespread distribution due to their formation by incomplete combustion of organic materials and are continuously released into the environment making them ever-present. The US EPA has earmarked 16 congeners that must be monitored and controlled because of their proven harmful effects on humans and wildlife. Anthropogenic activities largely increase the occurrence of these pollutants in the environment. A measurable amount of these PAHs are expected to find their way into aquatic ecosystems.
In a previous study completed for the Water Research Commission (Project no K5/1561) on persistent organic pollutants in freshwater sites throughout the entire country, the PAHs had the highest levels of all of the organic pollutants analysed for. According to this study Soweto/Lenasia was particularly burdened with high PAH levels which was the main motivation for further, in-depth investigation into this area, focussing on the PAHs only.
OBJECTIVES AND AIMS
In the current study (K5/2242) the potential exposure of humans and wildlife to the 16 priority PAHs, was investigated. The sites were selected in the suburban areas of Moroka, Lenasia, Fleurhof, Eldorado Park, Orlando West, Orlando East, Nancefield and Dobsonville. The sites were named after these respective areas.
The main aim of this study was to determine the levels of the 16 priority PAHs in the Klip River that flows through the densely populated urban areas of Soweto and Lenasia.
In addition, the pollutant profile of the 16 parent PAHs in the sediments was investigated, by comparing site PAH composition percentages to determine origin of the pollution, i.e. pyrogenic vs petrogenic.
The final aim of this study was to determine the toxicity posed by the PAHs in the study area. This was done by assessing the sediments against sediment quality guidelines and quality indices. A very specific mechanism of toxicity: that mediated via the cellular receptor the aryl hydrocarbon receptor, was also investigated. The biochemical responses and overall health of the fish was also investigated. Finally, the potential risk to human health was gauged using a health assessment index.
The levels of the PAHs in the sediment were determined by instrumental analysis in the sediment, fish tissue, and wetland bird eggs sampled within the study area. The PAHs were extracted using pressurised liquid extraction. The extracts was fractioned – to isolate the PAH containing fraction – using size exclusion. The extract was cleaned up with a silica/Florisil solid phase extraction (SPE) column. From the final extract, The PAHs were quantified with gas chromatography and time of flight mass spectrometry (GC-TOFMS).
The pollutant profile of the PAHs in the sediment was further extended into calculation of percentage congener contribution. Along with this, the percentage of low- and high molecular mass PAHs (LPAHs, HPAHs) and “carcinogenic” PAHs (CPAHs) were determined. The potential origins of PAHs measured were identified using diagnostic ratios.
The sediment toxicity was evaluated by comparing the levels to international sediment quality guidelines. The sediments were also assessed with sediment quality indices which describe sediment quality and ecological risk to benthic fauna. The investigation of the toxicity via the aryl hydrocarbon receptor was measured using the H4IIE-luc reporter gene bio-assay.
The effect on fish was explored by performing biomarker response assays. These included: acetylcholinesterase- and cytochrome P450 activity, malondialdehyde- and protein carbonyl content, as well as catalase and superoxide dismutase activity. Biomarkers reflect the biochemical responses to environmental stressors. Along with this, individual and community fish health was assessed using various indices: fish health assessment index and organo-somatic indices.
Finally, the potential health risk to the human population dependant on the water bodies in the study area was gauged by conducting a theoretical human health risk assessment.
RESULTS AND DISCUSSION
The chemical analysis of PAHs on the sediment, fish and bird eggs confirmed the presence of PAHs in the study area. The sediments showed significant levels of PAHs however analysis for PAHs in the biota produced little or no data – due to effective metabolism of the parent isomers.
The pollutant profile of the sediments indicated that the dominant sources of PAHs in the Soweto/Lenasia area are of pyrogenic origins, specifically from the burning of biomass and to a smaller extent, petroleum combustion, probably from vehicles in urban areas. The site of greatest concern was the site in the Moroka area. Other sites that were also of concern are Lenasia, Fleurhof, Eldorado Park and Orlando West as well as Orlando East. Moroka had the highest PAHs levels for both seasons and exceeded most of the sediment quality guidelines. The quality indices revealed the same results: Moroka scored the highest values for the sediment quality guideline index (SQG-I), indicating that this site poses a high ecological risk to benthic biota. Similarly, Fleurhof, Orlando West, Lenasia, and Eldorado Park showed a high probability of being toxic to benthic biota when considering the SQG-I. The sediment quality index (SQI) based on the chemical analysis data, indicate the quality of the sediments in terms of the PAH loads and according to this all the sites are of poor quality. In conjunction to these toxicity assessments the toxic equivalents (TEQs) of the study area (for both years) were all higher than the lower, interim sediment quality guideline (for the protection of fish) of Canada, except for Nancefield 2013. Moroka 2013 sediment exceeded also the higher probable effects level (PEL) of the Canadian guideline. The toxicity assessments identified Moroka’s sediment as the site with the highest probable toxicity to both benthic organisms and fish.
In comparison to the TEQs calculated for the sediments, the bio-assay equivalents (BEQs) of the sites also showed that Moroka (2013) elicited the highest response in the H4IIE-luc bio-assay. However, the 2014 samples had Lenasia as the highest BEQ, far higher than Moroka, which was the second highest.
The biomarker results cannot be exclusively ascribed to the PAHs in the aquatic environment, because other compounds also present might have been – or contributed to –the cause of the responses observed in the biomarkers. Also, it was impossible to tell how much PAHs the individual fish were exposed to as they metabolised the PAHs and due to budget constraints the metabolites could not be quantified Even though it would be theoretically possible to relate the cytochrome P450 (CYP450) results to the TEQ and BEQ – which measures the same toxic mechanism of action – no clear relationship was seen. The highest CYP450 response was from Orlando, and its sediment (Orlando East) had the second highest TEQ value for the season. The other biomarkers indicated that the fish were exposed to compounds that elicited selected responses, specifically the inhibition of acetylcholinesterase activity and the up-regulation of the catalase system.
The health assessment of the fish also corroborated that they most like were affected by xenobiotics. The condition factor showed that the fish were all in fair to good condition. The high prevalence of abnormalities in the livers (discolouration, deformation and fat deposition) seems to indicate to contaminant metabolism and the enlargement of the spleens in most fish – of which none had parasitic infections – supports the deduction that the health effects are from chemical contamination rather than natural factors (parasites, mechanical damage and malnutrition).
Possible health risk to humans consuming fish from the study area was investigated by conducting human health risk assessment by modelling risk from oral exposure. PAH levels in fish were extrapolated from levels found in the sediment. Benzo(a)pyrene and dibenz(a,h)anthracene were identified as the chemicals of concern even when they did not occur in high concentrations. The risk calculated at each site showed that there is no risk to humans living in the study area, contradictory to the previous WRC study in the area, on which this study is based.
The results obtained in this study indicate that there is a definite presence of PAHs in the Klip River of Soweto/Lenasia. The site that created the most concern based on the chemical analysis and toxicity assessments was Moroka followed by Lenasia and Eldorado Park. The biochemical responses and health assessment of the fish indicated that there are stressors present in the system – not necessarily PAHs – that activated the cytochrome P450s, inhibited neurotransmission enzymes, increased the anti-oxidation systems, and decreased the overall health. However, it seems that the humans in the area are not at risk of exposure to PAHs, at least not through ingestion of the fish from the area.
• The presence of PAHs in the sediments of Soweto/Lenasia was confirmed by the chemical analysis.
• The sources of these PAHs have been narrowed down to pyrogenic sources, mainly from biomass combustion. The ratios also identified petroleum combustion as a source of the HPAHs and this is most probably from vehicles as the study areas is situated in an urban area.
• The site of greatest concern is Moroka. Other sites that are of concern are Lenasia, Fleurhof, Eldorado Park and Orlando West as well as Orlando East. Even though Protea Glen had high CPAHs, according to the toxicological assessment of this site it ranked lower than the other sites. Moroka had the highest PAHs levels (ΣPAHs, ΣLPAHs, ΣHPAHs, and ΣCPAHs) of all the sites. It was also the site that ranked the highest in all the toxicological assessments – exceeding most guidelines, especially the PEL of the Canadian guidelines.
• For both the 2013 and 2014 seasons, Moroka had the highest SQG-I score, indicating that the site’s sediment posed a high ecological risk to benthic biota. The SQI corresponds to the chemical analysis and guidelines – scoring the sediment quality of this site as poor in terms of PAH pollution. Lenasia and Eldorado Park also had high levels of PAHs. The 2014 sediments of these sites exceeded both sets of guidelines.
• The toxic equivalent quotients (TEQs) of samples from the study area (2013 & 2014) were all higher than the lower guideline (ISQG: for the protection of fish), except Nancefield 2013. Moroka 2013 sediments had the highest TEQ – that exceeded the higher guideline (PEL) – and in conjunction with the other toxicological tests indicates that this site posed a serious threat to biota, specifically benthic organisms and fish.
• Even though the chemical analysis of the fish and bird egg samples produced little to no quantifiable data in terms of the parent PAHs, there is evidence that there are PAHs present in the system – high sediment loads.
• The biomarker responses are difficult to appoint to specific exposure due to the lack of chemical data in the fish. Cytochrome P450 activity in the fish can be compared to the TEQ and BEQ of the sediment data, seeing that the same mode of action is used (Ah-receptor mediated responses). One would expect the cytochrome activity to correspond to the TEQ and BEQ, but contradicting responses were observed for Fleurhof (2013) as well as Nancefield (2013): the lowest CYP450 response was in fish from Fleurhof (2013), which in turn had the highest TEQ and BEQ results for the sediment for the same year. The second highest 2013 CYP450 response was from the fish from Nancefield but its sediment had the lowest TEQ and BEQ values. Some of this discrepancy could be attributed to the fact that fish were sampled from dams and sediment from streams feeding the dams, and although the sampling sites were in close proximity to each other this might explain the observed differences. This discrepancy was unexpected as we assumed that the transportation of the PAHs to sites close together would be the same. The expectation of having high CYP450 responses from coinciding high TEQ and BEQ levels in surrounding sediment was met for Orlando: the highest CYP450 response was from Orlando, and its sediment (Orlando East) had the second highest TEQ value for the same year.
• The other biomarkers indicated that there were compounds present in the study area that elicited responses in the fish, specifically the inhibition of acetylcholinesterase activity and the up-regulation of the catalase system that could probably not be ascribed to PAH levels.
RECOMMENDATIONS FOR FUTURE RESEARCH
• The chemical analysis of the metabolised PAHs would complete the picture of what is happening to the parent PAHs after entering the animals’ bodies. This would however, necessitate more funding because these analytical standards are expensive and not always readily available in South Africa. Each of the 16 parent PAHs has more than two metabolites that could be quantified chemically increasing the analytical load and associated expenses.
• The biomarker response results could not conclusively be attributed to the PAHs, and therefore a broad spectrum screening for a much larger variety of organic chemical pollutants is advised for this densely populated area of Gauteng. Chemical compounds that can be considered include: polychlorinated biphenyls, brominated flame retardants, organochlorine pesticides, plasticisers, pharmaceuticals and personal care products and perfluorinated compounds, just to name a few compound classes.
• The number of bio-assays can be broadened to include assays capable of detecting endocrine disruptive effects.
• Evaluation of fish species composition and numbers to further describe pollution effects in the system.
• Add a social component to the study in which the human population’s physical interaction and dependence on the Klip River running through Soweto/Lenasia is quantified, i.e. using questionnaires and interviewing the citizens.
• Incorporating results from this study into management of this water catchment one must keep in mind that PAHs are mainly airborne. Therefore, a successful monitoring program of any water catchment for these compounds would require an integrated approach including air quality monitoring.
|Document Type:||Research Report
|Document Subjects:||Water Resource Management/IWRM - Water Governance
|Document Keywords:||Water Quality
|Document File Type:||pdf
|Research Report Type:||Standard
|WRC Report No:||2242/1/16
|Authors:||Pheiffer W; Pieters R; Genthe B; Quinn L; Bouwman H; Smit N
|Organizations:||North West University; Council for Scientific and Industrial Research; National Metrology Institute of South Africa
|Document Size:||3 026 KB