Assessment of Persistent Organic Pollutant accumulation in Sediments of the Klip River Wetland, Johannesburg

The accumulation of Polyaromatic Hydrocarbons (PAHs) and Polychlorinated Biphenyls (PCBs) was investigated in sediments of the Klip River wetland Johannesburg South Africa. The wetland serves as an important source of potable water to the Vaal Dam Johannesburg and purier of contaminants from different anthropogenic activities at the Witwatersrand basin. In this study, PCBs and PAHs were found to be spatially distributed in surface and core sediments of the two major tributaries entering into the wetland. The total concentration of PAHs and PCBs found in the surface sediments ranged from 45–95 mg/kg and 0.2–5.3 mg/kg respectively. Highlights of the distribution of PAHs indicated the upstream sites of both tributaries to be more polluted and were attributed to the proximity of these sites to heavy trac and industrial complexes. Conversely, the downstream sites were more polluted with residues of PCBs and were attributed to anthropogenic activities from the residential settlements of Soweto and Lenasia. The total amount of PAHs and PCBs found in the sediment core site were 2.4 − 6.3 mg/kg and 0.17–0.80 mg/kg respectively, which is signicantly lower than the concentrations found in the surface sample sites. Risk characterization of the sites indicated that about 60% of the investigated sites could be classied as moderately to highly polluted sites with potential ecological risk. A further investigation is therefore required for comprehensive ecotoxicological risk assessment.


Introduction
The negative repercussions associated with the long-standing use of Persistent Organic Pollutants (POPs) have been well documented (Jones and De Voogt, 1999). Some of these health-related issues include cancer (Karlsson and Viklander, 2008), gene mutation (Ghaeni et al., 2015), reproductive impairment, and endocrine disruption tendencies (Thomas et al., 2007). As a result of their environmental and human health challenges, there is a global effort through the Stockholm Convention to eliminate or reduce their production and use by 2025 (UNEP, 2001). Globally, their use for any purpose has been banned or Large quantities of fossil fuels continue to be used by industries to help boost the country's economy, with the combustion of coal being a major contributor to PAH and PCB emissions. It is estimated that the combustion of coal contributes 65% to total air pollution in South Africa (Balmer, 2017;Jeffrey, 2005). Soweto and Lenasia, two densely populated communities are situated downstream of the two tributaries. Additionally, a number of the industries that are dotted around the wetland rely heavily on coal as a source of fuel, while two major highways in Johannesburg are sited around the wetland, exacerbating the contaminant pressure of the wetland. Various studies have reported on different contaminants in sediments of the Klip River wetland catchments McCarthy and Venter, 2006b;Pheiffer et al., 2018). However, as important as this wetland is and the fact that several industries that potentially emit PCBs and PAHs are dotted around the basin, very limited studies reports on PCBs and PAHs and their potential impact on the ecology of the wetland. This study aims to provide useful information on contaminant status, potential sources, and possible ecotoxicological risks of PAHs and PCBs by assessing catchment and core sediment samples from the Klip River wetland.

Study area
The Klip River wetland is located south-west of Johannesburg and lies between latitude 26°10'-26°25 S' and longitude 27°45' − 28°05 E (Fig. 1). It extends from Lenasia and Soweto in the west towards Alberton in the south and receives runoff from several tributaries such as Klipspruit, Bloubosspruit, Harringtonspruit, Glenvistaspruit, Natalspruit, and Rietspruit. The wetland is covered by reed swamps (Phragmites australis) and is lled by organic-rich sediment/peat. Industrial growth in this region subsequently led to the discharge of e uents from mining activities, sewage treatment plants, and waste disposal, which accumulated in the Klip River wetland either by surface run-off or through groundwater input.

Sample collection
Surface sediment samples were collected from the two main tributaries (Klipriver and Klipspruit), which drain into the upper section of the Klip River wetland (Fig. 1). A sediment core from the upper section of the wetland was collected (site C 1 ) using a Russian corer. Coring was conducted to a depth of 3.5 m with subsamples taken at approximately 10 cm intervals to examine the vertical distribution of pollutants within the wetland. The pH, electrical conductivity, and redox potential were measured using a multipurpose electrode (ORION Thermo Scienti c Star A3255) in the eld. Sediment samples were kept on ice in the eld and later at 4 oC in the laboratory. Sediment samples were air-dried at room temperature and milled. The samples were then kept in amber bottles and stored under -18 o C until analysis. Total organic matter was estimated on each sub-sample as loss on ignition (LOI) at 500 oC temperature for 6 hours according to the method described by Byers et al. (1978).

Extraction and analysis
A modi ed QuEChERS method was adopted for the extraction of target PAH and PCB compounds (See Table 1 for the complete list) (Salem et al., 2016). In summary, 1 g of the sediment sample was accurately weighed into a 50 ml centrifuge tube and hydrated with 10 mL of millipore water. Acetonitrile (6 mL), containing 1% glacial acetic acid was added, followed by vigorous shaking using a vortex mixer for 2 minutes to obtain homogeneity. To this mixture, 3 g magnesium sulphate (MgSO 4 ), 0.7 g sodium acetate (C 2 H 3 NaO 2 ), and 0.7 g sodium acetate trihydrate (C 2 H 9 NaO 5 ) were added, followed by vigorous shaking to ensure complete dispersion. Samples were then centrifuged at 5000 rpm for 10 minutes. 7 ml of the organic extract was transferred into a centrifuge tube containing 0.5 g MgSO 4 , 0.2 g C18, and 0.2 g primary secondary amine (PSA). This mixture was vortexed, then centrifuged to isolate the clean extract. A 4 mL aliquot of the extract was evaporated to dryness under vacuum at a temperature ≤ 40°C, reconstituted in hexane (1 mL) before transferring into a GC vial for analysis.
The analysis was performed by two-dimensional gas chromatography − time-of-ight mass spectrometry (GC X GC-TOFMS) using an Agilent 7890 GC coupled to a Leco Pegasus 4D TOF mass spectrometer. The separation was achieved using a Restek Rxi-5Sil MS column with Integra-Guard (30 m × 0.25 mm i.d. × 0.25 µm lm thickness) coupled to a Rxi-17Sil MS (1.1 m × 0.25 mm i.d. × 0.25 µm thickness) secondary column. A 2 µl sample was injected in splitless mode using ultra high-purity helium as the carrier gas at a constant ow rate of 1.4 ml min -1 . The transfer line and inlet temperatures were set at 260 and 250 oC, respectively. The ion source temperature was set at 250 oC, with an acquisition delay time of 180 seconds and an acquisition voltage of 1850 V. The initial oven temperature was set at 80 oC, held for 0.5 min, and then increased to 180 oC at a rate of 20 oC/min, and nally to 300 oC at 5 oC/min. The secondary oven and modulator temperatures were offset at 5 oC and 15 oC, respectively, relative to the primary oven temperature. Peak identi cation and data processing were performed using the Leco ChromaTOF software and databases. Peaks were identi ed based on the retention time of speci c ions and con rmed by two identi er ions.

Quality assurance and quality control
Quanti cation was performed using high purity (> 98%) reference standards purchased from Sigma Aldrich. Linear regressions derived from matrix-matched calibration curves for individual PAH and PCBs were ≥ 0.99. Sample extracts were analysed in triplicate with relative standard deviations typically < 12% for PAH and PCBs respectively. Quality control standards were run after every third sample to monitor and correct for variations in instrument response. Detection limits were in the range of 0.04-1.1 µg/kg (PAHs) and 0.03-0.39 µg/kg for PCBs, respectively. Analysis of data was conducted using the two-tailed student t-test to investigate the difference in mean concentrations of sediment samples from the Klipriver tributaries and Klipspruit. Descriptive data analyses were conducted using Microsoft excel 2018, all test was considered signi cant at p < 0.05.

PAH and PCB concentrations
The PAH and PCB concentrations measured were normalized using total organic matter (TOM) estimates to account for the variability in the organic content of the samples and are quoted as dry weight (dw) concentration. PCB and PAH congeners investigated in this study were found to be widely distributed in sediments collected from the Klip River wetland and its tributaries. Total PCB (10.5 ± 0.2 mg kg − 1 ) and PAH (274 ± 4.9 mg kg − 1 ) concentrations in surface sediments collected from Klipspruit tributary was signi cantly higher than the ∑PCBs (6.4 ± 0.2 mg kg − 1 ) and ∑PAHs (156 ± 3.6 mg kg − 1 ) detected in sediments from Klipriver tributary. Chrysene (21 %), benzo [a] anthracene and pyrene (13 % respectively), and phenanthrene (9 %) were the four dominant PAH congeners detected at both tributaries and they accounted for > 55 % of ∑PAH concentration at both tributaries. While PCB 180 and PCB 195 (15 % respectively), PCB 118 (13 %) and PCB 101 (9 %) accounted for > 50 % of ∑PCB concentrations at both Klipriver and Klipspruit tributaries ( Table 1). The least detected PAH congener (acenaphthylene) contributed ∼ 2 % to ∑PAH concentration while PCB 52 contributed ∼ 3 % ∑PCB concentrations (Table 1).
Spatially, there was an inconsistent variation in ∑PAH and ∑PCB concentrations at both tributaries (Fig. 2). The highest ∑PAH concentrations were detected at site S1B of the Klipspruit tributary, with a total mean concentration of 95 ± 6.7 mg/kg, while site S3A of the Klipriver tributary recorded the lowest mean concentration of 44 ± 2.6 mg/kg across all the sites investigated (Fig. 2). ∑PAH concentrations decreased downstream towards the wetland at both tributaries; however, the decrease was not signi cant and was inconsistent. On average, the Klipspruit tributary (69 ± 4.6 mg/kg) showed a higher level of contamination of ∑PAH compared to the Klipriver tributary (52 ± 3.6) (Fig. 2, Supplementary Table S1). The total mean concentration of PAHs at both tributaries is signi cantly different at a 95% con dence level.
The highest and the lowest mean ∑PCB concentrations were recorded at Sites S3B located on the Klipspruit (5.3 ± 0.2 mg/kg) and S1A on the Klip River (1.1 ± 0.07 mg/kg) respectively. Additionally, site S4B (lower reach of the tributary) on the Klipspruit (2.4 ± 0.08 mg/kg) recorded a higher residual concentration of ∑PCB compared to site S3A the lower reach on the Klip River (1.2 ± 0.06 mg/kg), which is a similar trend of observation made for ∑PAH concentrations. Generally, average ∑PCB concentrations along the Klipspruit (2.6 ± 0.2) were higher than that detected along the Klip River (2.1 ± 0.09) ( Fig. 2, Table S1). There is no signi cant difference in the total mean concentration of PCBs at both tributaries. Table 1 Average concentration (mg/k ± SD, n = 3), percentage contribution, and potential source identi cation ratio of individual congeners and physico-chemical parameters present in the surface sediments from the Klip River and Klipspruit tributaries.

Downcore variation in the distribution of PAHs and PCBs
There was an inconsistent but gradual increase in mean ∑PAH concentrations with increasing depth to ~ 160 cm where the highest concentration of 6.3 ± 0.49 mg/kg was detected. THE Lowest ∑PAH concentrations (2.4 ± 0.22 mg/kg) were detected in the surface sample where the core sample was collected in the wetland ( Fig. 3a; Supplementary Table S2). The distribution of PAHs varied according to the number of aromatic rings. Four-membered ring compounds contributed the highest of 37 % followed by the two-membered ring compounds (30%). The three-and ve-membered rings recorded approximately 23% and 9%, respectively (Supplementary Table S2). The two and three rings showed similar downcore trends, with a substantial increase in concentration around 180 cm. The four-and ve-membered rings also showed similar downcore trends with the highest concentrations measured between 40-60 cm and 150-180 cm respectively (Fig. 3b).
Total PCB concentrations through the sediment core showed substantial variations, with concentrations ranging between 0.17 mg/kg and 0.8 mg/kg (Fig. 3c, Supplementary Table S3). The highest concentrations (0.8 ± 0.028 mg/kg) were detected at 160 cm but were lower than the measured concentrations in surface sediment samples (1.1-5.3 mg/kg). The distribution of PCBs according to the number of chlorine atoms revealed 4-6 chlorinated PCB compounds to be the dominant group. This group contributed 64% to ∑PCB concentrations followed by the 7-8 chlorinated PCBs at ∼25%. The 3 chlorinated PCBs contributed the least with about 10% (Fig. 3d, Supplementary Table S3). All three classes of PCBs showed similar patterns down the core with gradual increases in concentration from the surface to 180-200 cm (Fig. 3d).

Distribution and composition of PAHs and PCBs within the tributaries
The results of this study indicate that organic contaminants, speci cally PCBs and PAHs have accumulated in sediments of the Klipriver and the Klipspruit tributaries. This is consistent with a recent study that reported the presence of organic pollutants such as PAHs (Pheiffer et al., 2018), PCBs, PCDD, and PCDF (Rimayi et al., 2016(Rimayi et al., , 2017 in sediments of the Klip River system. The distribution of ∑PAHs within the study area indicates that the Klipspruit tributary is more contaminated compared to the Klipriver tributary (Fig. 2, Table 1). The highest PAH concentrations were recorded at the most upstream sites along the Klipspruit and Klipriver tributaries, with concentrations decreasing downstream towards the wetland (Fig. 2).
The upstream sites of the Klipspruit are proximal to several major highways including N1, R41, and to a lesser extent R12. In the case of Klipriver, the R41 is the only major highway that crosses this site. The N1 highway is known to be one of the busiest highways running through the city of Johannesburg and in combination with the two other highways may account for the greater concentration of PAHs detected at Klipspruit tributary relative to Klipriver tributary. According to Caricchia et al. (1999), 90% of the total PAH emissions worldwide are from stationary sources, but this may not be true in urban settlements where mobile sources from vehicular exhaust are dominant. Sibiya, (2012) reported a 5% increase in the amount of PAHs released from vehicular emissions in the city of Johannesburg from 2005 to 2011. This likely results from an increase in the number of vehicles that use these roads due to a growing middle class, particularly in Johannesburg. In addition, emissions from industrial activities, which are more predominant along the Klipspruit tributary may contribute to the high levels of PAHs at these sites compared to the Klipriver tributary.
The downstream sites of both tributaries are in close proximity to the residential settlements of Soweto and Lenasia (Fig. 2). Coal contributes 75% of the total energy consumption in South Africa, of which 3% is used in household cooking and heating.
However, this 3% is estimated to contribute to 65% of the total air pollution in South Africa (Balmer, 2017;Jeffrey, 2005).
Hence, the amount of PAH burden contributed downstream of both Klipriver and Klipspruit tributaries could be attributed to the domestic combustion of coal/wood for cooking and heating in and around the densely populated areas of Soweto and Lenasia (Fig. 2).
Similar to PAHs, ∑PCB concentrations revealed a similar pattern downstream with site S3B decreasing downstream toward the wetland (Fig. 2). Alberton is an industrial town located upstream of the Klip/Vaal River catchments, which is associated with many industrial activities (Rimayi et al., 2016). Jooste et al. (2008) reported Alberton to be a potential hotspot for dioxinlike compounds like PCBs. The concentrations of PCBs detected in the Klipriver and Klipspruit tributaries could be as a result of numerous anthropogenic activities in and around the industrial complex of Alberton, such as the incineration of PCB equipment, coal combustion, and leakages from transformer oil (Rimayi et al., 2017;Samara et al., 2006), municipal solid waste incineration, hospital waste, sewage waste, domestic coal combustion, and e uents from sewage plants located within proximity of the downstream sites (Brent and Rogers, 2002;Rimayi et al., 2016). Additionally, the National Energy Regulation of South Africa (NERSA) reported leakages of transformer oil (which is a major distributor of PCBs) as a common occurrence at electricity substations in Gauteng and other provinces (NERSA, 2015). Furthermore, due to a lack of air pollution regulations that were non-existent before April 2015 in South Africa, most industries located downstream were under no obligation to control air pollutions and resulted in the uncontrolled and unregulated release of incomplete combustible gasses into the environment (Brent and Rogers, 2002).

Vertical distribution of PAHs and PCBs in the Klip River wetland
Industrial and agricultural activities in ict high contaminant pressure on groundwater quality, which can affect groundwaterdependent wetland ecosystems (Dimitriou et al., 2008). Some sewage treatment plants are situated at the Olifontsvlie which is located along the upper reaches of the wetland system (Fig. 2).

Identi cation of potential sources of PAHs and PCBs
Overall, four-and three-member rings were the most dominant PAH congeners detected within the study area (Fig. 4a), this is similar to what was reported by Pheiffer et al. (2018). The Klipspruit tributary recorded higher contaminant burdens compared to the Klipriver tributary, with two membered-ring congeners dominating the wetland site (Fig. 4a). The presence of two and three membered-ring congeners is typically indicative of a petrogenic source (petroleum combustion), while four and ve membered-rings are often signature to pyrogenic sources (combustion of biomass) (Culotta et al., 2006). Both tributaries and wetland indicated a mix of petrogenic and pyrogenic sources of contamination.
Diagnostic source identi cation of PAHs in sediments of the Klip River wetland was performed using Fla/(Fla + Pyr), Ant/(Ant + Phe), and BaA/(BaA + Chr) ratios. Petrogenic processes promote the formation of phenanthrene over anthracene because it is thermodynamically more stable, leading to [An/(An + Ph)] ratios less than 0.1. Conversely, the high temperature required for pyrogenic processes in producing PAHs favours the formation of anthracene, thereby increasing the [An/(An + Ph)] ratio above 0.1 (Nieuwoudt et al., 2011;Yunker et al., 2002). Similarly, uoranthene and pyrene ratios can also be used to diagnostically identify PAH sources. Fl/(Fl + Py) ratios greater than 0.5 usually indicate pyrogenic sources, while a ratio of less than 0.5 indicates a petrogenic source (Nieuwoudt et al., 2011;Yunker et al., 2002).
The Ant/(Ant + Phe) values for all sites across both tributaries were 0.1, which suggests a pyrogenic source of contamination from the combustion of coal and biomass ( Fig. 4c; Table 1). Fla/(Fla + Pyr) values suggest approximately 63% of the site's pollution to be from dominantly petrogenic sources (incomplete combustion of petroleum), while the remaining 37% is from the combustion of biomass (Fig. 4c&d). The BaA/(BaA + Chr) values also suggest approximately 63% of all site contamination was from petrogenic sources while the remaining 37% from mixed petrogenic and pyrogenic sources ( Fig. 4c&d; Supplementary Table S2).
Hence, the petrogenic sources of contamination at upstream sites speak to their proximity to major highways (N1, R41, and R12), while the pyrogenic sources further downstream is indicative of anthropogenic activity such as sewage treatment, municipal waste incineration, and domestic coal combustion from nearby residential settlements of Soweto, Lenasia and to a lesser degree Alberton (Fig. 2).
At all catchment sampling sites, PCB congeners with 4-6 chlorine substituents (PCBs 101,118,153,138,and 149) were the most dominant contributing ∼ 45% to ∑PCB concentration, followed by congeners with 7-8 chlorine atoms (PCBs 180 and 195) ∼ 30 %. The percentage of 4-6 chlorinated PCBs were fairly evenly distributed across all sites (Fig. 4b). Contamination patterns characterizing the source and distribution of PCBs are not well understood in South Africa (Hu et al., 2011). Major sources of PCBs are attributed to Aroclor 1242, 1254, and 1260; trade names for commercial PCB mixtures (Rimayi et al., 2017). Aroclor 1242 is dominated by 2-3 chlorinated PCBs (PCBs 18, 31, 44, and 52) while Aroclor 1254 is dominated by 4-6 chlorinated PCBs, and 7-8 chlorinated PCBs are signature to Aroclor 1260 (Rimayi et al., 2017). Aroclor 1242, 1254, and 1260 were commonly used in electrical transformers imported by the energy utility provider of South Africa (ESKOM), most of which are still in use (Gray, 2004). The 4-6 chlorinated PCBs were the predominant congeners across all sites investigated and were widely detected in both tributaries and the wetland (Fig. 4b). The 7-8 chlorinated PCBs were more dominant upstream of both tributary sites, followed by a decrease downstream towards the wetland (Figs. 2 & 4b). The 2-3 chlorinated PCBs were the least dominant at all sites (Fig. 4b), which is consistent with previous studies conducted on the Klipriver and Jukskei tributaries (Rimayi et al., 2017). The distribution of PCBs in the environment can be in uenced by the loss of the less chlorinated PCB congeners through volatilization, sedimentation, and microbial degradations Gakuba et al., 2015). Low chlorinated PCBs (2-3 chlorines) are generally more soluble in water than the high chlorinated ones (Borja et al., 2005;Rimayi et al., 2017). The low concentrations of 3-chlorinated PCBs at all sites in this study may be attributed to their relatively higher solubility in water, losses through volatilization, and microbial degradation. Despite transformer oil leakages reported by NERSA (2015) in Gauteng province including areas surrounding Klipriver and its tributaries, Aroclor 1242 (a mixture of low chlorinated PCBs) were found in relatively low concentrations in this study.

Comparison with other relevant studies
Mindful of the fact that quantitative comparison between different data sets is complicated by differences in time of sampling, nature of the sediment analyzed, analytical methods used, storage conditions, and the number of congeners investigated, this nonetheless allows data from the current study to be placed within context. Compared to a study conducted by Pheiffer et al. (2018) within the same study area (Klip River system), ∑PAH concentrations reported in this study are three times higher. Other regional studies conducted within the Jukskei River (Sibiya, 2012) and the Vaal triangle (Nieuwoudt et al., 2011) report ∑PAH concentrations in sediments that are >2-3 times higher than the concentrations detected in the current study (Supplementary Table 2). The levels of ∑PAH reported in the current study are within the same order of magnitude as those detected in other international studies from Europe, the Americas, and Asia (e.g. Pereira et al., 1996;Anderson et al., 1996;Budzinski et al., 1997;Mai et al., 2002;Chen et al., 2015;Hussain et al., 2015). An exception is Shi et al., (2005) where ∑PAH was found to be about 100 folds more than was detected in the current study (Table 2). San Francisco Bay in USA 16 3-28 (Pereira et al., 1996) San Diego Bay in the USA 17 0.08-20 (Anderson et al., 1996) Arcachon Bay in France 16 0.03-4 (Baumard et al., 1998) Gironde Estuary in France 16 0.02-5 (Budzinski et al., 1997) Todos os Santos Bay in Brazil 16 0.01-2 (Nascimento et al., 2017) Generally, ∑PCB concentrations detected in this study were greater than the concentrations detected in samples from the Umgeni River in Durban, South Africa (Gakuba et al., 2015). Similarly, studies from the Americas (Alonso-Hernandez et al., 2014;Combi et al., 2013) and Yamuna River in Delhi, India (Kumar et al., 2008) had ∑PCB concentrations that were signi cantly lower than what was detected from the current study (Table 3). Conversely, the levels of ∑PCBs detected in other studies did not differ substantially from the current study, except for a study conducted by Li et al. (2005) on sediments from Sheboygan River, Wisconsin USA, where ∑PCB levels were more than 100 times the levels of the current study (Table 3).  (Gakuba et al., 2015) International studies Minjiang Estuary China 1.0 (Xing et al., 2005) Haihe Estuary, China n.d-0.3 (Zhao et al., 2010) Yangtze Estuary, China 0.002-0.2 (Gao et al., 2013) Liaohe River, North China 0.005-0.2 (Lv et al., 2015) Yamuna River in Delhi in India 0.0002-0.02 (Kumar et al., 2013) Eastern Romania 0.02-0.2 (Dragan et al., 2006) Urban river (Huveaune) in France 0.003-0.4 (Kanzari et al., 2014) Naple Harbour, Italy 0.001-0.9 (Sprovieri et al., 2007) Sheboygan River, Wisconsin in the USA 0.1-104  Sea Lots, Trinidad 0.06-0.6 (Mohammed et al., 2011) Gulf of Batabanó, Cuba 0.0001-0.0003 (Alonso-Hernandez et al., 2014) Guaratuba Bay, Brazil nd-0.006 (Combi et al., 2013) 4.5 Ecological Risk assessment A congener-based approach of analyzing PAH toxicity using effect range median (ER-M) and effect range low (ER-L) guideline values were used for all the investigated sites. According to Long and MacDonald, (1998), congener concentrations exceeding ER-M guideline values pose ecotoxicity to the biota and human health. PAH congeners such as uorene, pyrene, benzo(k) uoranthene, benzo(b) uoranthene, and benzo(a)pyrene all exceeded ER-M guideline values at one site each (Table 4). Of concern is the classi cation of benzo(a)anthracene and benzo(k) uoranthene, particularly as human carcinogens by the US-EPA (2002). Long and MacDonald (1998) recommended an approach to assessing the combined ecological risk from multi-toxic PAHs on the M-ERM-Q method, calculated according to the equation below.

M -ERM -Q =
Where Ci is the concentration of congener i in the sediment, ERMi is the ERM value for congener i, and n is the number of compounds. Values less than the ERM are below the lowest acceptable limit and are said to be non-toxic, while values exceeding the ERM are greater than the acceptable limit and are considered highly toxic. According to the benchmark suggested by Han et al. (2018), toxicity levels of 0.1, 0.1-0.5, 0.51-1.5, and 1.5 were classi ed as sites of low, medium-low, medium-high, and high toxicity, respectively (Fig. 5). The only site identi ed to display a high level of PAH toxicity was located upstream of the Klipspruit tributary (S2B) and is surrounded by heavy industrial activities and motorways. The other n upstream site of the Klipspruit tributary (S1B) and the two upstream sites of the Klipriver tributary (S1A and S2A) all showed medium levels of toxicity (Fig. 5).
All sites investigated exceeded the ERL guideline values for ∑PCBs with three of the sites exceeding the ERM guideline values ( The direct environmental impact of PAHs to nearby residents can be considered moderate; however, the presence of the two carcinogenic congeners which exceeded their respective ERM guideline values for multiple sites upstream may result in possible adverse impacts on both the ecology of the wetland as well as humans who rely on resources from the wetland. The overall assessment of all sites in the study area indicates PAHs and PCBs have the potential to cause acute toxicity such as carcinogenic and mutagenic effects to biota. PCBs like PAHs can bioaccumulate in benthic invertebrates, which later biomagni es in the food chain through sh predators (Clements et al., 1994). Aquatic life is susceptible to chronic effects of PAHs upstream of both river tributaries, and magni ed PCB concentrations could affect the diversity and population of these organisms (Pheiffer et al., 2018). Further investigations into the acute toxicity effects of PAHs and PCBs are required at the study site in order to better predict and understand the potential effects on human health.

Conclusions
This study was designed to investigate the bioaccumulation of PAH and PCB contaminants within the tributaries and wetlands of the Klip River. This is an important asset that protects and supplies water to residents in the city of Johannesburg, South Africa. Residues of PAHs and PCBs were found to be widely distributed in sediments of the Klip River wetland and tributaries. Residual concentrations reported here were among the highest found in South Africa, including those from an earlier study that was conducted within a similar area to this study.
Discharge of industrial e uent, proximity to busy highways, and possible leakage of oils from electrical transformers and capacitors were identi ed as potential sources of pollutants to the Klip River system. Furthermore, some sites were identi ed as posing moderate to high levels of risk, with the majority of congeners investigated in this study exceeding critical soil guideline values. This study highlights the importance of the Klip River system in ltering and sequestering pollutants that would otherwise likely enter the Vaal Dam further downstream and potentially affect the supply of potable water in the region.
Findings from this study raise some pertinent questions and present opportunities for future research, including a) in-depth investigation to better understand the accumulation of these compounds in different environmental compartments and their implication on the food chain within the study area and b) asses impact of these compounds on the health of residents of Soweto and Lenasia. Finally, the study highlights the need for a more regular and formal monitoring program in the area to help in management decisions regarding the wetland. Second author (Archibold Buah-Kwo e) conceptualised the idea, proof-read the writeup and gave correction for the nal draft of the manuscript.