Trace metal contamination and risk assessment of the Sand River, Limpopo Province, South Africa
Ngonidzashe A.G. Moyoa and Mmaditshaba M. Rapatsaa
aAquaculture Research Unit, School of Agricultural and Environmental Sciences, Faculty of Science and Agriculture, University of Limpopo (Turfloop Campus), Private Bag X1106, Sovenga, 0727, South Africa
Corresponding author: [email protected]
Abstract— The primary objective of this study was to determine trace metal contamination in water, sediment, grass and fish in Sand River which passes through an urban area. Ecological risk assessment and human health risk assessment were undertaken. The suitability of Sand River and the surrounding boreholes for irrigation was also determined. This study was undertaken between January, 2017 and January, 2018. Samples were taken on a monthly basis from the four sites. Site 1 was upstream of the Polokwane sewage treatment works, sites 2, 3 and 4 were downstream. Ten boreholes were randomly selected and the water was tested for anions and cations. The mean concentration of trace metals in Sand River water after sewage effluent discharge were Fe (0.43 mg/l), Mn (0.13 mg/l), Pb (0.09 mg/l), Cu (0.01 mg/l), Zn (0.01 mg/l), Cd (0.01 mg/l). Trace metal concentration in the sediment, grass and fish after discharge followed the order Fe>Mn>Zn>Cu>Pb>Cd. The geo-accumulation index showed that the sediment of the Sand River was not contaminated with trace metals. The health risk assessment index showed that consumption of fish from the Sand River was risky because of the high lead levels. Sodium adsorption ratio (SAR) and soluble sodium percentage (SSP) were used to determine the suitability of Sand River and borehole water for irrigation. The SAR for the Sand River was 2.54 and SSP was 49.7%. Both these values indicate the suitability of the Sand River for irrigation. The SAR and SSP for the all boreholes were below that of the Sand River. The borehole water is even more suitable for irrigation than the Sand River.
Keywords: crops, geo-accumulation index, sediment, sodium adsorption ratio
Trace metal contamination of rivers passing through urban areas is a growing major concern in Africa. Industrial and domestic effluents are the significant sources of trace metals of rivers passing through these urban areas (Moyo and Phiri, 2002). The Sand River is one such river that passes through the rapidly growing city of Polokwane in Limpopo Province, South Africa. This river is a major repository of wastewater. Many industries in developing countries do not have wastewater pre-treatment facilities. They discharge poor quality effluent in the municipal system. A number of industries are associated with both organic and inorganic pollutants. For example, the tanning and leather industry is associated with high levels of chromium. The sugar industry is associated with high level chemical oxygen demand. Textile industry is associated with high suspended solids, mercury and phenolics. The fertilizer manufacturing industry is associated with high levels of cadmium and lead. Poultry and red meat industry are associated with high levels of suspended solids, BOD and TDS. With rapid urbanization the problem of pollution has become more urgent, particularly trace metal contamination. The main problem with trace metal contamination in these rivers, is not only the human health risk associated with some of these metals, but also the damage that trace metals cause to aquatic life (Canli et al., 1999). Sediments are an important sink for these trace metals and several factors affect metal mobility between the sediment and water. These include pH, conductivity, redox potential and bioturbation (Bryan and Langston, 1992; Caussy et al., 2003). Metals in unpolluted sediments are mainly bound to silicates and primary minerals, and these metals are mostly immobile and are not biologically available (Pardo et al., 1990). However, in polluted sediments the metals are mobile and bound to different phases of the sediments. Most of the metals discharged into rivers flowing through urban areas are from sewage effluent, industrial effluent and storm water and precipitate at the bottom of rivers in the sediment.
The city of Polokwane is one of the fastest growing urban areas in South Africa (Seanego and Moyo, 2013). A number of new industries have been commissioned in the city in recent years. Both domestic and industrial effluents are discharged into the Sand River. Whilst the effect of organic pollutants discharged into the Sand River have been evaluated (Seanego and Moyo, 2013), no work has been undertaken on heavy meatal contamination. This is despite that a number of local people catch fish from the Sand River for household consumption. Furthermore, the Sand River is in close proximity to a metal processing plant, the Polokwane Smelter. Smelting processes can cause high levels of pollution in the soils and rivers (Ettler, 2016). A number of trace metals such as manganese, iron, lead, arsenic, chromium, cadmium, nickel and copper may be released from smelting plants. These metals may be released as fine particles into water bodies via a chimney or emissions from general operations. Dust particles bearing metals can travel long distances to pollute the soil and surface water ways. Thus, it is important to investigate the pollution levels in the Sand River because of its proximity to the Polokwane smelter
A number of farmers downstream of the Polokwane sewage treatment works (STW) irrigate their crops with Sand River water. This is a common practice in semi-arid areas where rainfall is inadequate. In South Africa, nearly half the effluent produced by local authorities is used for irrigation (Akpor and Muchie, 2011). In recent years the quality of effluent discharged into the Sand River from the Polokwane STW has deteriorated (Seanego and Moyo, 2013). The deterioration in the quality of sewage effluent is now a common phenomenon in most developing countries and this is largely attributed to the overloading of sewage treatment plants. (Morrison et al., 2001; Samie et al., 2009; Britz et al., 2013). However, the use of water that is polluted with domestic effluent for crop irrigation may result in high nitrate and chloride levels (Moyo, 2013). High salt levels in river waters in South Africa are exacerbated by the predominance of saline underlying geology (Halls and Gorgens, 1978). However, despite the widespread use of Sand River water for irrigation, no studies have been undertaken to ascertain its suitability for irrigation. Furthermore, the ecological risk to aquatic systems of trace metals in the sediments has not been evaluated.
Polokwane municipality artificially recharges groundwater with effluent from the final maturation ponds. The recharging of the Polokwane aquifer has been practiced for the last 30 years. It is possible to recharge the aquifer because the Sand River flows over a wide layer of alluvium. The alluvium are granite-gneiss rocks which have been weathered and fractured. It is through this weathering and fracturing up to a depth of 60 m that led to the formation of the Polokwane aquifer which is now being recharged with water from the maturation ponds. Boreholes are found along the Sand River and the Polokwane municipality uses these boreholes to draw water from the aquifer during times of drought. Local farmers also use the borehole water to irrigate crops However, no studies have been done to assess the suitability of borehole water from the recharged aquifer for irrigation.
The objectives of this study were to (i) determine the concentrations of selected trace metals in water, sediment, plants and fish in the Sand River, (ii) determine the suitability for irrigation of borehole and Sand River water using the sodium adsorption ratio (SAR) and soluble sodium percentage (SSP), (iii) conduct a human health risk assessment on people consuming fish from the Sand River and (iv) determine the ecological risk using the geo-accumulation index.
1.2 Materials and methods
1.2.1 Study Area
The Sand River is a tributary of the Limpopo River. Its source is Mokopane which is 45 km from Polokwane. The river flows on edge of Polokwane City. The Polokwane sewage treatment works discharges wastewater into the Sand River. Water, sediment, grass (along the margins of the river), and fish samples were collected at different sites along the Sand River. Samples were collected once a month at site 1, upstream of Polokwane WWTW discharge point, Site 2 (1.67 km after discharge) and sites 3 and 4 (6.60 and 10.67 km after discharge, respectively) (Fig 1).
1.2.2 Sample collection
Sampling took place from January, 2017 to January, 2018. Polyethylene sampling bottles (250 ml) washed with de-ionized water were used to collect water samples monthly from each site. Water samples were collected in duplicates just below the surface at a depth of 10 cm and stored in ice during transportation. At the Aquaculture Research Unit (ARU) Laboratory, 10 ml of 65% nitric acid was added to preserve the samples. The samples were then stored at 4°C until analysis. Water pH and conductivity were determined monthly on site using a YSI meter (MPS, 556).
Sediment samples from each site were collected in duplicates 20 cm below the river surface and placed in 250 ml polyethylene bags using a hand trowel. The samples were immediately kept in ice and transported to the ARU Laboratory, where they were kept in a freezer (-20°C). Ischaemum fasciculatum grass was sampled at each of the four sampling sites along the river. The sampling was also done in duplicates. I. fasciculatum were put in clean plastic bags that had been rinsed with de-ionized water. The grass was kept in a freezer at -20°C. a seine net of 50 mm mesh size was used to capture fish (Oreochromis mossambicus, Peters, 1852) at each site and placed in 5L buckets. This fish species is caught and consumed by local people. The fish were euthanized using MS-222 (tricaine methanesulphonate) before being sacrificed. Muscle tissue was excised and put in polyethylene tubes and frozen at -20°C.
1.2.3 Determination of trace metals in water, sediment, grass and fish
Sediment, grass and fish samples were dried at 80°C for 48 hrs and ground into a powder. Each sample for grass and fish were placed in a beaker with 20 ml reverse aqua regia (3:1 cHNO3: cHCl) and allowed to stand overnight. Sediment samples were passed through 60 µm mesh sieve to separate large fractions. The sieved samples were then used for determination of trace metal analysis. Each sediment sample was placed in a beaker with 20 ml aqua regia (1:3 cHNO3: HClO4) and allowed to stand overnight. The mixtures were heated until dry before 20 ml of a 5M HNO3 solution was added. The samples were allowed to stand overnight and then filtered through Whatman No. 41 filter paper. The filtrates were transferred to a 100 ml volumetric flask and made up to the mark with 0.5M HNO3. Trace metal determinations were performed on a sequential plasma spectrophotometer (ICP-OES) using the calibration curve method. A similar protocol was used for water samples. The mean concentration of the trace metals in the Sand River was determined by taking the average of each metal at different sites for water, sediment, grass and fish.
220.127.116.11 Quality assurance and quality control
Water, sediment, grass and fish samples were sent to an accredited lab in Pretoria (ISO/IEC17025:2005). Quality assurance (QA) and quality control (QC) procedures were conducted by using standard references materials. Each batch of samples had one blank and one standard. The accuracy of the analysis was determined using certified standards (De-Bruyn spectropic solutions 500 MUL20-50STD2) and recoveries fell within 10% certified values. The chemicals used were analytical reagent grade. All samples were subjected to the same QA/QC.
1.2.4 Geo-accumulation index of trace metals
The geo-accumulation index of trace metals in sediment was calculated using Muller (1969)’s formula given below:
Igeo= log2Cn1.5 Bnwhere Cn is the measure of the metal concentration in the sediment, Bn is the background concentration of the element (average shale concentration, Turekian and Wedepohl, 1961), and 1.5 is the factor compensating background data (correction factor) due to the lithogenic effect. The geo-accumulation index consists of seven grades (0–6) indicating the degrees of metal accumulation and ecological risk (Table 1).
1.2.5 Human health risk assessment
A human health risk assessment was conducted following the methods used by the US environment protection agency (US-EPA, 2000). Heath et al. (2004) subsequently modified these methods for the South African situation. The average daily dose (ADD) was calculated as follows:
ADD= average metal concentration in fish x mass of portionadult body weight mass x number of days between fish meals
This calculation is based on the following assumptions; (i) the population at risk consumed 150 g portion of fish muscle once a week and (ii) the adult body mass of the population at risk was 70 kg. A hazard quotient HQ was calculated using the formula: HQ= ADDRFD where RFD is the expected exposure of the population to the reference doses. A HQ;1 indicates a low possibility of adverse health effects and HQ;1 indicates high possibility of adverse health effects.
1.2.6 Suitability of Sand River and borehole water for irrigation.
Suitability of Sand River water for irrigation was determined at the site before discharge and the sites after discharge as previously described. Duplicate Samples were collected at a depth of 20 cm using polyethylene water bottles. Calcium and magnesium content of the water was determined by EDTA titration using Mordant black 11 as an indicator. Multiple aliquots were collected and stored for the different type of analysis. Sodium and potassium content were determined using a flame photometer. Chloride concentrations were measured by silver nitrate titration (APHA, 2005). Carbonate and bicarbonate content was measured by acid-base titration (APHA, 2005). Sodium adsorption ratio (SAR) was calculated using the equation below:
SAR = Na/(Ca+Mg/2)
where the ionic concentrations of sodium, calcium and magnesium are expressed in milli equivalents per litre. Soluble sodium percentage (SSP) was calculated by using the equation below:
SSP = (Na) (100)/(Ca+Mg+Na+K)
where the ionic concentrations of sodium, calcium, magnesium, potassium are expressed in milli equivalents per litre.
Ten boreholes were randomly selected from the 67 boreholes located along the Sand River to test the suitability of borehole water for irrigation. Calcium, sodium, bicarbonate, magnesium, potassium, conductivity and pH were determined both in the Sand River and borehole water. Sodium adsorption ratio and SSP were also determined for borehole water as previously described.
1.2.7 Data analysis
The trace metal concentration of water, sediment, grass and fish from site 2,3 and 4 was averaged. These sites are after sewage effluent discharge point. An independent t-test was used to test for the differences in trace metal concentration before and after discharge. The Statistical Package and Service Solutions (SPSS version 20.0) was used for this analysis.
1.3.1 Trace metal concentrations in the sediment, water, grass and fish from the Sand River
Iron dominated the trace metal profile in the Sand River registering a mean concentration of 0.43 mg/l after discharge (Table 2). Average concentrations of manganese in the water were 0.02 mg/l before discharge and rose significantly (P;0.05) to 0.13 mg/l after discharge. Lead concentrations in the water were the same before after discharge and averaged 0.09 mg/l. Concentrations of cadmium, copper and zinc averaged 0.01 mg/l (Table 2). Levels of iron, manganese, lead, zinc and copper in the sediment significantly (P;0.05) increased after effluent discharge (Table 2). Cadmium concentration remained more or less the same. Iron, manganese, lead, zinc and copper levels increased in the grass after discharge (Table 2). However, cadmium levels remained the same. In the fish, cadmium, copper, manganese, lead and zinc levels increased after discharge (Table 2). The levels of iron decreased in the fish after discharge.
1.3.2 Ecological risk and human health risk assessment
The geo-accumulation of trace metals in sediment were all below zero before and after discharge for all the trace metals. (Table 3).
Before discharge, the hazard quotient was less than 1 in cadmium, copper, iron, manganese and zinc. However, lead had a hazard quotient greater than 1 at 5.37 (Table 4). There were insignificant (P;0.05) increases in hazard quotient of cadmium, copper, manganese and zinc after discharge. Lead hazard quotient significantly (P;0.05) increased after discharge (Table 4). The lead values after discharge were also above 1 (Table 4). Iron hazard quotient declined after discharge and was below 1.
1.3.3 Suitability for irrigation of the water from the Sand River and boreholes
The calcium levels averaged 29.40 mg/l in the river and ranged between 37.10 to 51.90 mg/l in the boreholes (Table 5). Sodium levels were much higher in the river averaging 69.58 mg/l whilst in the boreholes they ranged between 5.97 to 8.95 mg/l. The mean concentration of magnesium in the river was 16.22 mg/l whereas in the boreholes it ranged between 34.80 to 74.20 mg/l (Table 5). Potassium levels were very low in the river averaging 9.85 mg/l whereas in the boreholes the levels ranged between 120.83 to 165.20 mg/l. Conductivity in the river was 10.49 mS/m whereas among boreholes it ranged between 10.01 and 12.69 mS/m (Table 5). The bicarbonate levels in the river (209.99 mg/l) were lower than in the boreholes which ranged between 320.68 mg/l and 440.00 mg/l. The pH in the river was 7.20 and it ranged between 7.30 and 7.60 in the boreholes.
The sodium adsorption ratio (SAR) and soluble sodium percentage (SSP) were 2.54 and 49.7% in the river respectively (Table 6). On the other the hand the SAR was much lower in boreholes ranging between 0.11 to 1.53. The SSP was also lower in boreholes ranging between 2.0 to 20.6%. (Table 6).
1.4.1 Trace metal concentrations in water, sediment, grass and fish
In this study, iron and manganese levels dominate the trace metal composition of Sand River water. This is consistent with Edokpayi et al., (2016) who also showed that iron and manganese dominated the trace metals composition of the Mvudi River in Limpopo Province, South Africa. The dominance of iron is because of its high occurrence in different geological formations in Limpopo Province. The level of both iron and manganese increased after discharge and this indicates that the effluent from the Polokwane sewage treatment works may be a significant source of these two trace metals. The manganese levels after discharge did not exceed the DWAF, (1996) regulations for aquatic ecosystems (0.18 mg/l). DWAF, 1996 does not have any recommended limits for iron in aquatic ecosystems. For irrigation purposes, the DWAF regulations stipulate that the iron levels must be between 5 to 20 mg/l (DWAF, 1996). The levels of iron in the Sand River are below the target suggested by (DWAF, 1996). DWAF suggest that manganese levels for irrigation must range between 0.02 to 10 mg/l. The manganese levels are also within the target range. Cadmium levels in the water remained the same before and after discharge. Cadmium levels also fell within the DWAF target range of 0.01 to 0.05 mg/l for irrigation. Copper levels were also the same before and after effluent discharge. For copper the DWAF guidelines for irrigation are between 0.2 to 5 mg/l. In this study the copper concentrations were 0.01 mg/l and fell within the target range. The lead levels were the same before and after effluent discharge at 0.09 mg/l. The lead levels were also within the target range (0.2 mg/l) for irrigation. Zinc levels remained the same before and after effluent discharge. The zinc levels were way below the target range (1-5 mg/l) for irrigation water. However, both iron and manganese are not a threat to the aquatic ecosystem of the Sand River. Edokpayi et al. (2016) reported that manganese levels exceeded the DWAF recommended limits in the Mvudi River. The lack of elevated levels of all the trace metals in the water is probably because there is no mining activity around Polokwane. There is no evidence from this study to show that the Polokwane smelter deposits airborne trace metals in the Sand River.
Trace metal concentrations in sediment followed the order iron ; manganese; zinc ; copper ; lead ; cadmium. This is consistent with most previous studies (Butu and Iguisi, 2013; Ladigbolu and Balogun, 2013). There was an increase in copper, iron, manganese, lead and zinc after effluent discharge. This is circumstantial evidence showing that the Polokwane sewage treatment works may be one of the sources of trace metal contamination of the sediments. The other sources of trace metal contamination may be farming activity downstream because fertilizers and pesticides are used by the local farmers. There are also a number of illegal dumping sites next to the River. The leachate from these sites may also be a source of trace metals. There are no sediment quality guidelines in South Africa. However, in comparison to the Canadian sediment quality guidelines (2001) all the trace metals were below the probable effect levels (PEL). The PEL is the concentration above which adverse effects are likely to occur. The Canadian PEL levels for cadmium, copper, lead and zinc are 3.5, 197, 91.3 and 315 mg/kg respectively. The results from this study were way below this threshold suggesting that the trace metals in the sediments are not a threat to aquatic life. There are no sediment quality guidelines for iron and manganese. The overall redox state of the sediments plays an important role in the precipitation of iron and manganese. Anoxic conditions will alter the chemistry of iron and manganese which in turn may affect other metals that would have been bound to oxides of iron and manganese. Oxidizing conditions tend to favour the precipitation of iron from the water (Fijalkowski et al., 2012). Discharge of poorly treated effluent leads to low dissolved oxygen levels. This may in part explain the high iron and manganese in the sediment.
The physical nature of the sediment is also important in sedimentation and transport processes of trace metals. Sediments are characterized as coarse material, clay/silt and sand fractions; this classification is based on particle size. The clay/silt fraction has a high surface area and it is more likely to adsorb trace metal contaminants. Although sediment texture, was not measured in this study, it was however observed that clay/silt particles dominated most of the sites and this may account for the high iron and manganese levels found in the sediment of the Sand River. The rate of oxidation of manganese is slower than that of iron hence it is more readily transported through aerobic environments.
Geo-accumulation index showed that the sediment was not contaminated with any of the trace metals before and after discharge. This probably indicates that the industries in and around Polokwane do not discharge significant amounts of trace metals into the Sand River. Thus, the ecological risk to the aquatic ecosystem is very low. Dahms et al. (2017) used the geo-accumulation index to evaluate the ecological risk of trace metals in sediments of the Nyl River in South Africa. The geo-accumulation indices for copper, iron, manganese and zinc were all less than 1. Pheiffer et al. (2014) also used the geo-accumulation index to determine metal contamination of sediments in the Vaal River in South Africa. Except for copper the geo accumulation index for iron, manganese, cadmium and zinc were below 1. Although the sediment in the Sand River is uncontaminated, it is important to regularly monitor trace metals in sediments because metals that are bound to the sediment layer in an aquatic system can be released through changes in pH and resuspension of particles into the water body (Soares et al., 1999). It has been previously speculated that the Polokwane smelter which is 15 km away from the Sand River may contribute to trace metal contamination. This does not appear to be the case because the Polokwane smelter does not discharge any effluent into any river and the possibility of wind borne pollutants contaminating the Sand River is also remote because of the distance between the Sand River and the Polokwane Smelter.
The concentration of trace metals in the grass also followed the same order (iron ; manganese ; zinc ; copper ; lead ; cadmium) as observed in the sediments. The plant transfer factor (PTF) is the ratio of metal concentration of plants to sediments (Wang et al., 2012). The PTF values for cadmium, copper, iron, manganese, lead and zinc were 0.71, 0.47, 0.32, 0.51, 0.50 and 0.68 respectively. The trend in PTF of trace metals for aquatic plants followed the order cadmium ;zinc ;manganese ;lead ;copper ; iron. This trend is very similar to Wang et al (2012) who also showed that cadmium had the highest PTF and iron at the lowest PTF. It is therefore, important to monitor cadmium levels because it had the highest PTF levels.
Trace metal concentration in the fish followed the same order (iron ; manganese ; zinc ; copper ; lead ; cadmium) as in the sediment and grass. Trace metals enter the fish through the gills, skin and diet of the fish. Oreochromis mossambicus is a herbivorous fish that feeds on phytoplankton and detritus. There is very little phytoplankton in rivers and O. mossambicus does not feed on I. fasciculatum. It therefore, appears that the major sources of trace metals in O. mossambicus are either directly from the water or the sediment in the form of detritus. It appears that the major source of trace metals in the fish might have been the sediment because trace metal concentration in the fish followed the same order as in the sediment. The human health risk assessment showed that only lead exceeded the levels for safe consumption. Jooste et al. (2015) also showed that lead exceeded safe consumption levels in Clarias gariepinus from the Flag Boshielo dam in South Africa. High concentrations of lead can cause kidney damage and high blood pressure (WHO, 2015). Even at low concentrations lead can cause mental retardation in children under the age of 6 (WHO, 2015). It is thus not advisable for local people to consume fish from the Sand River because of the potential risk of lead poisoning.
1.4.2 Suitability of Sand River water and surrounding borehole water for irrigation
The SAR for the Sand River was 2.54. DWAF stipulated that SAR should be below 15 because irrigation water with high SAR will result in a build-up of sodium levels in the soil. This build-up may lead to displacement of calcium and magnesium in the soil which will lead to the loss of soil structure. The SAR values of the Sand River from this study suggest that Sand River water is suitable for irrigation. The SAR borehole values were much lower than the river. It is therefore evident that borehole water from around the Sand river is suitable for the irrigation of crops. The SSP was 49.7% in the river and the highest value from the boreholes was 20.6%. An SSP of more than 60% will result in sodium accumulation (Ayers and Westcot, 1985). As with SAR, SSP shows that the Sand River and boreholes surrounding the river are suitable for irrigation.
The trace metals in the Sand River water did not exceed the DWAF stipulated targets for both aquatic ecosystems and irrigation. The geo-accumulation index indicated that the ecological risk of the sediment in the Sand River was very low since all the trace metals were below 1. The human health assessment risk showed that consumption of fish from the Sand River may pose a health risk because the lead levels exceeded the international safe limits. The SAR and SSP suggest that the use of Sand River water is not hazardous to the soils because both SAR and SSP were in line with the DWAF targets.
We would like to acknowledge the financial support from the Water Research Commission and the Aquaculture Research Unit. Mr Gavin Geldenhuys assisted with sample collection and his efforts are gratefully acknowledged.
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