Assessing heavy metal pollution in the Rietspruit, Far West Rand Goldfield, South Africa

doi:10.21203/rs.3.rs-1706599/v1

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Assessing heavy metal pollution in the Rietspruit, Far West Rand Goldfield, South Africa

https://doi.org/10.21203/rs.3.rs-1706599/v1

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Since the discovery of gold in the Witwatersrand Basin, the Basin has experienced over 100 years of mining activities. However, the removal of the precious stones buried several meters below the earth surface has led to water and sediment pollution by heavy metals. In this study, the concentration of arsenic, cadmium, mercury and lead were determined using inductively coupled plasma mass spectrometry in the water and sediment samples collected in the Rietspruit. The Rietspruit is a perennial stream located in the Witwatersrand Basin, the source of the water is attributed to the discharge of mine water by a gold and uranium mine located upstream of the river. High concentration of the studied heavy metals were recorded within and some distance away from the mining environment. In this study, the maximum concentrations of arsenic, cadmium, mercury and lead recorded were 45.5 µg/l, 0.43 µg/l, 11.8 µg/l and 3.5 µg/l respectively in Rietspruit water. In the sediment, the maximum concentrations recorded were 550.4 mg/kg, 3.88 mg/kg, 2.44 mg/kg and 184.3 mg/kg in arsenic, cadmium, mercury and lead respectively. In areas where human exposure was observed, the concentrations of mercury was higher than the guideline limit of the metal in water. As a result, the use of the Rietspruit water for agricultural, domestic and religious purposes must be discouraged and adequate intervention of the study area is needed.

Arsenic

cadmium

mercury

lead

heavy metal

Rietspruit

gold mining

piper plot

South Africa

Gold was first discovered in South Africa in 1884 in the Witwatersrand Basin. Witwatersrand Basin contains the largest gold deposit in the world (Phillips, 2013). Since the discovery of gold in the Basin, the Basin has experienced over 100 years of mining operations (Durand, 2012). Mining activities in South Africa has been a major contributor to the economy of the country [1]. Besides the large deposit of gold found in the Basin, uranium has also been mined in the Basin (Coetzee et al., 2006; Winde and de Villiers, 2002). In the study of Winde (2004), high concentration of uranium has been recorded in gold tailings. It was estimated that for every ton of gold mined, about 10 tons of uranium has been brought to the earth’s surface (Winde, 2004).

Despite the positive contributions of gold mining operations to the economy and development of South Africa, toxic heavy metals have been released into the immediate lithosphere, hydrosphere and atmosphere (Humphries et al., 2017; Du Preez et al., 2016; Oelofse et al., 2007; McCarthy and Venter, 2006). In the Witwatersrand Basin, there are more than 270 slime dams (also known as tailings) with an estimated surface area of 400 km² (Hobbs et al., 2010). Slime dams are sources of air, soil and water pollution (Winde, 2004; Oelofse et al., 2007; Van der Linde, 2008; Ashanti, 2004). An estimated 6 billion tons of tailings have been produced since the advent of gold mining in the Witwatersrand Basin. High concentration of toxic heavy metals has been reported in windblown dust from tailings dam (Maseki et al., 2017; Wright et al., 2014; Rösner and van Schalkwyk, 2000). Maseki et al (2017) reported elevated concentration of arsenic (As), cadmium (Cd), lead (Pb) and uranium (U) in air at the Central and East Witwatersrand Basin. They concluded that the concentration of heavy metals found at these locations could be of health concern to nearby inhabitants (Maseki et al., 2017).

Heavy metals are also released from gold tailings when such tailings are exposed to rainwater, pore-water and storm-water run-offs (Winde, 2004; Bakatula et al., 2012). Due to the establishment of slime dams to nearby surface water, heavy metals are introduced into the water system. High concentration of U has been recorded in rivers’ water and sediment within the immediate environment of slime dams and several kilometers away from the mining environment (Raji et al., 2021; Winde and de Villiers, 2002). Raji et al (2021) recorded high concentration of U about 24 km away from a mining site in the Witwatersrand Basin above the World Health Organization (WHO) permissible limit of U in river water (30 µg/l) and sediment (≤ 4 mg/kg) (Raji et al., 2021). This is because heavy metals are adsorbed to fine particles of clay and silt which are deposited to river’s bed when there is reduction in the water flow velocity. Also, the precipitation and co-precipitation of heavy metals with the oxides of iron and manganese promote the enrichment of heavy metals in river’s sediment. Heavy metals can be defined as metals that are at least 5 times denser than water (Fergusson, 1990). They are found naturally in the environment at a very low concentration. However, anthropogenic activities such as mining has increased the concentration of heavy metals in air, water and sediment (Tchounwou et al., 2012b; Mohammed et al., 2011).

Heavy metals can be classified as essential and non-essential heavy metals (Mohammed et al., 2011; Duruibe et al., 2007). Essential heavy metals are important for human metabolism, examples include iron (Fe), zinc (Zn). However, they become toxic at high concentration (Tchounwou et al., 2012b). Non- essential heavy metals are toxic to humans even at low concentration. Examples include mercury, Hg, Cd, As and Pb. World Health Organization regards Pb, Hg, Cd and As, as one of the most toxic heavy metals to human (Tchounwou et al., 2012a; Järup, 2003). Human exposure to As at high concentration can lead to drowsiness, seizures, brain damage, peripheral neuropathy, hyperkeratosis, hypotension, haemolysis, inflammation of sensory and motor nerves etc (Järup, 2003; WHO, 2003a). Symptoms of human over-exposure to Cd include headache, vomiting, fever, lung damage, erythrocytopenia, anosmia, renal tubular dysfunction, proteinuria, osteomalacia and changes in liver functioning. Depending on age and concentration of exposure, the symptoms of Pb exposure include anorexia, constipation, and anaemia (WHO, 2004). Changes in kidney function, ataxia, brain damage etc. Mercury exposure affects the lungs, kidney, brain and skin. The symptoms include headache, fatigue, dyspnoea, shock and permanent brain damage, ataxia, erythrism and so many more (Adams et al., 2010; Tchounwou et al., 2012a).

This study focused on the concentration of As, Cd, Pb and Hg in the Rietspruit water and sediment. This is important because the Rietspruit main source of water is the outflow from the Peter Wright (PW) dam. A gold and uranium mine located upstream of the Rietspruit discharges about 68 ML of both fissure and process water into the PW dam daily (Kritzinger, 2017). Downstream, Rietspruit water is used for agricultural (irrigation of farmlands, animal watering), domestic and religious activities which include drinking, bathing and cooking. All these activities were observed during field work. Due to human exposure to these chemicals and lack of published literature on the concentration of these heavy metals in the river, it is very important to determine the concentration of these toxic heavy metals in the Rietspruit.

This study will be used to encourage downstream users of the water from the Rietspruit to avoid the usage of the water and also to inform regulatory water bodies in the country for adequate intervention. Therefore, the objective of this study is to determine the concentration of As, Cd, Hg and Pb in the Rietspruit water as well as sediment and to determine the extent of the pollution.

The gold and uranium mine is located (26°21'46"S 27°42'52"E) in the Witwatersrand Basin. Gold is mined in the Upper and Middle Elsburg reef in the form of native gold with associated pyrite while, Uranium is mined in the Middle Elsburg reef in the fold of uraninite (Kritzinger, 2017). Witwatersrand Basin is a sedimentary Basin of about 2.7 billion years old. It contains a stratigraphic sequence about 6 km thick, rich in quartzites and shales with some volcanic units (Phillips and Law, 2000).

The gold and uranium mine was decommissioned in 2016, however, daily discharge of underground fissure and process water of about 68 ML into the PW dam through concrete canal and pipes is ongoing (Kritzinger, 2017). The outflow from the PW dam into the Rietspruit averages about 1.9 MCM monthly (Kritzinger, 2017). This is the primary source of the Rietspruit water. The Rietspruit flow is non-seasonal as a result of the outflow from the PW dam (Fig. 1). It is believed that the Rietspruit will be a non-perennial river if the mine should stop discharging waste water into the PW dam.

The Rietspruit extends southwardly for about 63 km before joining the Vaal River (Fig. 1). Located downstream are wetlands and farm dams. Informal settlers’ e.g. Lawley, farmlands as well as cattle breeders depend on the Rietspruit for their water needs. Contributing tributaries include Evaton Rietspruit, Leeuspruit and Klein-Rietspruit (Fig. 1). Other Rietspruit tributaries were dry during sampling (Rietspruit trib 1–5 in Fig. 2). A gold mine is located upstream of the Leeuspruit (26⁰24'29"S 27⁰40'52"E), however, no mining or industrial activity is established upstream of the Evaton Rietspruit and Klein-Rietspruit. As a result, the concentration of heavy metal of interest at these sites will be regarded as the natural background concentrations of As, Cd, Hg and Pb. The study area extends over four different quaternary catchments, C22H, C22J, C22K and C23B according to Department of Water Affairs and Sanitation (Fig. 2).

Twenty-two of the sampled sites are located in the C22H and C22J quaternary catchment which drains about 1122.86 km². Two sites are located in the C22K quaternary catchment with drainage size of about 433.79 km² and the last sampled site is located in the C23B quaternary catchment with drainage size of about 701.07 km² (Fig. 2).

The climate of the study area falls under the Moist Highveld Grassland with annual potential evaporation more than double the annual precipitation in the province (Kamunda et al., 2016; Tyson and Wilcocks, 1971). The study area is defined by moderate cold winter (April to August) and warm summer (October to March). Average temperature is about 13⁰C in winter and 24⁰C in summer (Kamunda et al., 2016).

Field work

For data collection, water and sediment samples covering the whole length of the Rietspruit, pre-confluence of associated tributaries where collected through field work. Sampling commenced on the 28th of August, 2020. A total of 30 water samples and 32 sediment samples were collected. Water samples were collected in a 100 ml brown water bottles that was submerged in water as well as sediment samples collected at the same spot as the water samples. An augur was used to collect the sediment samples and kept in a zip-lock plastic bag. Before water samples were collected, the water bottle was rinsed three times with the river water and decanted downstream. Water samples were collected at the middle of the river in order to avoid sampling river banks and once filled the bottle was capped under water. At each site, water samples were collected first before sediment samples. This was done to avoid river disturbance which could lead to the resuspension of sediment

Each water and sediment samples were labelled at the sites of collection and kept in a cooler box with ice blocks. The water pH, electric conductivity and temperature readings were taken at each of the site using WTW 350i probe-meter. The probe-meter was calibrated using the prescribed calibration fluid. A Garmin eTrex Touch 35 handheld GPS was used to record the coordinates of each sampled site. After sampling, all samples were taken to the Eco-Analytical Laboratory of North-West University, Potchefstroom campus for laboratory analysis.

Laboratory analysis

Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of As, Cd, Hg and Pb in the collected water and sediment samples. Prior to the analysis by ICP-MS, each water samples were filtered using 0.45 µg/l pre-nylon sieve to remove every suspended particle in the water. 1 ml of the filtered water was mixed with 9 ml of nitric acid in order to bring the volume to 10 ml and remove any dissolved organic material in the water. High performance Chromatography was used to determine the concentration of anions in the filtered water samples.

Sediment samples were dried first in the oven at a temperature of 50 ⁰C overnight. This was done to remove the moisture content of the sediment. Each sediment sample was placed in a labelled petri-dish and arranged in the pre-heated oven using an iron-tray. After drying, a mortar and pestle was used to break the dried sediment and a sieve of mesh size of 0.002 m was used to remove gravel and plant particles. 200 mg of the sieved sediment was then placed in a Teflon Tube, 9 ml of nitric acid and 3 ml of hydrochloric acid was added to the Teflon Tube and arranged into a Milestone, Ethos UP, and Maxi 44 microwave digestion system. After digestion, an Agilent 7500 CE ICP-MS fitted with CRC technology was used to determine the concentration of As, Cd, Hg and Pb in the samples.

The ICP-MS was optimized using a solution containing lithium, yttrium, cerium and titanium (1 ppb) for standard low-oxide (≤ 1.5%) while maintaining high sensitivity across the mass range. To obtain quantitative results, the instrument was calibrated using ULTRASPEC certified custom mixed multi-element stock standard (De Bruyn Spectroscopic Solutions, South Africa) solutions containing all the elements of interest. Quality Control Standards are employed to ensure the correct QC criteria were met.

3.1. Rietspruit water chemistry

3.1.1. Rietspruit water pH

The maximum pH reading recorded in the Rietspruit was 8.8 around midday when the sun was at his peak. The Rietspruit water pH gradually increased as the sun rises. The minimum pH recorded was 6 late in the evening after sunset. The increase in pH in stream water is due to photosynthesis of aquatic plants and algae. Winde et al., (2004) also reported similar trend in pH measurement within the Witwatersrand Basin (Winde and van der Walt, 2004). The pH range of the river is within the World Health Organisation (6–9 (WHO, 2012)) and South Africa (5–9.7 (SANS, 2015)) guideline limit of pH in water. It has been reported that Cd, As, Hg and Pb have a varying relationship with pH. An increase in pH increases the concentration of dissolved arsenic in water (WHO, 2003a). Increase in water acidity dissolves Cd and Pb causing an increase in the concentration of dissolved Cd and Pb in water (WHO, 2004; WHO, 2003b).

3.1.2. Rietspruit water electric conductivity

Electric conductivity (EC) of a water is the measure of the ability of water to conduct electric current (Meride and Ayenew, 2016). Rietspruit water EC range between 443–1358 µS/cm. The EC recorded was below South Africa permissible limit in water used for drinking (SANS, 2015). The average of EC in the study area is 785.1 µS/cm. Electric conductivity is directly proportional to total dissolved solids in water and it is affected by the abundance of cations such as sodium, calcium, magnesium, potassium and anions such as chlorine, carbonate, bicarbonate, sulphate and phosphate. The EC of sites close to the mining site is higher (1203 µS/cm) than downstream EC. This can be related to the amount of total dissolved solids in the water discharged by the mine into the PW dam.

Water treatment plants’ discharge also increased the amount of EC recorded downstream as observed in the EC measurements downstream of the two water treatment plants (Fig. 1) in the study area (Site 10). Rietspruit water chemistry is tabulated below (Table 1). Each site was given a name and a corresponding site number. The approximated distance of each site from the outflow of PW dam was expressed in kilometre. At each of the associated Rietspruit tributaries, the distance from PW dam was expressed as NA (not available).

Table 1

Sampled sites information and water chemistry
Site number	Site name	Distance from PW dam (km)	pH	EC (µS/cm)	Temperature (⁰C)
Site 1	PW dam inlet	0	6,4	1203,0	9,3
Site 2	Channel water	0	7,5	771,0	26,0
Site 2	Corner of dam	0	7,8	771,0	25,8
Site 3	PW dam outflow	1	7,5	843,0	14,2
Site 4	Wetland 1	2	7,5	847,0	14,1
Site 5	Wetland 2 inflow	3	7,4	880,0	20,9
Site 5	Wetland middle	3	7,4	896,0	23,1
Site 5	Wetland outflow	4	7,5	891,0	23,8
Site 6	Damlit outflow	6	7,6	1182,0	12,9
Site7	Between damlit	6	7,7	1180,0	13,4
Site8	Religious site	8	7,6	843,0	13,3
Site9	Post religious site	8	7,8	842,0	12,4
Site10	Post bridge	12	7,3	1250,0	15,1
Site11	Poortjie	14	7,9	783,0	15,3
Site12	Pre Evaton	24	8,8	745,0	16,5
Site13	Evaton Rietspruit	NA	7,4	521,0	17,1
Site14	Post confluence	32	7,3	530,0	15,8
Site15	Pre confluence Rietspruit & Leeuspruit	37	7,1	540,0	15,5
Site16	Leeuspruit	NA	7,6	1358,0	13,5
Site17	Post confluence Rietspruit & Leeuspruit	39	7,3	538,0	15,6
Site18	Post confluence Rietspruit & Leeuspruit	43	7,4	548,0	15,6
Site19	Rietspruit (Potch rd)	49	7,1	565,0	12,1
Site20	Pre-confluence Rietspruit & Klein-rietspruit	53	6,0	578,0	8,8
Site21	Klein-rietspruit	NA	7,6	443,0	12,1
Site22	Post-confluence Rietspruit & Klein-rietspruit	55	7,2	582,0	13,7
Site23	Lochvaal inflow	56	6,9	612,0	13,5
Site24	Lochvaal	60	7,8	606,0	14,9
Site25	Post confluence Rietspruit & Vaal river	63	8,2	704,0	14,7
Maximum			8,8	1358,0	26,0
Average			7,4	787,6	15,7
Minimum			6,0	443,0	8,8
Standard Deviation (SD)			0,5	251,7	4,4

3.1.3 Water anions and cations concentration (Rietspruit water chemistry)

The concentrations of anions and cations in water can be used to determine the source of water contamination (Raji et al., 2021). The most influential anions and cations are sulphate, carbonate and bicarbonate, chlorine, calcium, magnesium, sodium and potassium. The concentrations of these anions and cations were used to deduce the source of water contaminant by plotting a piper diagram (Fig. 3) using Grapher 18 (Piper, 1944). Piper plot is a graphical representation of sampled water chemistry. Piper plots has been used to interpret water chemistry in several studies (Kumar, 2013; Karmegam et al., 2011; Ravikumar et al., 2015).

The sodium-potassium and mixed type of cations dominate the magnesium and calcium cations in the Rietspruit (Fig. 3). The anion ternary shows that the anions in the sampled water are of the mixed type. In the Leeuspruit, calcium is the dominant cation and sulphate the dominant anion (Fig. 3) which can be linked to the partial dissolution of the dolomitic host rock. The Piper plot indicated that the source of water pollution in the Rietspruit is as a result of the gold mining operation upstream of the river. From the Piper plot, the dominant water is the calcium sulphate which is typical of mine drainage water (Fig. 3). The source of PW dam inflow water indicated sodium chloride water from effluent wastewater from sewage treatment plant and agricultural storm-water run-offs. The PW dam inflow contains water from the sewage plant and eroded mine materials. In the study area, strong acids exceeded weak acid and the alkaline earths exceeded alkalise in most of the sampled water (Fig. 3).

In the study of Ratombo (2019), it was reported that calcium and sulphate rich water dominate samples collected downstream of a mine in the Witwatersrand Basin (Ratombo, 2019). They concluded that high level of sulphate in the water is an indicator of mine pollution on surface water (Ratombo, 2019). Similar reports were given in the study of Hobbs and Cobbing (2007) and Du Preez et al. (2016) on the quality of water chemistry impacted by mining operations in the Witwatersrand Basin (Hobbs and Cobbing, 2007; Du Preez et al., 2016). These align with our findings despite the difference in study area but the studied streams are in the Witwatersrand Basin, an area with mining history spanning over a century.

3.2. Concentration of heavy metals in the Rietspruit

Under this session, the various concentrations of arsenic, As, cadmium, Cd, mercury, Hg, and lead, Pb in the Rietspruit water and sediment within the mining environment and further downstream was discussed. In addition, various factors influencing the studied heavy metals was highlighted.

3.2.1 Arsenic concentrations in the Rietspruit water

Within the mining environment, the mine discharge waste water into the PW dam through the concrete canal and the furrow. The concrete canal (site 2) takes fissure water discharged from the underground into the PW dam. While the furrow takes discharge from the Sewage Plant to the PW dam inlet (site 1). The furrow also takes the eroded mine wastes littering the mining environment as well as rainwater run-offs into the PW dam. At the PW dam inlet (site 1), the concentration of As recorded in the water was 45.52 µg/l (Fig. 4). This was the highest concentration of As recorded in this study which is almost twice the As concentration reported in the study of Du Preez et al (Du Preez et al., 2016). Site 1 can be regarded as non-point source pollutant. Leached mine waste and rainwater run-offs contributed to the concentration of As found at this site.

A concentration of 21.5 µg/l of As was recorded after a major rain event in the furrow leading to the PW dam inlet (Fig. 4). Before the rainfall, the furrow was totally dry and a rock dump was located adjacent the furrow. It is believed that the concentration of As recorded at this site was a result of the release of As when mine waste was in contact with rainwater and storm-water run-off. The concentration of As recorded in the fissure water at site 2 was 6.2 µg/l. Site 2 can be regarded as point-source pollutant.

However, from the PW dam outflow (site 3), the source of Rietspruit water, the concentration of As recorded in the water was 4.3 µg/l (Fig. 4). This is below the recommended guideline limit of As in water used for domestic (≤ 10 µg/l) (DWAF, 1996a) and agricultural (≤ 100 µg/l) purposes according the South African Department of Water Affairs and Forestry (DWAF, 1996b). Further downstream, the concentration of As was below 1 µg/l except at site 12 (Pre Evaton) where the concentration of As was 1.2 µg/l in the water (Fig. 4).

3.2.2 Arsenic concentrations in Rietspruit sediment

In the Rietspruit sediment, the concentration of As within the surrounding of the mining site is very high (Fig. 5). The highest concentration (550.4 mg/kg) was recorded in the rock dump located adjacent the furrow leading to the PW dam. With the guideline limit of As set at 6 mg/kg (Persaud et al., 1993), the concentration of As in the rock dumps is about 92 folds higher. Arsenic concentration of 6 mg/kg is considered a level of contamination which has no effect on the majority of the sediment-dwelling organisms.

The second highest concentration of As (415.3 mg/kg) was recorded in the scale formed on the wall of the concrete canal taking fissure water into the PW dam (Fig. 5). The scales were formed as a result of precipitation process which are narrowed to surface water systems with atmospheric interactions (Winde et al., 2004b). In the slime dam, a concentration of 61.74 mg/kg of As was recorded (Fig. 5). All these sites are the major sources of As in the Rietspruit. This is because As is released from the solid phase into nearby surface water under a reducing condition resulting in soluble mobile forms of As. At the PW dam inflow (site 1), the concentration of As was 214.2 mg/kg in the sediment. In the study of Welch et al., (1988), they concluded that higher level of As is found within the areas of contamination (Welch et al., 1988). The concentration of As recorded within the mining environment is higher than the ones recorded downstream of the river (Fig. 5). The maximum concentration of As reported in this study is higher than the concentrations reported within the Witwatersrand Basin (Du Preez et al., 2016; Kamunda et al., 2016)

Arsenic in the Rietspruit sediment has a positive strong correlation with both gold (+ 0.9) and uranium (+ 0.8) in the sediment (32 sediment samples). These are the minerals that were mined before the mine was decommissioned in 2016. From the outflow of PW dam, the concentration of As dropped to 58.8 mg/kg. This was the highest concentration of As recorded in the study area outside of the mining environment (Fig. 5). However, in downstream wetlands (site 4) and dam (site 6), higher concentrations of As were recorded (223.1 mg/kg and 62.6 mg/kg respectively). Wetlands and dams are regarded as sinks for heavy metals (Raji et al., 2021). This is due to the high content of organic carbon (OC), fine particles of clay and silt, iron and manganese content of the sediment and the reduction of water flow velocity. At the wetland and dams, the percentage of OC (6.7% and 0.6% respectively) and sediments’ clay and silt is very high. These favour the adsorption of As to OC and fine particles of clay and silt.

Also the concentration of Fe (35 111 mg/kg in wetland and 30 630 mg/kg in dam), manganese, Mn, (9332 mg/kg in wetland and 2029 mg/kg in dam) found at these sites are very high when compared with the concentrations of Fe and Mn recorded in sediment samples from other sites. Using excel, the correlation of As with Fe is + 0.8, + 0.7 with Mn and + 0.5 with organic carbon (in 32 sediment samples). Dissolved Fe and Mn forms gel when oxidised and settle as amorphous gels in the sediment pores with heavy metals such as As adsorbed by the gel forming a part of the sediment over a period of time. After the dam, the concentration of As dropped to 5.1 mg/kg in the Rietspruit sediment (Fig. 5). This is due to the reduction of clay and silt content of the sediment and the increase in the water salinity after the Ennerdale Water Treatment Plant thereby reducing the capacity of the sediment to adsorb heavy metals. In addition, the concentration of Fe and Mn in the sediment dropped more than 50% after the damlit (site 6).

3.2.3 Cadmium concentrations in the Rietspruit water

In the water, the concentrations of Cd were below the prescribed guideline concentration of Cd in water used for domestic (5 µg/l) and agricultural (10 µg/l) purpose [41] within the mining environment. The concentrations within the mining environment ranged between 0.02 µg/l – 0.43 µg/l (4 samples). This is because Cd is highly insoluble and therefore less mobile and high concentration is found in river bed [33]. Cadmium is easily adsorbed to river sediment. At most of the sites, the concentrations of Cd in the water were below the detection limit of the ICP-MS (26 samples). According to Langard et al (1986), it was concluded that Cd is found mainly in bottom sediments and suspended particles (Langard et al., 1986).

3.2.4 Cadmium concentrations in Rietspruit sediment

In the sediment samples, high concentrations of Cd were recorded within the first 6 km of the mining site (Fig. 6). The highest concentrations were recorded in the downstream dam and wetland (3.88 mg/kg and 3.53 mg/kg respectively). These were higher than the concentrations found in the mine wastes (slime dams and rock dumps) located within the mining environment (Fig. 6). It is believed that the same factors favouring the bioaccumulation of As and other heavy metals in wetlands and dams played a vital role in Cd level found in the wetland and dam. In this study, Cd has a positive correlation with As (0.7) in the 32 sampled sediments. Kamunda et al. and Wittmann and Forstner reported maximum concentration of 0.05 mg/kg and 0.86 mg/kg of Cd in their study respectively which is lower than the maximum recorded in this study (Kamunda et al., 2016; Wittman and Forstner, 1977).

This proves the ability of wetlands and dams to bio-accumulate heavy metals as described by Raji et al (2021). Cadmium concentration of 3.4 mg/kg was recorded in the scale of the concrete canal (site 2). With the high concentration of Fe (39 421 mg/kg), Mn, (1 724 mg/kg) and Ca (20 760 mg/kg) recorded in the scale of the concrete canal, it is believed that Cd precipitated along with this metals when the scale was formed.

In the study area, the concentrations of Cd within the mining site were mostly higher than the Canadian Guideline limit of Cd in sediment (0.6 mg/kg [38]). About 6.5 km from the mining environment, the concentrations of Cd downstream were all below 0.6 mg/kg (Fig. 6). Cadmium has a positive correlation with Fe (0.7), Mn, (0.7), organic carbon (0.6), gold (0.8) and uranium (0.9) in the 32 sampled sediment. With the F-value (40.9) greater than the F – Critical value (2.14), and the P value less than 5 (3.9 E-33), the statistic is significant.

3.2.5 Mercury in the Rietspruit water

The highest concentration of Hg (11.8 µg/l) in the water samples was recorded in the PW dam inlet (Fig. 7). This site is regarded as a non-point source pollutant. As aforementioned, the PW dam inlet takes the water discharged from the sewage plant and rainwater run-offs into the PW dam. Rainwater run-offs contain the leached mine waste materials from the slime dams and rock dumps littering the surface of the mining environment.

The concentration of dissolved Hg recorded in the PW dam outflow water (site 3) was 3.45 µg/l (Fig. 7). With an estimated average monthly dam outflow of about 1 912 250 m3 (1 m3 = 1000 l), it can be estimated that the amount of mercury released into the Rietspruit average 6 597 200 mg/l monthly. At sites where human exposure was witnessed (site 6 in Table 1), the concentration of Hg recorded was 1.5 µg/l which is higher than the guideline limit of U in drinking water (1 µg/l (DWAF, 1996a)). Similar to As and Cd concentrations in Rietspruit water, the concentration of Hg is high above the recommended limit within the mining environment. Mercury concentrations further downstream (after about 6.5 km) are below the recommended limit of Hg in water at most of the sites.

3.2.6 Mercury in Rietspruit sediment

In the Rietspruit sediment, high concentration of Hg was recorded within the mining environment above the recommended limit of Hg in sediment, 0.2 mg kg (Persaud et al., 1993). The concentrations of Hg in the slime dams, rock dump and the scale of the concrete canal were all below 0.45 mg/kg (Fig. 8). The highest concentration of Hg was recorded at site 9 about 7 km downstream of the mining site (2.4 mg/kg) and it is lower than the maximum concentration (8.5 mg/kg) recorded in the study of Wittmann and Forstner (Wittman and Forstner, 1977).

Further downstream, the concentration of Hg recorded was below the concentration found within the mining area and below the recommended Hg limit in sediment with only one site as an exception (Fig. 8). Mercury has zero correlation with Fe, Mn, OC, gold and U despite the statistic being significant.

3.2.7 Lead concentrations in Rietspruit water

The concentrations of Pb recorded within the mining area and the entire Rietspruit water were below the recommended guideline limit of Pb in water used for domestic and animal watering, 10 µg/l and 100 µg/l respectively (DWAF, 1996a; DWAF, 1996b). A maximum concentration of Pb (6 µg/l) has been reported within the Witwatersrand Basin which triple this study’s maximum Pb concentration (2.1 µg/l).

3.2.8 Lead concentrations in Rietspruit sediment

In the Rietspruit sediment, the highest concentration of Pb was found in the rock dump (184.3 mg/kg) within the mining environment (Fig. 10). The second highest Pb concentration was recorded in the wetland after PW dam (120.9 mg/kg). These concentrations are higher than the maximum concentrations of Pb reported by Kamunda et al (2016) and Du Preez et al., (2014) but lower than the maximum concentration reported by Wittmann (Wittman and Forstner, 1977; Kamunda et al., 2016; Du Preez et al., 2016). Further downstream, the trend of Pb concentration in the Rietspruit sediment was hapharzardic but below 26.25 mg/kg (Fig. 10). With the recommended guideline of Pb in sediment below 31 mg/kg, 4 sites have higher level of Pb in the sediment while the remaining 28 sites have Pb level below the recommended limit.

The correlation between Pb and Fe (+ 0.8), Pb and Mn (+ 0.8) and Pb and OC (+ 0.5) indicated that Fe, Mn and OC positively contributed to the concentration of Pb in sediment (32 sediment). Studies have confirmed that Fe, Mn and OC play a critical role in the mobility and bioaccumulation of Pb in sediment (Gao et al., 2018; Zarcinas and Rogers, 2002; Todorović et al., 2001). In the study of Todorović et al., 2001, they concluded that Pb favorably bond to hydrous iron and manganese oxide by adsorption and co-precipitation process. Positive correlation of Pb with microelements like Mn, Fe was also reported in their study (Todorović et al., 2001).

Piper plot indicated that the source of water pollution in the Rietspruit is as a result of mining activities. Majority of the Rietspruit water corresponded with calcium sulphate typical of mine drainage water.

High concentrations of the heavy metals were recorded within the mining environment. The concentrations of the heavy metals decreased downstream in the Rietspruit water. Bioaccumulation of As, Cd, and Pb were recorded away from the mining environments in wetlands and dam. This confirms the ability of wetlands and dam to acts as sinks for heavy metals. High percentage of organic carbon, fine particles of silt and clay aid the retention of the heavy metals in the wetlands and dam through the adsorption of heavy metals to OC and fine particles. Co-precipitation of heavy metals with oxides of Fe, Mn contribute to the retention of the heavy metals in the wetlands and dams.

Since heavy metals are adsorbed to the fine particles of silt and clay, these heavy metals could be released into the water column as suspended particles. With human drinking the water for religious rites, they may ingest some of the suspended particles which could be detrimental to their health. As a result, the Rietspruit needs a strategic intervention.

Immediate remediation of the mining environment should be encouraged. This should involve the mining company and relevant government departments like Department of Water Affairs and Sanitation. In addition, continuous usage of the water for irrigation, animal watering and religious activities should be discouraged. The perimeter of the mining area should be fenced to prevent animal grazing. Downstream users of the Rietspruit water should be sensitized about heavy metal pollution and the health impacts. The erection of sign-posts in different South African Home Languages should be used to pass information discouraging downstream users from using the stream.

Author Contribution: "Conceptualization, Raji, I.B. and Bett S.K.; Methodology, Raji I.B. and Bett S.K.; Investigation, Raji I.B.; Resources, Raji I.B. and Bett S.K.; Writing – Original Draft Preparation, Raji I.B.; Writing – Review & Editing, Raji I.B. and Bett S.K..; Visualization, Raji I.B.; Supervision, Bett S.K.; Funding Acquisition, Winde F. and Hoffmann E. All authors have read and agreed to the published version of the manuscript.”

Funding: The study was funded by Sasol Research.

Institutional Review Board Statement: Not applicable

Informed Consent Statement: Not applicable

Acknowledgement: We would like to acknowledge the Faculty of Natural and Agricultural Sciences at North-West University for providing us with the necessary facilities to carry out this research.

Conflict of Interest: The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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Assessing heavy metal pollution in the Rietspruit, Far West Rand Goldfield, South Africa

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Abstract

Figures

Introduction

Materials And Methods

Results And Discussion

3.1. Rietspruit water chemistry

3.1.1. Rietspruit water pH

3.1.2. Rietspruit water electric conductivity

3.1.3 Water anions and cations concentration (Rietspruit water chemistry)

3.2. Concentration of heavy metals in the Rietspruit

3.2.1 Arsenic concentrations in the Rietspruit water

3.2.2 Arsenic concentrations in Rietspruit sediment

3.2.3 Cadmium concentrations in the Rietspruit water

3.2.4 Cadmium concentrations in Rietspruit sediment

3.2.5 Mercury in the Rietspruit water

3.2.6 Mercury in Rietspruit sediment

3.2.7 Lead concentrations in Rietspruit water

3.2.8 Lead concentrations in Rietspruit sediment

Conclusions

Declarations

References

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