?Mathematical formulae have been encoded as MathML and are displayed in this HTML version using MathJax in order to improve their display. Uncheck the box to turn MathJax off. This feature requires Javascript. Click on a formula to zoom.ABSTRACT
The environmental impact of the abandoned Monarch Gold Mine in Botswana was analysed based on the concentrations of As, Mn, Cu, Cr and Zr in tailings and its vicinity, and the plants growing in the area. Results showed that the soil in the tailings dam (TD) and in the vicinity of the tailings dam (VTD), and the river sediments (RS) were severely contaminated with pollution load index (PLI) ranging from 1.89 to 2.86 in decreasing magnitude from TD>VTD> RS. The main contaminant is As but Cu, Cr and Zr are all also slightly above the critical values for soil. The TD has fewer plant species than VTD and accumulated elevated levels of these heavy metals (HM). The livestock grazing on these plants also consume the soil which could result to HM bioaccumulation. Therefore, proper management of the site is recommended to prevent the spread of pollutants and exposure to HM by animals and humans.
1. Introduction
Botswana’s economy is based on natural resources, and the extraction of these resources leads to environmental pollution and contamination and health risk to humans [Citation1]. The most common problems are heavy metal contamination due to mining activities and water pollution from industries, which threaten both human health and the ecosystem [Citation2]. A study by Ultra and Manyiwa [Citation3] used daily intake estimates for ruminants like cattle, sheep and goats and found that these animals were at risk due to heavy metal pollution from the BCL Copper and Nickel mine which is associated with lead (Pb) and copper (Cu) contamination in the surrounding areas. These environmental problems continue to rise because some mining operations are not compliant with environmental health and safety regulations [Citation1].
Soil contamination from heavy metals continues to be one of the most devastating environmental problems worldwide. The cause is mainly from mining activities and poor environmental management [Citation2,Citation4]. Heavy metal pollution and contamination pose a great risk to human health and the ecosystem at large. If ignored, it could pose an even greater danger for future generations because heavy metal pollution can cause chronic diseases like cancer [Citation1]. In this regard, Maleki et al. [Citation5] found that humans are being exposed to heavy metals through air, water and soil. People living near mines or smelters like Monarch could be exposed to high metal levels by breathing air and touching contaminated soil. Previous analysis there has found that total levels of As are 200 to 1700 ppm and Sb is 40 to 400 ppm [Citation6]. These values for As and Sb exceed the intervention action values set by Albanese et al. [Citation7].
Heavy metals generally occur naturally in the soil, but mining activities increase the exposure potential through excavation, crushing, treatment and disposal of mine tailings in the area [Citation6]. Furthermore, Vogel and Kasper [Citation6] also indicated that Monarch Gold Mine tailings have high levels of Cu which range from 865 to 2125 ppm. The intervention value for copper in non-agricultural areas is 200 ppm. Even though copper is an essential trace element for animals, plants and humans, high concentrations are considered to be phytotoxic and may be detrimental to human health. Other heavy metals that were found to have high concentrations include Ni, Zn and Ba. This clearly indicates the danger these tailings pose to people residing near the mine and the water sources around the mine. High levels of arsenic in water have been found to cause several types of cancer [Citation8]. Arsenic could also lead to cardiovascular diseases, neurological diseases and skin hyperpigmentation and depigmentation [Citation6].
Worldwide, mine tailings release vast amounts of heavy metals into the surrounding areas through surface erosion and deposition of Particulate Air Matter (PAM) [Citation9]. Studies conducted in mine tailings at the Selibe-Phikwe Cu-Ni Mine (BCL) indicated that PAM near the mine contained quartz, pyrrhotite, chalcopyrite, albite, as well as djurleite polymorphs, a secondary pollutant which is formed from the mineralization of chalcocite and the H2S and SO2 gases which are released through the concentration and smelting processes [Citation1]. These gases react with atmospheric moisture and form H2SO4 to create acid rain which damages plants and affects the ecosystem [Citation2]. Studies conducted by both Ekosse [Citation9] and Ultra [Citation2] indicated that soil and plants in the vicinity of Cu-Ni Mine (BCL) in Selibe Phikwe had elevated levels of heavy metals mainly due to erosion from mine tailings. Plants that accumulate these metals become a part of the food chain when they are consumed by animals. Consumption of vegetation with elevated metal concentrations can be a major exposure pathway for the animals or humans who consume them. Although some of these metals are not phytotoxic, they may pose extreme risk to some humans who consume them [Citation8]. With respect to selenium and molybdenum, their uptake into the edible portion of plants is generally not sufficient to cause plant toxicities, but it has created toxicities in the animals who consume them [Citation10]. Thus, measuring plant uptake of metals from the soil is a means to evaluate metal exposure to higher organisms. Information regarding the degree of bioaccumulation in plants is essential for ecological risk assessment because of the compounding effect it has on wildlife.
To date, studies conducted on Monarch Gold Mine tailings have focused on the presence of heavy metals in mine tailings, but they have not analyzed the effects and accumulation of these heavy metals on plants and their potential risk to grazing animals. As a result, more knowledge is needed about the availability of heavy metals in soils, the uptake and accumulation of pollutants by plants and the possible harm that heavy metal buildup in plants and soil poses to nearby grazing animals.
2. Materials and methods
2.1 Description of the study area
Francistown, the oldest city in Botswana, is the site of the Monarch Gold Mine, which is located at the confluence of the Tati and Ntshe rivers. It commenced gold mining in the 1890s and continued until mid-1998 when it was permanently closed [Citation6]. In Francistown, where the study was conducted, the abandoned Monarch mine with a latitude of 21̊ S and longitude of 27̊ E and an average elevation of 988 m above sea level is situated near the Ntshe River which leads to Tati and Shashe rivers that supply the Shashe dam. Water from the Shashe dam is used for domestic consumption by residents in the local and surrounding areas. Therefore, if not properly monitored, heavy metals from the abandoned mine could lead to contamination of water sources and the environment. Monarch Gold Mine tailings pose a huge health hazard, primarily due to the mine’s proximity to built-up areas. For example, children use the site as a playground, despite the high levels of As and Sb that have been recorded there.
2.2 Sampling and sampling points description
A total of 22 soil samples were collected from topsoil at a depth of up to 15 cm, in and around the abandoned mine tailings. These samples were stored in well-labeled zip-lock bags. A Garmin GPS was used to record the coordinates of each sample collection point. The samples were placed into three groups based on their location. The groups included (1) tailings dumps (TD) which comprised five samples; (2) vicinity of tailings dump (VTD) consisting of twelve samples; and (3) river sediment (RS) from the river with five samples collected at intervals of 500 m along the Ntshe river. The shoots of representative plants growing in each sampling point were collected, and the identification of the plants was conducted onsite. Duplicate samples were gathered for verification of the plant’s identity at the Botany laboratory. shows a map of the study area with sampling points.
Figure 1. Location of the study area and sampling sites.
2.3 Soil sample processing and chemical characterization
Soil samples were air-dried for 3 days in the laboratory and then sieved through a 2 mm stainless steel mesh wire prior to chemical characterization [Citation11]. The chemical characterization of the soil followed the same procedures outlined by Ultra [Citation2]. Briefly, the soil pH and electrical conductivity (EC) were measured in water suspension (1:2, soil: water) after mechanically shaking for 2 hours using a portable Hanna pH/EC Multiparameter. Five grams of the plant sample was weighed in a 250 mL conical flask, and 5 mL of nitric acid and 15 mL of hydrochloric acid (HNO3:HCl mixture (1:3)) were added to the sample and covered with a watch glass. The sample was placed on a hot hotplate at 120°C for 6 hours, and the total contents of As, Mn, Cu, Cr and Zr were quantified using ICP-MS [Citation2]. The available fractions of these heavy metals in soil were assessed by determining the exchangeable ammonium acetate. Ten grams of the sample was weighed, and 100 mL of ammonium acetate was added to the samples. Samples were mechanically shaken with 1 N ammonium acetate (pH 7) at a 1:100 ratio for 2 hours at room temperature. After shaking, the samples were filtered with Whatman filter paper No. 42 and 0.45 µm syringe filters. The exchangeable fraction was analyzed for different heavy metal concentrations using ICP-MS (Thermo Scientific™ iCAP Q ICP-MS, Illinois 60060, U.S.A.).
2.4 Plant sample processing and chemical analysis
The air-dried shoots were oven-dried at 70°C for 72 hours and ground into powder using a Philips home blender to homogenize samples prior to chemical analysis. To assess the heavy metal content, the powdered plant tissues were digested using a nitric acid and hydrogen peroxide mixture in an open vessel covered with a watch glass at 120°C for a minimum of 4 hours and then analyzed for heavy metals using ICP-MS [Citation10].
2.5 Validation of the analytical method (ICP-MS)
The method was validated for linearity, limits of detection (LODs), limits of quantification (LOQs), accuracy and precision. For each analyte, a calibration curve was generated by plotting peak area versus concentration at eight different concentrations between 10 and 1000 pmol/mL under optimum extraction conditions. indicates ICP-MS instrumental parameters employed for heavy metal determination. For all chemical analyses of soil and plant samples, precision and accuracy were assured through batch inclusion of internal reference samples (soil and corn tissue) whose concentrations were cross verified by three laboratories in Botswana and by repeat analysis of samples when the coefficients of variation of replicate analysis (n = 2) were more than 10%. The recovery of the internal standard sample was more than 95% for all elements measured (). All the chemical analyses were conducted at the Soil Science Laboratory of the Department of Earth and Environment Sciences, BIUST.
Table 1. ICP-MS instrumental parameters employed for heavy metal determination and % recovery of concentrations of internal reference sample.
2.6 Risk assessment of ruminants grazing in the area
The daily intake of metals from the different ruminants was estimated based on the formula by Manyiwa et al. [Citation10] and calculated using the formula below as:
[Metal]Grass, [Metal]Shrubs, [Metal]herbs are the average concentrations of metals for grasses, shrubs and herbs, respectively (mg/kg −1) presented in ; and Igrass, Ishrubs, Iherbs are the equivalent intake of grasses, shrubs and herbs, respectively, and daily intake of metals was calculated as the daily dietary intake multiplied by the percentage intake of grasses, shrubs and herbs for each ruminant. Cattle diets consist of 75% grasses, 23% woody plants and 2% herbs; goat diets consist of 20% grasses, 78% woody plants and 2% herbs, while sheep diets consist of 74% grasses, 21% woody plants and 5% herbs. Incorporating the proportion of these diets and their equivalent average metal concentration in plants collected from the study site was used in the estimation of the daily intake.
Table 2. The pH, EC and total As, Mn, Cu, Cr, Zr contents and PLI of soils from abandoned Monarch Gold Mine, Francistown, Botswana.
Table 3. Available As, Mn, Cu, Cr and Zr contents of soils from abandoned Monarch Gold Mine, Francistown, Botswana.
Table 4. Heavy metal contents in shoots of different plants growing in the tailings dumpsite of abandoned Monarch Gold Mine, Francistown, Botswana.
Table 5. Bioaccumulation factor of heavy metal contents in shoots of different plants growing from the tailings dumpsite of abandoned Monarch Gold Mine, Francistown, Botswana.
Table 6. Heavy metal contents in shoots of different plants growing within 0.5 to 2 km away from the tailings dumpsite of abandoned Monarch Gold Mine, Francistown, Botswana.
Table 7. Bioaccumulation factor of heavy metal contents in shoots of different plants growing within 0.5 to 2 km away from the tailings dumpsite of abandoned Monarch Gold Mine, Francistown, Botswana.
Table 8. Maximum and estimated daily intake of heavy metals of cattle, goat and sheep grazing on plants inside and near tailings dumpsite of abandoned Monarch Gold Mine, Francistown, Botswana.
2.7 Determination of contamination factors, pollution load index and bioaccumulation factor in soil
Using the total concentrations in soils and plants, contamination factors (CF), pollution load index (PLI) and bioaccumulation factor (BAF) of heavy metals in the soil samples were determined as indicated in Equation 2, 3, 4 [Citation10,Citation12].
Where Cm is the measured concentration of heavy metal in the soil, and Bm is the local background concentration value of the heavy metals (). The background data was obtained from soil samples collected at about 1.5 km along the leeward side of the Cu-Ni smelter area (therefore assumed to mimic pre-industrial data) which has similar texture and mineralogy to the study sites.
Pollution Load Index (PLI), proposed by Tomlinson et al. (1980), was calculated using Eq 3.
where N is the number of metals studied and CF is the contamination factor calculated as described in (EquationEq 2(2)
(2) ).
Bioaccumulation factor is the ratio of the concentration of heavy metals in plants and in soils. It is an indicator of a plant’s capacity to accumulate heavy metals. The BAF was calculated using Eq 4 proposed by Aladesanmi et al. [Citation12].
Where Pi is the concentration of a heavy metal in plants (mg kg−1), and Si is the concentration of the same heavy metal in the soil where the plant was collected (mg kg−1).
2.8 Statistical analysis
The data was analyzed using a single-factor ANOVA in CRD to determine the differences of heavy metal concentration between sampling sites (TD, VTD and RS). When significant differences were observed, Tukey’s test was performed for treatment mean (sampling site) comparison at 5% level of significance. Each parameter was analyzed separately per plant and soil samples.
3. Results
3.1 Soil chemical properties and heavy metal concentration and availability
The pH, electrical conductivity and total concentration of As, Mn, Cu, Cr and Zr in soils from the three different sampling sites (TD, VTD (0.2–2 km) and RS) are shown in . The pH of soil was alkaline, ranging from 7.18 to 8.76 except for one sample in the TD and one sample in the VTD which had a slightly acidic pH of 6.49 and 6.84, respectively. The average electrical conductivity (EC) for TD was 2507.65 µS m−1 with a range of 1236.75 to 3540 µS m−1. The average EC for soils in VTD was 556.04 µS m−1 with a range of 258.80 to 871.50 µS m−1; whereas the average EC for RS was 36.70 µS m−1, with a range of 26.4 to 55.5 µS m−1.
The average concentration of the total As in TD was 1784.87 mg kg−1 which is more than 10 times the average of the total As in soils in VTD (175.28 mg kg−1), while the average of the total As in RS was 24.08 mg kg−1. The average concentration of Mn in the TD was 577.72 mg kg−1, whereas in VTD and RS they were 813.46 and 748.28 mg kg−1, respectively. The Cu concentration in TD was 304.44 mg kg−1, 238.02 mg kg−1 in VTD and 174.84 mg kg−1 for RS. Total Cr in TD was 116.36 mg kg−1, while in VTD it was 159.15 mg kg−1 and 145.18 mg kg−1 in RS. The Zr in TD was 200.06 mg kg−1, whereas in VTD and in RS they were 151.24 and 149.28 mg kg−1 , respectively.
The available fraction of heavy metal in soils is shown in . The average concentration of exchangeable As (exch-As) in TD was 2.44 mg kg−1 with a range of 1.75 to 3.77 mg kg−1. This was about 0.137% of the total As in TD. The average As concentration for VTD was 0.72 mg kg−1 (0.41%) and it was 1.21 mg kg−1 (5%) for RS. The Exch-Mn was about 3.70% of the total concentration ranging from 13.98 to 25.47 mg kg−1 for TD, whereas for VTD the exch-Mn ranged from 4.59 to 16.17 mg kg−1 making up 1% of the total Mn, and for RS the exch-Mn ranged from 9.60 to 14.07 mg kg−1making up 1.6% of the total Mn. The Exch-Cu in TD was about 0.9% of the total Cu concentration with a range of 2.12–3.46 mg kg−1. In VTD, the exch-Cu was about 1% of total Cu concentration with a range of 1.19 to 4.26 mg kg−1 and in RS, the exch-Cu was 0.67% of total Cu, with a range concentration of 0.89 to 1.61 mg kg−1. In TD, the exch-Cr was about 1.9% of the total Cr, with a range of 1.66 to 2.97 mg kg−1. In VTD, the exch-Cr was 1.1% of the total Cr concentration with a range of 0.62 to 2.03 mg kg−1 and in RS, the exch-Cr was 0.9% of the total Cr, with a range concentration of 1.07 to 1.61 mg kg−1. The Exch-Zr was 0.43% of the total concentration ranging from 0.63 to 1.35 mg kg−1 for TD, whereas for VTD, the exch-Zr ranged from 0.49 to 0.82 mg kg−1 making up 0.42% of the total Zr, and for RS, the exch-Zr ranged from 0.59 to 0.88 mg kg−1 making up 0.5% of the total Zr.
3.2 Pollution load index
The contamination factor was used to determine the pollution load index (PLI) of the three different sites (). In all the sites, the pollution index was greater than 1 based on the combined CF of the metals under consideration. TD had the highest pollution index ranging from 2.67 to 3.25, followed by the VTD dump with a PLI ranging from 2.01 to 2.15 and then the RS with 1.84 to 1.91 PLI.
3.3 Composition, classification, heavy metal concentrations and bioaccumulation factor of plants in the tailings dump of Monarch Gold Mine
A total of 94 plant samples which were collected from Monarch tailings dump belonged to grass, herbs, trees and shrubs. A total of 24 samples of Cenchrus biflorus were collected from the TD indicating a dominance of this grass in the area. Other plant species growing in the TD included three herb species: Portulaca oleracea (13 samples) Gossypium herbaceum (6 samples) and Withania somnifera (4 samples); three species of shrubs: Lantana dinteri, Maytenus senegalensis and Monechma leucoderme; eight tree species: Dichrostacys cineria, Schinus molle, Colophospermum mopane (Kirk Ex Benth.), Melia azedarach, Boscia albitrunca (Burch.) Gilg & Gilg-Ben., Acacia mellifera (M.Vahl) Benth, Acacia dudgeonii (Craib ex Holl) and Acacia schweinfurthii (Brenan & Exell) were identified with the most dominant species being D. cineria, C. mopane and B. albitrunca. All these species are commonly grazed and browsed by livestock and wild animals.
Heavy metal content in the shoots of these different plants growing in the tailing’s dumpsite of the abandoned Monarch Gold Mine is presented in . The only grass growing in the TD is Cenchrus biflorus with an As concentration of 47.14 mg kg−1 and a range of 15.11–135.03 mg kg−1, Mn at 56.68 mg kg−1, Cu at 115.94 mg kg−1, Cr at 77.38 mg kg−1 and Zr at 37.39 mg kg−1. Among the different plant groups, herbs had the highest average Mn concentration at 108.48 mg kg −1, while the grass had the highest average Cu concentration. Grasses and herbs have higher average Cr concentrations than trees and shrubs, while for Zn, there were no significant differences between plant groups. Among the different species, the As concentration was highest in one of the C. bicolor samples with an amount of 135.03 mg kg−1, while for Mn, the highest value was recorded from Portulaca oleracea at 240.14 mg kg−1. The highest concentrations of Cu and Zr were recorded in C. biflorus at 300.25 and 112.90 mg kg−1, respectively, while for Cr, the highest concentration was recorded in Gossypium herbaceum.
The bioaccumulation factor (BAF) for heavy metals in plants growing in the TD of the Monarch Gold Mine is presented in . Among all the plant species, the BAF for As was in the range of 0.001 to 0.076 wherein a higher BAF value was recorded in Cenchrus biflorus. For Mn, the BAF ranged from 0.047 to 0.24 with the highest BAF recorded for Portulaca oleracea. For Cu, the BAF ranged from 0.07 to 0.381 while for Cr, the range was 0.28 to 0.783 with the highest BAF in Schinus molle. For Zr, the BAF ranged from 0.05 to 0.27.
3.4 Composition, classification and heavy metals concentration of plants in the vicinity (0.5 to 2 km) of tailings dumpsite of the abandoned Monarch Gold Mine
There were 166 plant samples collected in the VTD (0.5 to 2 km) of the abandoned Monarch Gold Mine (). A total of seven grass species were identified: Cenchrus biflorus, Bolboschoenus nobilis (Ridl.) Goetgh. & Simpson, Oxycaryum cubense (Poepp. & Kunth), Dactyloctenium aegyptium (L.) Willd., Stipagrostis uniplumis (Licht.), Bothriochloa insculpta (Hochst.) and Panicum congoense Franch. Cenchrus biflorus was the only grass that grew in both the TD and in the VTD (0.5–2 km). Three different types of herb species were found near the TD: Portulaca oleracea Gossypium herbaceum and Withania somnifera. Four different shrubs (Fadogia homblei De Wild, Lantana dinteri, Maytenus senegalensis and Monechma leucoderme) were also identified, and a total of 10 different trees (Dichrostachys cinerea, Schinus molle, Colophospermum mopane (Kirk Ex Benth.), Melia azedarach, Boscia albitrunca (Burch.) Gilg & Gilg-Ben.), Acacia mellifera (M.Vahl) Benth.), Acacia dudgeonii (Craib ex Holl), Acacia schweinfurthii (Brenan & Exell), Acacia luederitzii (Engl.var.) and Grewia bicolor A. Juss.) were found in the VTD. The most dominant shrub was Lantana dinteri with seven samples, whereas the most dominant tree species was Melia azedarach with twelve samples.
Heavy metal contents in shoots of different plants growing in the VTD of the abandoned Monarch Gold Mine is also presented in . In general, grasses had a higher As content compared to herbs and trees. The highest concentration of As for the grasses was recorded in Panicum congoense Franch. (56.57 mg/kg) and for the trees it was Fadogia homblei De Wild. With respect to Mn, trees have a higher average concentration than herbs and grasses. Among the shrubs and tree species, Grewia bicolor Juss. had the highest Mn concentration at 393.74 mg/kg, Dactyloctenium aegyptium (L.) Willd. at 190.37 mg/kg for grasses and the Gossypium herbaceum (165.7 mg/kg). The shrubs and trees had higher average concentrations of Cu than herbs and grasses.
3.5 Heavy metal accumulation and the bioaccumulation factor in plants found in the vicinity (0.5 to 2 km) of tailings dumpsite of the abandoned Monarch Gold Mine
The heavy metal concentration in the shoots of plants growing in the VTD of Monarch Gold Mine is presented in . Based on the plant type, the total As concentration in the Cenchrus biflorus grass species ranged from 0.008 to 0.076 mg kg−1; Mn concentration ranged from 0.014 to 0.343 mg kg−1; Cu ranged from 0.121 to 0.986 mg kg−1; the Cr range was 0.232 to 0.937 and the Zr concentration ranged from 0.041 to 0.546 mg kg−1. There were three different types of herbs found in the TD of the mine. Of the three different species of herbs, Cr had the highest concentration of heavy metal accumulation. Trees and shrubs were the most dominant species in the TD. Schinus molle and Colophospermum mopane (Kirk Ex Benth.) had high concentrations of Cr. In Schinus molle, Cr ranged from 0.970 to 0.510 mg kg−1, whereas for Colophospermum mopane (Kirk Ex Benth.), the Cr concentration ranged from 0.808 to 0.608 mg kg−1. For trees and shrubs, Cu had the second-highest heavy metal concentration.
3.7 Metal intake by grazing animals
The maximum and estimated daily heavy metal intake of grazing animals based on the concentration of heavy metals from the different plant groups and the average daily intake of forage and soils are shown in . Assuming that cattle will consume 16.9 kg of food per day consisting of 75% grasses, 23% woody plants and 2% forbs from the TD and the VTD, the cattle daily intake of heavy metals would be 1363.45 mg/day for As, 1130.09 mg/day for Mn, 2321.58 mg/day for Cu, 1223.52 mg/day for Cr and 725.58 mg/day for Zr. For goats, which have a daily feed consumption of 0.90 kg/day with a diet that consists largely of shrubs (78%) and grasses (20%) and herbs (2%), the daily intake of heavy metals would be 191.93 mg/day for As, 94.87 mg/day for Mn, 180.46 mg/day for Cu, 57.76 mg/day for Cr and 55.89 mg/day for Zr. With an average daily forage intake of 2.50 kg for sheep which consists of 74% grasses, 21% of shrubs and woody species and 5% of herbs, the daily intake of heavy metals would be 281.20 mg/day for As, 224.03 mg/day for Mn, 294.51 mg/day for Cu, 197.20 mg/day for Cr and 112.35 mg/day for Zr.
As for the heavy metal intake by cattle, goats and sheep from plant species which are in the VTD, the daily intake of heavy metals by cattle would be 771.94 mg/day for As, 2281.96 mg/day for Mn, 1660.35 mg/day for Cu, 2421.73 mg/day for Cr and 449.04 mg/day for Zr. For goats, the daily intake of heavy metals would be 48.42 mg/day for As, 206.48 mg/day for Mn, 141.69 mg/day for Cu, 138.22 mg/day for Cr and 38.12 mg/day for Zr. With an average daily forage intake of 2.50 kg for sheep, the daily intake of heavy metals would be 11.29 mg/day for As, 368.68 mg/day for Mn, 210.64 mg/day for Cu, 322.57 mg/day for Cr and 74.01 mg/day for Zr.
4. Discussion
The 100-year operation of the Monarch Gold Mine in Francistown, which ended with its permanent closure in 1996 [Citation13] has altered the soil properties not only in the mine site but also for the surrounding areas and for the river sediments. The current data shows this alteration is due to the presence of elevated heavy metals, which has led to serious environmental pollution as a consequence of ineffective waste management and a lack of enforcement of environmental laws. In the TD, samples showed a relatively high pH of up to 8.7 while in the VTD, the pH ranged from 6.84 to 8.38 (). The alkaline pH of the tailings is attributed to the extraction process of gold which uses an alkaline solvent of sodium cyanide solution. During the extraction process, the ore is alkalinized with sodium hydroxide and calcium carbonate or calcium oxide before the addition of the cyanide solution to leach the gold [Citation14]. This alkaline condition was observed even on the surface sediments collected from the adjacent Ntshe river, and these values are comparable to the pH recorded in the study of Vogel and Kasper [Citation6].
The electrical conductivity (EC) of the samples from the TD site () exceeded the optimal range for EC for most crops with a range of between 800 and 1800 µs/cm and an upper limit of 2500 µs/cm. In contrast, the soils from the VTD and from RS had EC which were relatively compared to those in the TD, and they were within the optimum EC range for plant growth. The high EC in the TD implies there is a high concentration of soluble salts in the tailings that may inhibit the growth of the plants. A high level of EC in the TD would also imply there is a high risk of transporting soluble substances into vulnerable environmental compartments, e.g. ground and surface water. On the other hand, the low EC level coupled with a high pH in the VTD, and the sediments implies that the concentration of soluble heavy metals and metalloids are low compared to those at TD [Citation15]. High EC in the TD could be attributed to the high concentration of residual sodium since sodium is used during the gold extraction process. In addition, the low EC observed in the river sediment could be attributed to its light texture which has a low capacity to retain soluble substances; hence, soluble salts are being leached down to lower levels [Citation16].
On average, As can be regarded as the main contaminant in the TD and VTD as its high quantities and available concentrations () exceed the critical concentrations (0.5-50ppm) proposed by Albanese et al. [Citation7]. It is followed by Mn, Cu, Zr and lastly Cr. Most heavy metals followed a decreasing trend based on the distance from the mine with the highest levels found at the TD; the next highest in the VTD and the lowest at the RS which is farthest from the mine. The TD serves as a point source of contamination of heavy metals in the abandoned Monarch area. Elevated levels of heavy metals found in the VTD are due to the dispersion of these elements both downstream and downslope by wind and water. The occurrence of heavy precipitation during the short rainy season (December to February) may have contributed to the dispersion of contaminants in the vicinity of the abandoned gold mine [Citation42]. In addition, the strong wind velocity during the dry season could also disperse the heavy metal-laden particles from the TD to the VTD because of the low vegetation cover in the study area.
The high PLI in the tailings dump indicates that heavy metal concentrations exceed the background concentrations and show potential heavy metal contamination. The PLI in the TD is considered as highly contaminated with an average value of 2.86 (2.67–3.25), which implies that this area should be subjected to active decontamination and rehabilitation. Similarly, the PLI in the VTD and in RS is slightly lower at 2.11 and 1.89, respectively, but they are still considered highly contaminated and should be subjected to the same remediation treatment as the TD. The high PLI values in the TD could be attributed to the high contribution of the As contamination factor (CF) followed by the contamination factor of Zr (10.03) > Cu (7.61) > Cr (5.82) > Mn (1.93). Similar to the TD, the PLI value in the VTD was also increased mainly by As concentration but at a lower magnitude (CF of AsTD = 178.5 vs CF of AsVTD = 17.5). The CF for Cr (7.96), Zr (7.56) and Cu (5.95) remained relatively constant as compared to its CF in TD, contributing more to the observed PLI in the VTD. Based on the total concentration and the CF of the different metals under consideration, the concentration of As significantly decreased from the TD to the VTD by almost 10 times, while other elements remained relatively unchanged. This would imply that As has emanated from the mine tailings after gold extraction, thus serving as the point source. The As originated from gold ore which is an arsenopyrite and is able to release As upon oxidation [Citation17]. Considering that there is a big difference between the As concentration in the samples from TD and VTD, it appears that the main mode of As dispersion is through the transport of particles which remain after gold extraction. The other metals are inherently common between the tailings and the surrounding soil as evidenced by almost similar concentrations in the samples from the TD and the VTD. Environmental factors like erosion also contribute to the increase of heavy metals, especially As in the VTD because the tailings are partially exposed to environmental factors [Citation18]. Observation of the study site during sampling also showed the prevalence of gullies along the slope of the TD. Particle dispersion from the TD occurs during both the rainy season and the dry season especially because of low vegetation cover. Therefore, gully erosion disperses the tailings to the surrounding area, thereby increasing the concentration of these heavy metals in surface soils. This is also evident in terms of Cu, Mn and Cr which have higher concentrations than the background concentrations (). For the river sediments, the PLI was 1.89, which was due to high concentrations of Mn and Cu. This indicates that Mn and Cu concentrations may be associated with sand and silt particles that are dominant in the river sediment. The lower concentration of As may be due to its solubilization and loss from the matrix during deposition process [Citation19]. The significance of finding high amounts of Zr in the study area is that this element has been found to significantly decrease plant growth and alter enzyme activity in plants [Citation20]. Plants that are exposed to high amounts of Zr tend to develop various defense mechanisms to cope with Zr toxicity [Citation21]. Some of these mechanisms include among others, Zr sequestration on plant roots and activation of numerous antioxidants [Citation21]. Its presence may also have contributed to abiotic stress on plants [Citation22]; hence, only a few plants have been able to survive in the TD site.
The concentration of heavy metals in the available fraction is considered a better index for assessing the potential risk to living organisms. The available fraction of As, Cu and Cr is much higher in TD than VTD and RS, while for Mn and Zr, the available fraction is much higher in TD than VTD. Comparing the availability of these metals, Mn is the most common element in all three sites (TD, VTD, RS) followed by Cu > Cr > As > Zr. The high availability of As at the TD was attributed to the presence of soluble salts. With respect to concentrations, the available As is higher in the sediment than in soils in the VTD. This may have occurred because the soluble salts which are carried from the TD cannot be retained in the surface soil due to its sandy texture, so they dry up in the sediment and are then retained in the sediment matrix [Citation23]. The high availability of Mn may be due to its soluble nature at a pH of 7.0 and above [Citation24]. The pH in all three sites was above 7. In addition, the gold particles are associated with sulfide minerals such as pyrite, arsenopyrite, chalcopyrite and bornite which have led to a high concentration of As in the tailings [Citation25,Citation26]. Furthermore, the cyanidation process converts several As species into As(V). Paktunc et al. [Citation27] found that As and Fe bearing minerals were composed of iron(III) oxyhydroxides, scorodite, ferric arsenates, arseniosiderite, Ca-Fe arsenates, pharmacosiderite, jarosite and arsenopyrite, and that As occurred exclusively as As (V) resulting from cyanidation which is more easily transported within a soil column and also preferentially fixed with iron oxides and sulphides. Mn was the highest available element recorded in the TD, VTD and RS because Mn could be easily mobilized upon changes in chemical properties of the host environment, and it can also be easily extracted from an oxidizable phase [Citation43,Citation44] stated that Mn in poorly aerated soils and at a pH 6 will have high availability and can be toxic to plants.
After being abandoned for almost 20 years, the vegetation in the TD has reestablished itself with one grass species (C. biflorus), three herb species (P. oleracea, G. herbaceum, W. somnifera), eight tree species dominated by (D. cineria, C. mopane, B. albitrunca) and three shrub species (M. senegalensis, L. dinteri, M. leucoderme). All these species were able to accumulate As, Cu, Cr, Mn and Zr above the normal concentration in plants indicating their ability to withstand very high concentration of these elements. In addition, these plants can tolerate high concentrations of these metals simultaneously. Based on the bioaccumulation factor (BAF) for the different elements, it appears that these plants could regulate the uptake of these elements at very high concentrations, resulting in their ability to thrive in the TD. Trees like C. mopane, D. cineria and B. albitrunca are able to survive because they belong to the Fabaceae family which are known as hyperaccumulators [Citation28]. As for the herbs, P. oleracea and G. herbaceum, these plants are heavy metal tolerant and can bioaccumulate As, Cu, Cd and Pb [Citation29]. These trees and herbs are common in other heavy metal-polluted areas such as the BCL mine in Selebi Phikwe, Botswana [Citation10]. With respect to the composition of the vegetation in the VTD, six additional grass species (B. nobilis, O. cubense. D. aegyptium S. uniplumis, B. insculpta and P. congoense Franch.), two more shrub species (G. bicolor and F. De Wild) and one tree species (A. leuderitzii) were also found in the VTD. Grasses like B. nobilis, O. cubense and B. insculpta are hyperaccumulators that can accumulate high levels of Cu, Zn and Pb. These heavy metals are commonly found in metalliferous environments and in areas that are contaminated by heavy metals due to mining activities [Citation30]. Furthermore, the tolerance of heavy metals like Pb displayed by O. cubense may be linked to the root anatomy of the grass which has an exodermis that acts as an apoplast barrier [Citation31]. For plant species that possess an exodermis, ionic selectivity occurs very close to the periphery of the root, blocking the accumulation of potentially toxic substances in the cortex [Citation32]. Tree species like A. leuderitzii, an acacia species, including those in the subgenera of Aculeiferum and Acacia, as well as those in the series Filicinae and other mimosoid legumes, contain hydrolyzable tannins that act as adsorbents of heavy metals [Citation33].
There is also an increasing number of plants surviving in the VTD compared to the TD. Certain plant species found in the VTD but not in TD are considered more sensitive and vulnerable to elevated concentrations of heavy metals. For example, in the TD, only one grass species survived compared to eight grass species in the VTD. The C. biflorus, which has survived in an As polluted environment, can accumulate up to 47.14 mg Kg−1 of As and has 0.26 BAF. Its ability to thrive with high As concentrations has been attributed to exclusion mechanisms which prevent the uptake of As [Citation45]. Similarly, except for Acacia dudgeonii and Gossypium herbaceum and some plants with high BAF, all plants in TD were able to survive with high levels of HM concentration, especially As. Therefore, these plants are non-hyper [Citation34].
At the time of sampling, the abandoned Monarch Gold Mine did not have a security fence to prevent the entry of ruminants. Livestock in the area frequent the abandoned mine and graze on the vegetation in the area, which puts them at risk for exposure to the contaminated vegetation. Among the heavy metal of interest in this study, only As is estimated to exceed the maximum allowable daily intake (MADI) of ruminants grazing on the plants from the TD and VTD, while Mn, Cu and Cr are below or comparable to the MADI. No reference value was established for Zr. The estimated daily intake of As in TD is much higher than in VTD for cattle, goat and sheep, which may be attributed to having a much higher concentration of As in the soil/tailings and in the plants growing in the TD. Although ruminants with chronic exposure to As do not usually exhibit toxicity symptoms, there is a risk for human exposure to As through the consumption of meat or milk products from these ruminants [Citation35]. Therefore, there is a need for assessment and close monitoring of the As levels of the ruminants in the study area. Such information is necessary for the prevention of As exposure to humans.
Overall, the study has shown that the TD, VTD and the sediments from the Ntshe river near the abandoned mine tailings dam of the Monarch Gold Mine are severely contaminated based on the PLI ranging from 1.89 to 2.86. The high PLI was primarily due to very high As concentrations in tailings and also in part due to high concentrations of Cu, Cr and Zr. The solubility of As, Cu, Cr and Zr in the TD is much higher than the VTD and the RS which could indicate higher risk for these metals to be absorbed by plants or transported to nearby environments and to the ground water. Due to high concentrations of As in the tailings, as well as the alkaline pH and very high electrical conductivity, only a few species were thriving in the TD which was dominated by C. biflorus, three herbs and eleven trees and shrubs. The grass and herbs in the TD have concentrations of As, Cu and Cr which are above the critical levels, while the trees have concentrations within or slightly above the critical levels of plants. The number of plants thriving in the VTD is much higher than in the TD. The BAF of these plants is less than one indicating that these plants can survive in these environments by using exclusion [Citation36]. Furthermore, estimates of the daily intake of these metals by cattle, goat and sheep through grazing and browsing of these plants and intake of soil indicate potential As exposure risk and probable transfer of As to humans through the consumption of these ruminants. It is therefore recommended that proper management of the tailing dam and its vicinity should be implemented to prevent the spread of pollutants and the exposure of animals and humans to heavy metals, especially As emanating from the TD.
5. Conclusion
The main contaminant in Monarch Gold Mine is As and some Cu, Cr and Zr which were found to be slightly above the critical values in soil. TD recorded high availability of these metals compared to VTD and the river sediments. The most dominant species in The TD was C. biflorus, three herbs and eleven trees and shrubs, and the concentration of As, Cu and Cr was above the critical level estimates of the daily intake of these metals by cattle, goat and sheep through grazing and browsing of these plants. The intake of soil indicates a potential risk of As exposure and probable transfer to human though the consumption of these ruminants. It is recommended that proper management of the tailings dam and its vicinity should be implemented to prevent the spread of pollutants and exposure of animals and humans to heavy metals especially As emanating from the tailings dam.Citation37,Citation38
Acknowledgments
The authors would like to express their gratitude to the Department of Earth and Environmental Science at Botswana International University of Science and Technology, Botswana, for providing us with the laboratories to carry out analysis of soil and plant samples and providing us with transport to conduct sampling.
Additional information
Funding
References
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