Bulk sampling and source apportionment of heavy metals within a gold mine area, South Africa

Abstract

Studies have indicated that heavy metals originate from both natural and anthropogenic sources. Anthropogenic sources of heavy metals include mining, industries, and agricultural activities. The most common heavy metals found in soil are lead (Pb), mercury (Hg), cadmium (Cd), manganese (Mn), nickel (Ni), zinc (Zn), and arsenic (As). The heavy metals find their way to the ground, thus contaminating the soil. This study was divided into two (2) components: determining the concentrations of heavy metals in the soils collected within the mining area and identifying their sources. A total of ten (10) topsoil samples were collected around Evander and eMbalenhle. The soils were collected at depths between 0 and 15 cm. The inductively coupled plasma mass spectrometry (ICP-MS) method (Agilent 7900) was used to analyze the heavy metals in the soil samples. The samples were prepared in a 1% nitric acid (HNO₃) matrix. About 0.5% hydrochloric acid (HCl) was routinely added to soil samples to ensure the stability of the heavy metals. The absolute principal component score–multiple linear regression (APCS-MLR) model was used to calculate the contribution rate of the heavy metals at each sampling point. The enrichment factor (EF) and contamination factor (CF) were calculated to determine whether the heavy metals in the soil samples originated from natural sources or were influenced by anthropogenic activities. The upper earth’s crust (UEC) values were used as the soil background values of heavy metals. Mn was chosen as the reference element. The mean Cr, As, Pb, U, and Au concentrations were higher in tailings storage facilities (TSFs) (89.2 ± 58.8, 22.9 ± 25.7, 10 ± 10, 5.72 ± 4.78, and 0.53 ± 0.54 mg/kg) compared to residential sites (79.7 ± 24.6, 2.6 ± 1.33, 8.98 ± 5.05, 1.55 ± 1.42, and 0.08 ± 0.07 mg/kg), respectively. There was a significant difference in Mn concentrations (rrb = 0.833, p = 0.038) between the residential sites and TSFs. Results of the APCS-MLR of the heavy metal concentrations at the TSFs sites indicated that Au (92.7%), As (93.3%), U (93.3%), and Cr (88.3%) mainly originated from mining activities. The study findings confirm that TSFs are basically waste soil and may have affected the surroundings. This is the first study to assess heavy metal contamination in soils around Evander and eMbalenhle. The study recommends integrating other methods alongside bulk sampling and APCS-MLR in future studies to capture heterogeneity within the soil matrix and improve source apportionment resolution.