Abstract

Groundwater chemistry in and around an abandoned metal mine within a large dam construction site was evaluated. For this purpose, a total of 50 groundwater samples were collected and chemically analysed for major ions and toxic metals. The pHs of the groundwaters were near neutral to slightly basic, with an apparent increase along the flow direction away from the mine waste dump. Higher values of electrical conductivity with a mean of 532 μS cm−1 were observed in the mine area. Dissolved oxygen concentrations over 5 mg l−1 clearly indicated an oxygenated groundwater environment. The high concentration of sulphate was derived from weathering and oxidation of sulphide minerals. Hydrochemistry of the groundwater samples is characterized by the relatively significant enrichment of Ca2+ and SO42−. Results of factor analysis also indicated that the dominant mechanisms or continuing processes affecting the distribution of the chemical parameters in the study area are various geochemical reactions including dissolution of sulphide and carbonate minerals, and removal of calcium and sulphate by dilution through mixing with Ca2+− and SO42−-poor groundwaters. In the mine area, substantial groundwater contamination by toxic metals including Zn, Al, Fe and Mn was observed. Most of the toxic metals decreased gradually or dramatically with distance from the mine, although some metals were also found in high concentrations in the downstream areas. Compared with the contamination levels of the surrounding farmland soils and stream sediments, the levels of contamination of groundwater by toxic metals were relatively low. The low concentrations are ascribed to the high pH and highly oxygenated conditions, which reduce toxic metal mobility. Significant groundwater contamination with toxic metals was strictly limited within the immediate vicinity of the mine waste dump but the ubiquitous distribution of the toxic metals with slightly elevated levels all over the study area may be attributed to the same geology and mineralogy as in the mine area.

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