Response of antimony and arsenic in karst aquifers and groundwater geochemistry to the influence of mine activities at the world’s largest antimony mine, central China

Abstract

The elevated levels of antimony (Sb) and arsenic (As) in groundwater caused by mine activities is of particular global concern. Understanding pollution mechanism of Sb and As related to mine activities and their influence on karst groundwater geochemistry is of great significance. However, it is still unclear. To address such questions, Sb and As and major ions, δ34S and δ18O of dissolved sulfate as well as δD and δ18O of water were analyzed for groundwater sampled from three main karst aquifers at the Xikuangshan (XKS) antimony mine from 2013 to 2017. Spatial and temporal variations of Sb and As of the three main karst aquifers were characterized as that, (1) Sb concentrations, rather than As, varied with aquifers, with high Sb (1–50 mg/L) of the D3s2 and C1y3 aquifers and low Sb (0.01–1.0 mg/L) of the D3x4 aquifer; (2) As concentrations had significant seasonal effect that was higher in winter and lower in summer, whereas Sb concentrations did not differ seasonally. Here we applied a recently developed cluster analysis method as well as geochemical indicators to identify pollution types of Sb and As in groundwater related to complex mine activities, and further to reveal main effects on karst groundwater geochemistry. The cluster analysis yielded a classification of four clusters, which was confirmed by principal component analysis. The groundwater of cluster 3 is of weakly alkaline HCO3-Ca type with low Sb (<0.015 mg/L) and low As (<0.005 mg/L), which is typical karst groundwater controlled by limestone dissolution. Whereas the other three clusters with high concentrations of Sb and As were identified to represent three pollution patterns dominated by oxidative dissolution of stibnite, leaching of mine wastes, and mixed pollution, respectively. Moreover, the effects of long-term mine activities on karst groundwater geochemistry are revealed, including: (1) Hydrogeochemical compositions of groundwater changed from HCO3-Ca type to SO4-Ca type. (2) Sulfuric acid significantly participated in carbonate weathering. (3) Weathering of silicate (main as silicified limestone) by H2CO3 and H2SO4 was accelerated. (4) Cation-exchange was enhanced to dominate groundwater geochemistry. Another striking finding is that sulfur and oxygen isotopes of sulfate can effectively distinguish sulfate sources related to mine activities and to trace Sb sources in karst groundwater.