Controls on mobility of heavy metals of mine tailings in a karst area as shown by multi-stable isotopes tracing: δ18O/δ2H in soil water and δ34S of soil soluble SO42-

The collapse of a tailings dam of a Pb-Zn mine, caused by a storm in 1978, resulted in severe heavy metals contamination in the valley downstream the mine, located in the Guangxi Province, southwest of China. The metals still pose a risk to the adjacent fragile karst environment. Especially, the potential for leaching of the heavy metals to the adjacent environment is of concern due to the high average annual precipitation of >1500 mm in the subtropical climate. Previous studies have classified areas of the valley as slightly (SP), moderately (MP) or heavily polluted (HP) based on heavy metals content (Pb, Cd, As, Cu, Zn) of the upper 20 cm of the soils. We analysed soil and sediment profiles up to 2 m deep, obtained in areas of the three pollutions levels, for basic chemical and physical parameters including pH, total organic carbon (TOC), soil moisture, particle size, total metals concentrations (Pb, Zn, Cd, and Cu), and δ¹⁸O and δ²H of soil moisture. Further, we measured the δ³⁴S of soil extractable sulphate, and the content of chromium-reducible sulphur (CRS) and soluble sulphates (SS), to investigate the link between sulphur cycling and heavy metals mobilization. Today, four decades after the dam collapse, heavy metal concentrations are still highly elevated in the valley. In the HP profile concentrations of Pb, Cd, Cu and Zn range between 800–8120, 8–132, 156–616, and 2647–12250 mg/kg, respectively, between surface and 2 m depth. Concentrations of CRS in the HP profile of 287–5530 mg/kg were observed, while no CRS could be extracted from the SP and MP soil profiles. The δ³⁴S-SO₄^2- of the HP profile (0.4‰–16.0‰) matches values previously measured in the original tailing. The matching δ³⁴S-SO₄^2- and elevated CRS values of the HP profile indicate that the valley contains thick deposits (up to at least a 2 meters) of resettled tailings sediments of the original upstream tailings dam. However, these sediments are clayey, with >50%wt being <0.002 mm in particle size, allowing only a slow advective water and solute movement to or from (leaching) the sediment. A currently low, yet possibly significant(!), heavy metals leaching is further indicated by the only slightly acidic pH (6-6.5) which indicate a lack of oxygen intrusion into the sediments and reaction with the CRS content. Also, the HP profile had soluble sulphate concentrations of 532–1156 mg S/kg which were reasonably comparable to the values measured in the less polluted areas, implying a history without large amounts of CRS oxidation. Further, Pb and Cu concentration in the HP profile shows a continuous (high) distribution vs. depth which also suggests a history without extremely low pH. Finally, deuterium-excess values can be interpreted as showing diffusive, rather than advective, water and solute movement. While, accordingly, the heavy metals currently appear relatively well stabilized towards leaching, any management that increases oxygen intrusion or water exchange will impose a high risk of immediate and severe environmental pollution to the adjacent aqueous environment.