Tracing and quantifying contributions of end members to karst water at a coalfield in southwest China

Abstract

Karst water, which provides 25% of the world's drinking water, is especially vulnerable to anthropogenic contamination. Such is the case in southwestern China with trace element pollution in important karst aquifers. Approximately 20% of the total study area consisted of abandoned mine tailings with elevated concentrations of Fe, S, Mn, As, Cu, and Cr. Acid mine drainage (AMD) water originating from pyrite oxidation of the tailings was characterized by low pH and high concentrations of Fe, SO₄^2-, and As. Concentrations of Fe, Mn, Al, SO₄^2-, As, Cd, and Pb in spring water in wet and dry seasons were greater than WHO and USEPA drinking water guidelines. Based on the results of mineral characterizations, hydrogeochemistry, and isotopic compositions (δ³⁴S_SO4, δ¹⁸O_SO4, and δ¹⁸O_H2O), the chemistry of AMD water was primarily controlled by pyrite oxidation, river water by atmospheric precipitation, and spring water by carbonate rock dissolution and mixing with river and AMD waters. A three-end-member mixing model identified the contributions of these different end members to spring water quality. Although AMD water was characterized by the lowest mixing percentages during the wet (14.1%) and dry (26.9%) seasons, it played a very important role in degrading spring water quality. Based on these findings, an investigation strategy was developed for illuminating seasonal water quality and potential remediation methods corresponding to the contaminants in the spring water are also proposed to manage this seriously polluted karst system. Results could benefit remediation planning for these distinctively complex and vulnerable systems in other regions of the world.

Keywords: A three-end-member mixing model; Coalfield; Hydrogeochemistry; Mineral characterization; Stable isotopes.