Signature of oxygen and sulfur isotopes of sulfate in ground and surface water reflecting enhanced sulfide oxidation in mine areas

Abstract

Aqueous geochemistry data together with sulfur and oxygen isotope ratios of minerals and dissolved sulfate in ground and surface water were assessed in six mining areas including five metalliferous mines and one coal mine in various regions in South Korea. The delta S-34(sulfate) values effectively reflected characteristic sulfur isotopic ratios of minerals in each mine area, and the delta O-18(sulfate) values of mining-affected water (adit water and leachate) were depleted than those of background water. A plot of delta O-18(sulfate) values versus the logarithmic sulfate concentration revealed a good linear negative relationship (R-2 = 0.78) for 219 samples spanning six mines in different regions. The observed range of delta O-18(sulfate) values spanned the region where most of oxygen is derived from molecular oxygen for samples with low sulfate concentrations and the region of 100% oxygen derived from water molecules when sulfate concentrations were highest. This suggests that enhanced sulfide oxidation by ferric iron incorporating water-derived oxygen exponentially increases sulfate concentrations in mining areas. In addition, the lower and upper limits of sulfate in the plot could be defined as the atmospheric sulfate input and saturation of gypsum, respectively. The observed relationship between delta(18)O(sulfat)e and sulfate concentration based on field data can further be used for semi-quantitative interpretation of oxygen isotopic ratios of sulfate in mine areas in terms of pyrite oxidation pathways.