δ34S and δ18O of sulfates and Zn/Cd ratios reveal the cause of soil and groundwater contamination in metalliferous mining areas

Abstract

In mining areas, discriminating the contamination sources and pathways of soil and groundwater downstream of mine drainage and tailings is often difficult. Vertical profiles of the groundwater geochemistry and soil-contamination data were investigated for the abandoned Dalseong mine in South Korea. Zn and Cu from a contaminant plume were nearly depleted at some depths by sulfide precipitation following sulfate reduction. Secondary metal sulfides, possibly from contaminated adit seepage, may have acted as soil contaminants. The extraordinarily low δ34SSO4 between −7.5 and −7.1‰ in shallow groundwater suggests the oxidation of secondary sulfides to contaminate shallow groundwater. Additionally, the high δ18OSO4 between 6.5 and 6.8‰ in shallow groundwater indicates slow oxidation, mainly by atmospheric oxygen (54–74%), and the high δ18OH2O and δ2HH2O indicate water from uncontaminated groundwater or precipitation. The Zn/Cd ratios of soil (46–218) in the area approached those of water samples (54–121) and were distinguished from the Zn/Cd ratios of soil (339–1014) which are considered to have been contaminated by tailings. The lower Zn/Cd may have been related to the enrichment of Cd from water. δ34SSO4 and δ18OSO4 can discriminate between the soil and groundwater contamination sources (mine water and solid waste) especially in paddy field with possible sulfate reduction. Besides, Zn/Cd ratios can be utilized where soil contamination sources have distinct Zn/Cd ratios.