Interpreting complex geochemistry of groundwater in a coastal paddy field near a mine using isotopic signatures of sulfate and water

In the area around the abandoned Seoseong mine, South Korea, coastal paddy fields undergo seawater intrusion and possible sulfate reduction. Here, channel water is used for irrigation, fertilizers are applied, and some paddy fields are contaminated by mining activities, which subsequently contaminate a groundwater well with arsenic. In this complex environment, the isotopic signatures of sulfate and water in water samples were assessed to reveal sources of sulfate, water and processes in the groundwater system. Sulfur and oxygen isotopes of sulfate indicated three major sources of sulfate—namely the mine including tailings, intruded seawater, and fertilizer—and an additional process of sulfate reduction. The sulfate sources and sulfate reduction could be distinguished more clearly after the variable of sulfate contribution from seawater was introduced. According to the analysis results of hydrogen and oxygen isotopes of water, areas affected by irrigation from a reservoir and its downstream channel were distinguished, possibly because the reservoir underwent evaporation effect. A schematic diagram was proposed to explain complex sources and processes in the studied area. Especially, a suggested plot of δ 34 S SO4 against the sulfate contribution from seawater [f(SO 4 2− seawater )] could efficiently differentiate various contamination sources (e.g., mining activity and fertilizer) and processes (e.g., seawater intrusion and sulfate reduction) in coastal aquifer.