Determination of contamination sources and geochemical reactions in groundwater of a mine area using Cu, Zn, and S–O isotopes

To determine contamination sources and pathways, the use of multiple isotopes, including metal isotopes, can increase the reliability of environmental forensic techniques. This study differentiated contamination sources in groundwater of a mine area and elucidated geochemical processes using Cu, Zn, S–O, and O–H isotopes. Sulfate reduction and sulfide precipitation were elucidated using concentrations of dissolved sulfides, δ³⁴S_SO4, δ¹⁸O_SO4, and δ⁶⁶Zn. The overlying contaminated soil was possibly responsible for the contamination of groundwater at <5 mbgl, which was suggested by low δ⁶⁵Cu values (0.419–1.120‰) reflecting those of soil (0.279–1.115‰). The existence of dissolved Cu as Cu(I) may prevent the increase in δ⁶⁵Cu during leaching of contaminated soil in the sulfate-reducing environment. In contrast, the groundwater at >5 mbgl seemed to be highly affected by the contamination plume from the adit water, which was suggested by high SO₄²⁻ concentrations (407–447 mg L⁻¹) and δ⁶⁵Cu (0.252–2.275‰) and δ⁶⁶Zn (−0.105‰–0.362‰) values at a multilevel sampler approaching those of the adit seepages. Additionally, the O–H isotopic ratios were distinguished between <5 mbgl and >5 mbgl. Using δ⁶⁵Cu and δ⁶⁶Zn to support the determination of groundwater contamination sources may be encouraged, particularly where the isotopic signatures are distinct for each source.