Spatial patterns of Zn, Cd, and Pb isotopic compositions of ground and surface water in mine areas of South Korea reflecting isotopic fractionation during metal attenuation

As application of multiple metal isotopes can effectively constrain geochemical behavior of contaminants and assess contamination sources and pathways, field-scale studies on the geochemically interlinked fractionation of Zn and Cd isotopes in groundwater are needed. In this study, we collected groundwater samples from multi-level samplers downstream of tailings dumps as well as surface water, ore mineral, precipitate, and tailings samples at the Sambo and Buddeun metallic ore mines in South Korea, and analyzed their Zn, Cd, Pb, and sulfur isotopic compositions. Furthermore, isotopic ratios of ore mineral samples from additional four mines in South Korea (Dangdu, Dongbo, Gomyeong, Samgwang) were compared. A dual isotopic approach using Zn and Cd isotopes was used to assess fractionation processes, and Pb isotopic signatures reflecting their sources were assessed. Increasing trends of δ⁶⁶Zn and δ¹¹⁴Cd with decreasing Zn and Cd concentrations were observed in groundwater, which was saturated with respect to ZnS (amorphous and sphalerite) and CdS (greenockite). Moreover, for some groundwater samples, δ⁶⁶Zn showed a positive relationship with δ³⁴S_SO4. These results suggest that Zn and Cd are precipitated as sulfide following sulfate reduction. In the plot of δ⁶⁶Zn against δ¹¹⁴Cd, relatively high and/or increasing δ⁶⁶Zn in groundwater suggested the effect of fractionation due to sulfide precipitation, while variable and high δ¹¹⁴Cd values suggested the fractionation by adsorption and/or sulfide precipitation, which were based on positive fractionation factors for δ⁶⁶Zn and δ¹¹⁴Cd during sulfide precipitation and mostly negative and positive fractionation factors for δ⁶⁶Zn and δ¹¹⁴Cd, respectively, during adsorption. This study shows that the combined use of Zn and Cd isotopes in groundwater can effectively differentiate between adsorption and sulfide precipitation following sulfate reduction in groundwater. Additionally, the ²⁰⁸Pb/²⁰⁶Pb ratios of most water samples reflected those of ore and tailings samples, which verified usefulness of Pb isotopes in water in investigating Pb contamination sources.