Fractionation behaviors of Cu, Zn, and S-O isotopes in groundwater contaminated with petroleum and treated by oxidation

Fractionation behaviors of Cu and Zn isotopes have been increasingly studied at the field scale, but those in various redox conditions of groundwater contaminated with petroleum and treated by oxidation have not been assessed. In this study, δ65Cu and δ66Zn as well as δ34SSO4 and Δδ18OSO4-H2O were assessed in wells undergoing contamination by total petroleum hydrocarbons (TPH) and oxidation using H2O2 in 2021 and 2022. High δ34SSO4 and relevant parameters (e.g., dissolved sulfide and HCO3-) indicated the occurrence of sulfate reduction. The plot of δ65Cu versus δ34SSO4 effectively indicated precipitation of Cu sulfides and their reoxidation at oxidation wells. Although the plot of δ66Zn versus δ34SSO4 could also indicate reoxidation of Zn sulfides, the Zn isotopic fingerprint of sulfide precipitation may have been masked by fractionation by sorption. The advantage of using δ65Cu in the redox reactions resulted from the wider range of δ65Cu owing to the redox behavior of Cu. The plot combining isotopic fractionations of Cu and S can assist in assessing sulfide precipitation and oxidative treatment in TPH-contaminated groundwater. [Display omitted] •δ65Cu and δ66Zn during precipitation and dissolution of sulfides were elucidated.•δ65Cu was more efficient than δ66Zn in assessing sulfide precipitation and oxidation.•The δ65Cu-δ34SSO4 plot was very useful to assess the oxidative treatment of TPH.