Behavior of stable isotopes of dissolved oxygen, dissolved inorganic carbon and nitrate in groundwater at a former wood treatment facility containing hydrocarbon contamination

The speciation and mobility of a variety of chemical species in groundwater are strongly influenced by redox and pH conditions. Dissolved O2 (DO) and dissolved inorganic C (DIC) concentrations are significant controls of these conditions, respectively. It is not always clear what the major processes are that influence changes in the concentration of DO and DIC across a groundwater flowpath. The combined use of the stable isotope compositions of DO (δ18O–DO) and DIC (δ13C–DIC) has the potential to help investigators discriminate between sources and sinks of DO and DIC in groundwater systems. A total of 31 monitoring wells were sampled to investigate changes in DO, DIC, δ18O–DO and δ13C–DIC in shallow groundwater at the Montana Pole and Treating Plant (MPTP), Butte, Montana, USA. The MPTP site contains significant quantities of petroleum hydrocarbons in soil and groundwater from historical operations as a wood treatment-preservative facility. Dissolved O2 concentrations decreased downgradient across the site (340–6.6 μmol L−1) while DIC concentration increased (3.6–12.8 mmol L−1). A general enrichment in δ18O–DO (16.8–31.6‰) and depletion in δ13C–DIC (−12.7‰ to −18.7‰) along the flowpath is consistent with aerobic microbial respiration. However, the correlation between δ18O–DO and δ13C–DIC was poor which may be caused in part by the remediation treatments being used to mitigate soil and groundwater contamination. The presence of an extensive suboxic zone near the center of the field area suggests that some DIC of unknown isotopic composition could have been added to the groundwater by anaerobic degradation of hydrocarbons. This idea is consistent with a limited amount of NO3- isotope data which suggest that denitrification is occurring at the MPTP site.