Dissolved O₂ (DO) is an important geochemical agent in groundwater, particularly in controlling the oxidation state and hence mobility of contaminants including metals. A large body of research on O₂ in the unsaturated zone has shown that the major processes affecting its concentration ([O₂]) and isotopic composition (d¹⁸O) are diffusion and microbial respiration.  However, this remains to be adequately extended to the saturated zone, where the relationship between DO concentrations and its d¹⁸O may be useful in identifying redox processes involving contaminants. Here, we present concentrations and d¹⁸O of DO in a groundwater system below the Key Lake Uranium Mine in northern Saskatchewan. This site was selected because it is both well-characterized and extensively monitored for its physical and geochemical properties, making it an ideal site for this investigation. 

CO₂ and O₂ pore-gas concentration and d¹³C_CO2 profiles through the unsaturated waste rock piles that cover much of the Key Lake site have previously shown that decreasing [O₂] with depth is predominantly due to oxidation of organic matter at the base of the piles rather than sulfide mineral oxidation.  Increasing d¹⁸O values from 23.5 ‰ (atmospheric) to 31 ‰, with decreasing [O₂] (and increasing depth) is also consistent with this.  Our observations in the groundwater system were that DO concentrations below the main waste rock pile continue to decrease along a groundwater flow path.  However, in contrast to the unsaturated zone situation, d¹⁸O values decrease from 25 ‰ to 19 ‰ at low (0 mg/L) DO concentrations.  In groundwater on the up-gradient side of the waste rock pile, a different trend, previously observed in the unsaturated zone, was identified.  Here, d¹⁸O of pore-gas O₂ again decreased from 23.5 ‰ to 20.0 ‰ with depth, whilst [O₂] remains close to atmospheric concentrations.  In the groundwater system, d¹⁸O values as low as 14.6 ‰ were observed, with similar DO concentrations.  Such decreases in d¹⁸O below atmospheric have not previously been observed and suggest that a process other than diffusion or respiration may control d¹⁸O_DO.  A dramatic drop in pH from 6.8 to 3 along the groundwater flow path suggests that inorganic redox reactions may be involved.