SEASONAL AND SPATIAL GRADIENTS IN STABLE ISOTOPES OF DISSOLVED OXYGEN AND DISSOLVED INORGANIC CARBON IN A FLOODPLAIN AQUIFER

The geochemistry and microbiology of shallow aquifer systems is greatly influenced by the concentration of dissolved oxygen gas (DO). DO concentrations are often progressively depleted along groundwater flow paths; however, the mechanisms that consume DO (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of DO (δ¹⁸O-DO) in conjunction with the stable isotope composition of dissolved inorganic carbon (δ¹³C-DIC) has potential to discriminate between the various mechanisms causing DO depletion in the subsurface. Because aerobic bacteria consume isotopically light O₂ (¹⁶O:¹⁶O) at a faster rate than heavier O₂ (¹⁶O:¹⁸O) as an electron acceptor, it is hypothesized that DO should become progressively enriched in ¹⁸O as its concentration diminishes. At the same time, since microbial consumption of DO is coupled to the release of isotopically-light biogenic CO₂, we should observe an inverse relationship between δ¹⁸O-DO and δ¹³C-DIC.

Our field work took place at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana. The site has a well-maintained set of groundwater monitoring wells that is being used by other groups for ongoing geochemical and biological studies. We report the results of spatial and seasonal changes in δ¹⁸O-DO and δ¹³C-DIC over a short, well constrained flow path (~ 200 m) near the upper end of the floodplain. These results are then compared to isotopic data from a set of wells distributed over the entire floodplain (several km). Over the short flow path, δ¹⁸O-DO consistently increased as DO concentrations decreased with distance from the river recharge zone. Concurrently, DIC concentrations increased and δ¹³C-DIC values decreased. These observations are explained by microbial respiration coupled with dissolution of calcite from limestone cobbles in the aquifer matrix. However, over the scale of the entire floodplain, a much less predictable relationship was observed between DO concentration and δ¹⁸O-DO. Many wells with low DO concentrations (e.g., < 4 mg/L) had anomalously low δ¹⁸O-DO values. The origin of the isotopically light DO is an unsolved puzzle, since all known processes that fractionate δ¹⁸O-DO, including microbial respiration and the abiotic oxidation of reduced Fe and S species, consume ¹⁶O faster than ¹⁸O.