Tracing dissolved O2 and dissolved inorganic carbon stable isotope dynamics in the Nyack aquifer: Middle Fork Flathead River, Montana, USA

The geochemistry and microbiology of shallow groundwater aquifers is greatly influenced by the concentration of dissolved oxygen gas (DO); however, the mechanisms that consume DO in groundwater (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of molecular O₂ (δ¹⁸O-DO), in conjunction with stable isotopes of dissolved inorganic carbon (δ¹³C-DIC), has potential to discriminate between the various mechanisms causing DO depletion in subsurface waters.Here we report the results of spatial and seasonal changes in δ¹⁸O-DO and δ¹³C-DIC at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana, USA. Over a short, well constrained flow path (∼100 m) near a main recharge zone of the floodplain, the δ¹⁸O-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and δ¹³C-DIC values decreased. These observations are explained by community respiration coupled with dissolution of calcite from cobbles in the aquifer matrix. When these results are compared to data from wells distributed over the entire floodplain (several km) a much less predictable relationship was observed between DO concentration and δ¹⁸O-DO. Many wells with low DO concentrations (e.g., <125 μmol L⁻¹ or 4 mg L⁻¹) had anomalously low δ¹⁸O-DO values (e.g., <20‰). Mass balance calculations show that approximately equal amounts of O₂ may be contributed to the aquifer by diffusion from the vadose zone and by advection from the river recharge. Calculations presented here suggest that diffusion across a narrow air-water interface can contribute isotopically light δ¹⁸O-DO to the saturated zone. Possible contributions of light δ¹⁸O-DO from other processes, such as isotopic exchange and radial oxygen loss from plant roots in or near the water table, are compared and evaluated.