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 O-2 (delta O-18-DO), in conjunction with stable isotopes of dissolved inorganic carbon (delta C-13-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 delta O-18-DO and delta C-13-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 (similar to 100 m) near a main recharge zone of the floodplain, the delta O-18-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and delta C-13-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 delta O-18-DO. Many wells with low DO concentrations (e.g., <125 mu mol L-1 or 4 mg L-1) had anomalously low delta O-18-DO values (e.g., <20 parts per thousand). Mass balance calculations show that approximately equal amounts of O-2 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 delta O-18-DO to the saturated zone. Possible contributions of light delta O-18-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. (C) 2011 Elsevier Ltd. All rights reserved.