Floodplain aquifers are a critical component of natural river ecosystems as well as being assets for agricultural, industrial and domestic uses. The preservation of our groundwater reserves is a prerequisite for sustainable growth of the national and global economy. The water quality and microbiology of shallow aquifer systems is greatly influenced by the concentration of dissolved oxygen gas (DO). Numerous studies have shown that concentrations of DO are often depleted along groundwater flow paths; however, the mechanisms (e.g., biotic or abiotic) that consume oxygen are often ambiguous. The use of stable isotopes of molecular O2 has great potential to discriminate between the various mechanisms causing DO depletion in the subsurface, and is the crux of this study. Since this field of study is relatively new, there is a need for a better fundamental understanding of the mechanisms that contribute to O2 consumption in the subsurface before such an approach gains widespread utility. In particular, it is essential to establish functional relationships between changes in £_18O-DO (the stable isotopic composition of DO) with reaction progress for different biotic and abiotic processes. The fractionation of £_18O-DO during microbial respiration is large, and is already known through numerous published laboratory studies. However, there are virtually no published data on the fractionation of 18O accompanying inorganic reactions such as oxidation of dissolved Fe(II) or dissolved sulfide (H2S, HS‾). The current proposal has a major experimental component, the objective of which is to determine kinetic and equilibrium fractionation factors for these inorganic DO-consuming reactions under differing conditions of pH, temperature, and reductant concentration. Experimentally-determined equilibrium and kinetic isotopic fractionation factors are the ¡§mass action constraints¡¨ that are needed to interpret changes in £_18O-DO in natural groundwater settings. However, it is also vital to demonstrate to the scientific community that large variations in £_18O-DO of groundwater do indeed exist in nature, and that plausible explanations for these isotopic gradients can be formulated. This is the impetus for the field segment of the current proposal. The two proposed field sites are the Nyack aquifer along the Middle Fork of the Flathead River floodplain near Kalispell, MT and the Silver Bow Creek aquifer in Butte, MT. The hydrogeology, geochemistry, and microbiology of the Nyack aquifer system has been well characterized by a diverse group of scientists with major funding from the NSF Biocomplexity Program. A well-maintained set of groundwater monitoring wells exists and the oxygen dynamics of the system have been well characterized. One of our main hypotheses is that DO will become progressively enriched in 18O with increased residence time in the subsurface, as microbial processes preferentially consume 16O . Since microbial consumption of DO is coupled to release of isotopically-light biogenic CO2, we also predict an inverse relationship between £_18O-DO and ?Ô13C-DIC (isotopic composition of dissolved inorganic carbon). The Butte field site is a well monitored aquifer in an area that has been heavily impacted by mining and smelting activities for over 100 years. The processes affecting DO depletion in this system are expected be strongly influenced by abiotic mechanisms (e.g., oxidation of reduced metals and sulfide). One outcome of this project will be the development of a new set of tools that can be used to assess the various chemical and biological processes acting on groundwater resources. Additionally, interpretation of groundwater data that includes following £_18O-DO, ?Ô13C-DIC and ?Ô13C-DOC as progress variables will produce a better understanding of processes that affect these underground reservoirs.
