T78 Groundwater in Ecosystems: Effects of Physical, Chemical, and Biological Processes and Feedback Mechanisms

Monday, 19 October 2009
8:00 AM - 12:00 PM
D137/138

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

Paper number: T78-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

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.