Dissolved oxygen and dissolved inorganic carbon stable isotope composition and concentration fluxes in shallow groundwater

Oxygen and carbon isotopes have been used to provide insight into the biogeochemical mechanisms affecting the dissolved O2 (delta 18O-DO) and dissolved inorganic carbon (delta13C-DIC) concentrations in a marine, fresh and groundwater systems. Previous studies on the Nyack aquifer near West Glacier, Montana reported that there were two different processes affecting the introduction, consumption and transport of the DO throughout the floodplain aquifer. This conclusion was based on the contrasting trends observed in delta18O-DO behavior throughout this aquifer. Microbial respiration is one process known to affect the DO within groundwater that will increase delta18O-DO values as DO is consumed. This relationship between delta18O-DO and DO was observed at sites proximal to a major river recharge area (within 100m). Across the broader range of the floodplain aquifer (3--4 km) the delta 18O-DO values decreased as DO concentration decreased. At the same time, the delta13C-DIC values consistently decreased and the DIC concentration increased across the whole floodplain which is consistent with microbial respiration as a significant component affecting the DIC concentration.;In order to better understand the causes of the decreasing delta 18O-DO values as DO concentration decreased, described above, this study used 3H-3He dating to measure groundwater ages for samples collected across the Nyack site. As the groundwater moves down-gradient there is an increase in specific conductance and DIC concentration with increasing age. Results also showed a consistent trend of decreasing delta 18O-DO, delta13C-DIC and DO concentration with increasing groundwater age. Previous studies indicated that the age of the groundwater in the lower reaches of the floodplain was no older than 1.5--2 years, the 3H-3He data from this study reported that water as old as 10 years in age can be found in the lower reaches the Nyack floodplain.;A laboratory tank experiment was performed as part of this study to simulate the diffusional fractionation s of O2 as it moves across an unsaturated/saturated zone interface. Observations showed that as the O2 diffused across the simulated capillary fringe the DO concentration slowly increased, starting out isotopically depleted and becoming progressively enriched as the concentration increased.;Additionally this study investigated the possible effects of root oxygen loss (ROL) over a diel period on the delta18O-DO in shallow groundwater. Results showed that there was little to no diel change on the isotopic composition of DO in in the surrounding groundwater. A series of wells along the Silver Bow Creek floodplain were sampled for delta 18O-DO and delta13C-DIC compositions and showed that the delta18O-DO values were all below atmospheric equilibrium and the delta13C-DIC decreased with distance down the floodplain.