Dissolved oxygen and dissolved inorganic carbon stable isotope composition and concentration fluxes across several shallow floodplain aquifers and in a diffusion experiment

Recent studies have documented the occurrence of dissolved molecular oxygen (DO) in shallow groundwater that is isotopically lighter than can be explained by atmospheric gas exchange or by biogeochemical reactions that consume ¹⁶O¹⁶O faster than ¹⁶O¹⁸O. In the present study, spatial gradients in the isotopic composition of DO (δ¹⁸O-DO) and dissolved inorganic carbon (δ¹³C-DIC) were measured in three shallow floodplain aquifers: (1) the Nyack aquifer, of the Middle Fork of the Flathead River in northwest Montana; (2) the Silver Bow Creek floodplain in southwest Montana; and (3) the Umatilla River floodplain in northeast Oregon. The field data show general trends of increasing DIC concentration, decreasing δ¹³C-DIC, and decreasing DO concentration with increase in groundwater path length. These trends are consistent with consumption of DO and production of DIC by microbial respiration. Although the expected trend of an increase in δ¹⁸O-DO with increase in path length was found at an area adjacent to hyporheic recharge at the Nyack floodplain, the majority of groundwater samples collected at Nyack and from the other sites distal to recharge zones had anomalously low δ¹⁸O-DO values well below 24.2 ‰, the value corresponding to atmospheric isotopic equilibrium. At the Nyack site, ³H-³He dates were used to estimate groundwater travel time: all groundwater samples with apparent age >1 year had δ¹⁸O-DO<24.2 ‰. Previously it has been suggested that diffusion of O₂ could be a viable mechanism to explain the existence of isotopically light DO in shallow groundwater. To test this hypothesis, laboratory experiments were conducted to measure the isotopic fractionation of O₂ as it diffuses from air across a simulated capillary fringe (made from a floating layer of foam beads) into a stirred, initially anoxic, water column. As expected, ¹⁶O¹⁶O diffused faster than ¹⁶O¹⁸O, and the magnitude of isotope fractionation associated with diffusion increased with a decrease in temperature. Fractionation factors (α) calculated from these diffusion experiments were 1.0030 at 15–19 °C and 1.0048 at 8 °C. The combined field and laboratory data suggest that diffusion is an important mechanism to maintain aerobic conditions in shallow groundwater systems, allowing microbial respiration to continue at long distances (km scale) from the source of groundwater recharge.