Stable isotope composition of molecular oxygen in soil gas and groundwater: A potentially robust tracer for diffusion and oxygen consumption processes

We have measured the concentration and isotopic composition of molecular oxygen in soil gas and groundwater. At a site near Lincoln. Nebraska, USA, soil gas oxygen concentrations ranged from 13.8 to 17.6% at depths of 3-4 m and the delta(18)O values ranged mostly from 24.0 to 27.2 parts per thousand (SMOW). The concentration of dissolved oxygen in a perched aquifer in the Texas Panhandle (depth to water similar to 76 m) was about 5 mg/L and the delta(18)O values were 21.2-22.9 parts per thousand. The delta(18)O of soil gas oxygen in our study are higher and those of dissolved oxygen an lower than the delta(18)O of atmospheric oxygen (23.5 parts per thousand). A model for the oxygen concentration and isotopic composition in soil gas was developed using the molecular diffusion theory. The higher delta(18)O values in soil gas at the Nebraska site can be explained by the effects of diffusion and soil respiration (plant root and bacterial) on the isotopic composition of molecular oxygen. The lower delta(18)O of dissolved oxygen at the Texas site indicates that oxygen consumption below the root zone in the relatively thick unsaturated zone here may have occurred with a different fractionation factor (either due to inorganic consumption or due to low respiration rates) than that observed for the dominant pathways of plant root and bacterial respiration. It is concluded that the use of the concentration and isotopic composition of soil gas and dissolved oxygen should provide a robust tool for studying the subsurface gaseous diffusion and oxygen consumption processes. Copyright (C) 1998 Elsevier Science Ltd.