Stable Isotope Composition of Molecular Oxygen in Soil Gas and Groundwater: A Potentially Robust Tracer for Diffusion and Oxygen Consumption Processes

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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 δ18O values ranged mostly from 24.0 to 27.2‰ (SMOW). The concentration of dissolved oxygen in a perched aquifer in the Texas Panhandle (depth to water ∼76 m) was about 5 mg/L and the δ18O values were 21.2–22.9‰. The δ18O of soil gas oxygen in our study are higher and those of dissolved oxygen are lower than the δ18O of atmospheric oxygen (23.5‰). A model for the oxygen concentration and isotopic composition in soil gas was developed using the molecular diffusion theory. The higher δ18O 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 δ18O 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.