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 molecul ar oxygen in soil gas and groundwater. At a site near Lincoln. Nebrask a, USA, soil gas oxygen concentrations ranged from 13.8 to 17.6% at de pths 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 pe r 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 atmosphe ric oxygen (23.5 parts per thousand). A model for the oxygen concentra tion and isotopic composition in soil gas was developed using the mole cular diffusion theory. The higher delta(18)O values in soil gas at th e 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 Texa s site indicates that oxygen consumption below the root zone in the re latively thick unsaturated zone here may have occurred with a differen t 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 gaseou s diffusion and oxygen consumption processes. Copyright (C) 1998 Elsev ier Science Ltd.