Journal of Hydrology, 111 (1989) 133--143 133 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands [2] DIFFUSION AND CONSUMPTION OF METHANE IN AN UNSATURATED ZONE IN NORTH-CENTRAL ILLINOIS, U.S.A. ROBERT G. STRIEGL and AUDREY L. ISHII U.S. Geological Survey, P.O. Box 25046, Mail Stop 413, Denver, CO 80225 U.S. Geological Survey, 102 E. Main St., Urbana, IL 61801 (U.S.A) (Received October 24, 1988; accepted after revision February 20, 1989) 'U.S.A.) ABSTRACT Striegl, R.G. and Ishii, A.L., 1989. Diffusion and consumption of methane m an unsaturated zone in north-central Illinois, U.S.A.J. Hydrol., 111: 133-143. The distribution of CH 4 in unsaturated glacial and eolian deposits adjacent to buried low-level radioactive waste was measured, and movement of the gas from the waste source was simulated using a two-dimensional finite-difference model for gas diffusion in the unsaturated zone. Mean PCH 4 was greatest (1.56 Pa) in a pebbly-sand deposit 11.6 m below the land surface and 12 m from the waste, and generally decreased with increased horizontal distance from the waste. Mean PCH 4 was least (0.07 Pa) at depth of 1.8 m below land surface, regardless of distance from the waste. PCH 4 at the land surface averaged 0.17 Pa. Depth versus PCH 4 profiles suggest consumption of both waste-produced and atmospheric CH 4 in the upper unsaturated zone, presumably by methanotroph- ic microorganisms. Numerical simulations of methane movement support the consumption obser- vation; inclusion of a term in the model for consumption of CH 4 in the upper 2 m of the unsaturated zone resulted in simulated PCH 4 within 30% of mean PCH 4 at eleven of thirteen sampling locations. A similar fit of the data was obtained for only four locations when consumption was not considered. INTRODUCTION Recent increases in the CH4 composition of the atmosphere have evoked extensive characterization of global CH4 sources and sinks (Khalil and Rasmussen, 1983; Blake and Rowland, 1986, 1988; Matthews and Fung, 1987; Mooney et al., 1987). A related area of importance is determination of how existing CH4 sinks may accomodate future CH4 increases. Microbial consump- tion of CH4 in soils was first recognized near sources of large CH4 concentra- tions (Adamse et al., 1972; Mancinelli et al., 1981). More recently, consumption of atmospheric or subatmospheric concentrations of CH4 has been observed in a variety of unsaturated soils and climates (Harriss et al., 1982; Keller et al., 1988). However, the taxonomy, the depth distribution, and the consumptive capacities of the responsible methanotrophs have not been fully characterized (Conrad, 1984). In this paper we describe the partial-pressure distribution of CH4 in the 0022-1694/89/$03.50 © 1989 Elsevier Science Publishers B.V.