Hydrogeology and redox geochemistry of CH4 in a Late Wisconsinan Till and Loess Sequence in central Iowa

Abstract

The hydrogeology and redox geochemistry of CH4 in two hydrogeological settings within a late Wisconsinan till and loess sequence in central Iowa were investigated using CH4, Eh, dissolved O2, NO3‐N, NH4‐N, dissolved Fe, SO4, H2S, dissolved organic carbon (DOC), particulate organic carbon (POC), δ13C‐CH4, δ2H‐CH4, δ13C of dissolved inorganic carbon (DIC), and 14C−CH4. Depleted δ13C‐CH4 and δ2H‐CH4 values, enriched δ13C‐DIC values, and the lack of higher hydrocarbons suggest that the CH4 is microbial and is formed by CO2 reduction. Carbon 14 dates of CH4 indicate that 14,000‐ to 17,000‐year‐old POC and DOC in the 1oess and till compose the substrate for methanogenesis. Production of CH4 probably began after deglaciation at about 12,000 years B.P. and has continued to the present. Groundwater recharge, particularly in the upland topographic position at the site, has decreased CH4 concentrations in the upper part of the till.