Hydrogeology and redox geochemistry of CH₄in a Late Wisconsinan Till and Loess Sequence in central Iowa

Abstract

The hydrogeology and redox geochemistry of CH 4 in two hydrogeological settings within a late Wisconsinan till and loess sequence in central Iowa were investigated using CH 4 , Eh, dissolved O 2 , NO 3 ‐N, NH 4 ‐N, dissolved Fe, SO 4 , H 2 S, dissolved organic carbon (DOC), particulate organic carbon (POC), δ 13 C‐CH 4 , δ 2 H‐CH 4 , δ 13 C of dissolved inorganic carbon (DIC), and 14 C−CH 4 . Depleted δ 13 C‐CH 4 and δ 2 H‐CH 4 values, enriched δ 13 C‐DIC values, and the lack of higher hydrocarbons suggest that the CH 4 is microbial and is formed by CO 2 reduction. Carbon 14 dates of CH 4 indicate that 14,000‐ to 17,000‐year‐old POC and DOC in the 1oess and till compose the substrate for methanogenesis. Production of CH 4 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 CH 4 concentrations in the upper part of the till.