CONTEMPORARY GROUNDWATER METHANE PRODUCTION FROM PLEISTOCENE CARBON

Relative to surface systems, microbial activities in groundwater have been poorly studied, especially with regard to methanogenesis. Past studies have reported high concentrations of CH4 in groundwater in the late Wisconsinan till and Wisconsinan loess of central Iowa. The objectives of this work were to assess whether CH4 is presently produced in the till and loess, and, if so, to characterize the spatial variability of CH4 production rates. Groundwater and sediment samples were collected at several locations within the Des Moines Lobe in central Iowa. Methane concentrations in groundwater ranged from 0.04 mu mol L(-1) at 1 m to 2600 mu mol L(-1) at 7 m. Incubations of deep sediment obtained from core samples revealed active CH4 production was occurring, although high variability was observed. Highest specific CH4 production rates were associated with buried particulate C fragments of Wisconsinan age. Results of our study indicate methanogenesis is an active process in the sediments beneath the Des Moines Lobe, that the substrate for methanogenesis is of late Wisconsinan age, and that the high spatial variability of CH4 production is controlled by the heterogeneous distribution of particulate organic C fragments in the glacial sediments.