14C Dating of Groundwater Containing Microbial CH4

Abstract

Groundwater samples were collected from several different depths in Illinois glacial deposits at a site in eastcentral Illinois. Dissolved gases were extracted from many of the water samples, measured volumetrically, and analyzed by gas chromatography. The DIC was precipitated as barium carbonate and analyzed for both delta-13C and 14C. Due to the formation of microbial CH4, some of the DIC had unusually heavy delta-13C values of -3 to -1 per mil. The standard groundwater 14C-age correction models were developed to account for carbon dissolution as the primary carbon input beneath the soil zone. If the heavy delta-13C values observed in this study are used in readily available groundwater dating models without accounting for the effects of microbial methane formation, many of the resultant calculated ages are negative (future ages) or mathematically unsolvable. Isotopic and analytical results show a positive correlation (r2 = 0.90) between the delta-13C of the DIC and the concentration of methane in the groundwater. With this correlation, we were able to correct the delta-13C values of the DIC which were altered due to microbial CH4 formation. This adjustment of delta-13C values, along with estimations of dead carbon input from the redox processes, allowed us to calculate 14C ages using standard groundwater age correction models.