CONCENTRATIONS AND ISOTOPE RATIOS OF DISSOLVED INORGANIC CARBON IN DENITRIFYING ENVIRONMENTS

We measured the concentration and isotope ratio of dissolved inorganic carbon (DIC) in groundwater associated with denitrification (C-org NO3- = CO2 + N-2) in an agriculturally impacted site in southwestern M ichigan. Samples with the lowest nitrate levels also had low dissolved oxygen content and were more depleted in delta(13)C than background g roundwater. All the samples had DIC concentrations in excess of titrat able alkalinity. The magnitude of this DIC in excess of alkalinity cor related with a decreasing delta(13)C attesting to the presence of CO2 derived from organic carbon, Carbon dioxide is one of the byproducts o f denitrification. Since the carbon isotope ratio of carbon dioxide pr oduced during denitrification can be expected to be similar to that of the substrate consumed in the process, this can serve as a useful too l in monitoring denitrification. The estimated carbon isotope ratio of source carbon added to the DIC pool via denitrification was close to the value obtained for the organic carbon in the aquifer solids, sugge sting that the carbon substrate used in denitrification came from soil organic matter. The results provide support for the hypothesis that t he concentration and carbon isotope ratios (delta(13)C) of dissolved i norganic carbon (DIG) in groundwater might be useful to monitor denitr ification.