IMPACT OF LAND USE CHANGE ON CARBONATE WEATHERING RATES IN A SHALLOW GLACIAL AQUIFER; CHEBOYGAN WATERSHED, MI

Chemical weathering rates play an important role in the carbon cycle. Quantifying these dissolution rates is essential to a comprehensive understanding of modern and past carbon cycles. In this study, groundwater age tracers were used in conjunction with geochemical measurements to understand dissolution reactions in a Midwestern aquifer. Residence times were calculated based on chlorofluorocarbon (CFC) concentrations for eight groundwater wells sampled from the Cheboygan watershed in the northern Lower Peninsula, Michigan. In addition, groundwater samples were analyzed for standard cation and anion content. Preliminary data from this shallow groundwater aquifer in the carbonate-dominated Cheboygan watershed exhibit an inverse relationship between mineral solute concentration and groundwater residence time. Cheboygan groundwaters with shorter residence times are more concentrated in dissolved inorganic carbon (DIC) and solutes derived from mineral weathering reactions than groundwaters with longer residence times. This inverse relationship suggests that the flux of DIC to groundwater may be increasing in response to human activities such as land use change. Groundwater residence times range from 16 to 48 years and hence provide an approximately 30-year record of change. We hypothesize that low Mg-calcite precipitation is limited, as is suggested by the chemical composition of these waters. Therefore, the inverse relationship between groundwater age and solute concentrations is likely to result from fundamental changes in ecosystem behavior. The consequence of these important changes is an increasing flux of DIC to groundwater and increasing mineral weathering rates. The rapid response of shallow groundwater to changes at the Earth's surface is likely to play an important role in short-term carbon cycling.