Carbon dioxide respiration in the deep vadose zone: Implications for groundwater age dating

In the deep vadose zone at the Dalmeny site, subsurface gas samples were collected and analyzed for CO2 and the C-13 and C-14 ratios of that CO2. High concentrations of CO2 depleted in C-14 near the water table necessitate the use of an open-system model of calcite dissolution to match observed dissolved inorganic carbon C-14 ratios just below the water table. Groundwater age-dating models assuming closed-system calcite dissolution predict incorrect groundwater age dates at the Dalmeny site. These results and our field observations suggest that such errors may generally occur where deep-vadose generation of nonmodern CO2 is not accounted for in groundwater age-dating exercises.