In the deep vadose zone at the Dalmeny site, subsurface gas samples were collected and analyzed for CO2 and the 13C and 14C ratios of that CO2. High concentrations of CO2 depleted in 14C near the water table necessitate the use of an open-system model of calcite dissolution to match observed dissolved inorganic carbon 14C 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. ¿ 1998 American Geophysical Union