Cold-water geysers at Green River, in east central Utah are sourced by natural springs situated along the Little Grand Wash and Salt Wash faults zones, as well as abandoned oil and water wells. The intermittent geysers are driven by degassing of CO2 from CO2-rich waters sourced from the Jurassic Navajo sandstone. Here we discuss use of del13C analyses of water and gases to estimate the depth at which the ascending waters become CO2 saturated. This is a potentially important control in the unlikely event of rapid CO2 escape from CO2 reservoirs. In Green River, it is also important for calculation of the kinetics of fluid-mineral reactions in the Navajo sandstone as an analogue for predicting the fate of CO2 in carbon storage reservoirs