Seasonal variability and long term trends of chlorofluorocarbon mixing ratios in the unsaturated zone.

To investigate processes that might affect chlorofluorocarbon (CFC) mixing ratios at the water table, a time series was obtained of unsaturated zone soil gas CFCs to depths of ca. 4 m at a site near New York City (NYC). Observed CFC 11, 12, and 113 mixing ratios were lower in winter than expected from either a local, high-resolution time series or remote atmospheric mixing ratios. A diffusion model, which includes seasonal changes in soil temperature, moisture, and CFC solubility, reproduces to first order the observed soil gas mixing ratios for CFC 11 and 12. Underestimation by the model of the seasonal cycle of CFC 11 points to changing levels of sorption to soils due to seasonal changes in temperature as an additional cause of the cycle seen in CFC 11 mixing ratios in soil air. In the case of spring recharge, low CFC mixing ratios in soil air caused by increased solubility may result in low CFC 11 concentrations in groundwater and, when dating groundwater recharged before the 1990s with CFCs, older apparent ages by up to 4 years. Attempts to observe average atmospheric CFC levels from soil gas are also significantly hindered by these seasonal fluctuations. Our results indicate the importance of considering seasonal changes in soil temperature when making precise observations of even very moderately soluble gases in the unsaturated zone and shallow groundwater.