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.