CFC DESORPTION IN AQUIFERS AND THEIR IMPLICATIONS AS TRACERS OF VOC TRANSPORT IN GROUNDWATER

Chlorofluorocarbons (CFCs) may be useful as reactive tracers that show where and how volatile organic compounds (VOCs) are retarded in groundwater. Recent work by our group has demonstrated CFCs adsorption -at very high concentrations- to black carbon (BC). The potential effect of retarded transport on groundwater age determinations in BC-containing aquifers was also illustrated. In the present study, groundwater CFC sorption at ambient and lower concentrations is evaluated. The initial sample selected for study is the Marcellus shale, which is representative of kerogen-containing rocks and sediments typical in the Northeastern and Central US. Initial experiments were performed using 40 and 160 mg sample sizes. Additional experiments using 10, 20, and 40 mg sample sizes are ongoing. The contact time to achieve equilibrium for CFC-11, CFC-12 and CFC-113 at less than atmospheric concentrations was two weeks, similar to the patterns observed in sorption experiments at high concentrations for the same material. Desorption of CFCs exhibited a nonlinear behavior. The calculated K_oc,C for CFC-12 (2.9E+05 L/Kg) is four orders of magnitude higher than the one estimated from a linear energy relationship. Ongoing experiments are documenting CFC desorption isotherms for CFC-11 and CFC-113. These results will help us to elucidate the potential for CFCs to serve as retarded tracers of VOCs transport in kerogen-containing aquifers and to estimate the effect on groundwater age determinations in such systems.