STRONG IMPACT OF CFCS DESORPTION IN AQUIFERS. TRACERS OF VOCS GROUNDWATER TRANSPORT

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 CFCs sorption at ambient and lower concentrations is evaluated. For this purpose, sediment samples that were initially equilibrated with ambient concentrations are purged intermittently and total CFC mass is quantified from each purge. The short purge cycles (4 minutes) hence provide two functions: they allow us to quantify the CFC mass in the air and water phases and they remove CFC mass from the sealed batch reactors lowering the system concentration and perturbing the equilibrium. Experiments were conducted in a CFC-free chamber to prevent sample contamination. VOCs were trapped into a cold trap and analyzed by a gas chromatograph with electron capture detection following standard methods for CFC analysis at typical environmental concentrations. Experiments were done to confirm that desorption during purging was insignificant.

The initial sample selected for study is Marcellus shale which is representative of kerogen-containing rocks and sediments typical in the Northeastern and Central US. The contact time to achieve equilibrium for CFC-11, CFC-12 and CFC-113 at less than atmospheric concentrations was similar to the patterns observed in sorption experiments at high concentrations for the same material. Ongoing experiments are documenting CFC desorption isotherms. 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.