IMPACT OF CHLOROFLUOROCARBONS SORPTION TO CARBONACEOUS MATTER ON GROUNDWATER AGE-DATING

The chlorofluorocarbons (CFC), CFC-11, CFC-12, and CFC-113, have been used as environmental tracers for groundwater age-dating. Most studies of CFC age-dating assume either conservative behavior (no sorption) or linear sorption with low K_OC. However, sorption of other hydrophobic organic chemicals to thermally altered carbonaceous matter such as char, soot, and kerogen is exceptionally stronger than to bulk soil organic matter. Although CFCs have low water solubility, sorption studies of CFCs to various forms of carbonaceous matter (CM) have not previously been reported.

This study focuses on determining the sorption of CFCs to two forms of CM, which is the first step in evaluating the effects of sorption on groundwater age-dating. Sorption experiments using batch techniques were performed with wood char as a representative of thermally altered CM forms and commercial humic acid as a representative of amorphous organic matter. Batch experiments using five different aqueous phase CFC concentrations ranging over four orders of magnitude were employed to develop sorption isotherms. The humic acid sorption reached equilibrium quickly while equilibration with wood char required about 100 times longer. Nonlinear sorption behavior to char was observed, but humic acid showed relatively linear sorption behavior. Comparison between wood char and humic acid K_OC values indicates that sorption of CFCs to wood char is ³ 100 times stronger than sorption to humic acid at low dissolved concentrations. Considering the strong sorption to thermally altered CM, the retardation factor was greatly increased in comparison to the estimates from previous studies. These results suggested that CFCs can be significantly retarded even in low thermally altered CM contents. Therefore, this study shows that groundwater age-dating on the basis of CFC techniques may require consideration of retardation in aquifer environments containing thermally altered CM.