Concentrations of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 were determined in groundwater from coastal plain sediments of the Delmarva Peninsula. CFC-modeled ages were calculated independently for CFC-11 and CFC-12, and agreed to within 2-3 years in the majority of the waters. Recharge temperatures, determined from dissolved nitrogen and argon concentrations, varied from 9 -+ 2øC over most of the peninsula to 14 +_ 2øC at the southernmost tip of the peninsula in Virginia. The CFC-modeled ages were examined in relation to the known hydrogeologic environment, both on regional scales and in more intensively sampled local scale networks. The CFC-modeled recharge years and measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritium plus 3He distribution. Most of the present distribution of tritium in Delmarva groundwater is consistent with low dispersivities. The results of the study strongly support the use of CFCs for dating shallow, aerobic groundwater. INTRODUCTION Chlorofluorocarbons (CFCs, Freons, (The use of brand names in this report is for identification purposes only and does not represent endorsement by the U.S. Geological Survey) chlorofluoromethanes, halomethanes, or halocarbons), specifically CFC-11 (CCI3F) and CFC-12 (CC12F2), are chemically stable, manmade compounds that have been manufactured since the !940s and 1930s, respectively, for use as propellants in aerosol cans, as foaming agents in plastics, and as refrigerants and solvents. They have received widespread attention in recent years because of their long atmospheric lifetimes, their contribution to ozone destruction in the lower stratosphere, and their contribution as atmospheric greenhouse gases. Growth curves of atmospheric concentrations of CFC-11 and CFC-12 have been reconstructed using production records and release data prior to 1977 and worldwide atmospheric measurements thereafter [Elkins et al., 1993; Busenberg et al., 1993]. Based on these growth curves and CFC solubility relations [Warner and Weiss, 1985], concentrations of CFCs found in groundwater have been used as indicators of the time since the infiltrating groundwater was recharged and isolated from the soil air assuming that the soil air is in equilibrium with the troposphere [Busenberg and Plumruer, 1992]. Results from previous studies confirm the utility of CFCs as dating tools and groundwater tracers [Randall and Schultz, 1976; Schultz et al.; Plummer et al., 1993a, b]. Busenberg and Plumruer [1992] introduced a sampling procedure for collection and storage of water samples in borosilicate glass ampules that are flame sealed in the field. CFCs remain stable in these ampules for many months and can be returned to the laboratory for analysis. Plumruer et al. [ 1993a] summarize the CFC dating method, evaluate effects of hydrodynamic dispersion on CFC dating, and compare CFC dating to dating with other environmental tracers. The present study uses CFC-11 and CFC-12 to date shallow groundwater from largely u...
