CHLOROFLUOROCARBONS (CCL3F AND CCL2F2) AS DATING TOOLS AND HYDROLOGICTRACERS IN SHALLOW GROUNDWATER OF THE DELMARVA PENINSULA, ATLANTIC COASTAL-PLAIN, UNITED-STATES

Concentrations of the chlorofluorocarbons (CFCs) CFC-11 and CFC-12 wer e determined in groundwater from coastal plain sediments of the Delmar va Peninsula. CFC-modeled ages were calculated independently for CFC-1 1 and CFC-12, and agreed to within 2-3 years in the majority of the wa ters. Recharge temperatures, determined from dissolved nitrogen and ar gon concentrations, varied from 9 +/- 2-degrees-C over most of the pen insula to 14 +/- 2-degrees-C at the southernmost tip of the peninsula in Virginia. The CFC-modeled ages were examined in relation to the kno wn hydrogeologic environment, both on regional scales and in more inte nsively sampled local scale networks. The CFC-modeled recharge years a nd measured tritium concentrations were used to reconstruct a tritium input function that was compared to the modeled tritiUM plus He-3 dist ribution. Most of the present distribution of tritium in Delmarva grou ndwater is consistent with low dispersivities. The results of the stud y strongly support the use of CFCs for dating shallow, aerobic groundw ater.