VARIABILITY IN ATMOSPHERIC CHLOROFLUOROCARBONS (CCL3F AND CCL2F2) NEAR A LARGE URBAN AREA - IMPLICATIONS FOR GROUNDWATER DATING

Chlorofluorocarbons (CFCs) a re frequently used as tracers for age dat ing of young groundwaters. However, in urban environments with many CF C point sources, uncertainties in the delivery of CFCs to groundwater (input function) complicate quantitative interpretation of observed CF C distributions. To assess the potential impact of elevated atmospheri c CFC mixing ratios on CFC-based groundwater ages near a large coastal urban center, we measured atmospheric mixing ratios of CFCs in Palisa des, NY, 25 km north of New York City. We present and discuss a 16-mon th record of atmospheric CCl3F (CFC-11) and CCl2F2 (CFC-12) obtained f rom gas,chromatographic analyses taken at intervals of approximately 1 6-min. Nearly all measured Values are in excess of remote Northern Hem isphere (NH) atmospheric mixing ratios. The mean mixing ratios of CFC- 11 and CFC-12 are 6 and 13% higher, respectively, than those measured at a remote NH location during the same time period. The temporal tren ds of CFC-11 and CFC-12 differ from those of the remote atmosphere. Di urnal, weekly, and seasonal patterns are evident in the measured CFC d istributions, in addition to variations resulting from regional meteor ological conditions. These observations indicate that, to effectively use CFCs as groundwater dating tools near local or regional sources, t heir local atmospheric input functions must be explicitly defined.