Variability in atmospheric chlorofluorocarbons (CCl3F and CCl2F2) near a large urban area: Implications for groundwater dating

Publication Type  Journal Article
Year of Publication  1998
Authors  Ho, D. T.; Schlosser, P.; Smethie, W. M.; Simpson, H. J.
Journal Title  Environmental Science & Technology
Volume  32
Issue  16
Pages  2377-2382
Journal Date  Aug 15
ISBN Number  0013-936X
Accession Number  ISI:000075380800003
Key Words  shallow groundwater; hydrologic tracers; coastal-plain; transport; tools; water
Abstract  

Chlorofluorocarbons (CFCs) a re frequently used as tracers for age dating of young groundwaters. However, in urban environments with many CFC point sources, uncertainties in the delivery of CFCs to groundwater (input function) complicate quantitative interpretation of observed CFC distributions. To assess the potential impact of elevated atmospheric CFC mixing ratios on CFC-based groundwater ages near a large coastal urban center, we measured atmospheric mixing ratios of CFCs in Palisades, NY, 25 km north of New York City. We present and discuss a 16-month record of atmospheric CCl3F (CFC-11) and CCl2F2 (CFC-12) obtained from gas,chromatographic analyses taken at intervals of approximately 16-min. Nearly all measured Values are in excess of remote Northern Hemisphere (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 trends of CFC-11 and CFC-12 differ from those of the remote atmosphere. Diurnal, weekly, and seasonal patterns are evident in the measured CFC distributions, in addition to variations resulting from regional meteorological conditions. These observations indicate that, to effectively use CFCs as groundwater dating tools near local or regional sources, their local atmospheric input functions must be explicitly defined.

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