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

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Environmental Science & Technology
Journal Date: 
Aug 15
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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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