Measurement of sulfur hexafluoride and CFCs in the environment: Application to groundwater dating, soil air dynamics and atmospheric mixing

This thesis investigates the application of SF6 and CFCs for dating of recent groundwater, and as tracers of soil air dynamics and atmospheric mixing. To identify processes, which control SF6 and CFCs in soil air, mixing ratios were observed over several years at Sparkill, NY (near NYC) and compared to atmospheric levels. SF6 in soil air was also measured near six large cities. To observe temporal trends as well as sources and transport of these gases, a long atmospheric time series of SF6 and CFCs collected at LDEO was analyzed. Lastly, SF6 and CFCs were applied to date groundwater at urban-influenced sites in southern NJ.; Soil air SF6 mixing ratios were controlled by diffusive transport and corresponded to atmospheric levels averaged over several months. In contrast, CFC mixing ratios were lows in winter and high in summer not explained by diffusion alone and attributed primarily to changing air/water partitioning of the gases with temperature. For shallow water tables or seasonal recharge, this can significantly effect tracer ages, and hinders observation of atmospheric mixing ratios from soil gas profiles. Within large cities and through much of the northeastern U.S. strong emissions result in atmospheric SF6 20% or more elevated above remote atmosphere levels hindering application of SF6 for groundwater dating near urban areas.; The atmospheric time series collected at LDEO indicates that from 1996 to 2004 mixing ratios of CFC 12 decreased rapidly, those of CFC 11 slowly in parallel with the remote atmosphere. From 1998 to 2004 SF6 decreased in parallel with known emissions from NYC. Estimates of local emissions derived from the time series indicated similar trends. Regional scale transport of these gases over 200 km to central Massachusetts was observed. In southern NJ, SF6 appeared to function successfully as a transient tracer giving groundwater ages in agreement with those derived from CFCs. Tracer ages indicated that CCl4 is slowly broken down in this aquifer with a half-life on the order of 20 years.