Dating ground com
Information about ground-water age can be used to determine recharge rates and refine hydrologic models of ground-water systems (Reilly and others, 1994; Szabo and others, 1996) and thus to predict the contamination potential and estimate the time needed to flush contaminants through a ground-water system.The 0- to 50-year time scale is particularly relevant to environmentally sensitive shallow ground-water systems.A closed path is established between the well or pump to a valve system that is used to fill glass ampoules with water, creating a headspace with CFC-free, ultra-pure nitrogen gas. Age is determined from CFCs by relating their measured concentrations in ground water back to known historical atmospheric concentrations and/or to calculated concentrations expected in water in equilibrium with air.As with any environmental tracer, age applies to the date of introduction of the chemical substance into the water, and not to the water itself.The accuracy of the determined age depends in part on how perfectly the CFCs are transported with the water.Chemical processes, such as microbial degradation and sorption during transit, can also affect the concentration of CFCs and other compounds used in dating.USGS scientists (Busenberg and Plummer, 1992) adapted analytical procedures developed by the oceanographic scientific community for ground-water studies and designed sampling equipment and procedures for collection and preservation of water samples in the field.
During the past 50 years, human activities have released an array of chemical and isotopic substances to the atmosphere.Chlorofluorocarbons are stable, synthetic organic compounds that were developed in the early 1930s as safe alternatives to ammonia and sulfur dioxide in refrigeration and have been used in a wide range of industrial and refrigerant applications. N., Busenberg, Eurybiades, Drenkard, Stefan, and Schlosser, Peter, 1996, Age-dating of shallow groundwater with chlorofluorocarbons, tritium/helium 3, and flow path analysis, southern New Jersey coastal plain: Water Resources Research, v. Production of CFC-12 (dichlorodifluoromethane, CF). CFC-11 and CFC-12 were used as coolants in air conditioning and refrigeration, blowing agents in foams, insulation, and packing materials, propellants in aerosol cans, and as solvents. CFC-113 has been used primarily by the electronics industry in semiconductor chip manufacturing, in vapor degreasing and cold immersion cleaning of microelectronic components, and surface cleaning. Probably better known to the public as Freon, CFCs are nontoxic, nonflammable and noncarcinogenic, but they contribute to ozone depletion. J., and Busenberg, Eurybiades 1994, Estimation and corroboration of shallow ground-water flow paths and travel times by environmental tracer and hydraulic analyses A case study near Locust Grove, Maryland: Water Resources Research, v.
The detection of chlorofluorocarbons and tritium in ground water provides valuable information that can be used for dating and tracing young ground watertechniques that help water-resources managers develop management strategies for shallow ground-water systems.