Environmental tracers age dating young groundwater
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.Water samples for CFC analysis are now routinely collected from domestic, irrigation, monitoring, and municipal wells, and from springs. Nuclear Regulatory Commission-licensed USGS laboratory for analysis of CFC content by gas chromatography to a detection limit of about 0.3 picograms per kilogram (0.3 pg/kg) of water, which is equivalent to 0.3x10 Ground-water dating with CFC-11, CFC-12 and CFC-113 is possible because (1) their amounts in the atmosphere over the past 50 years have been reconstructed, (2) their solubilities in water are known, and (3) concentrations in air and young water are high enough that they can be measured.Therefore in 1987, 37 nations signed an agreement to limit release of CFCs and to halve CFC emissions by 2000. This agreement, the Montreal Protocol on Substances That Deplete the Ozone Layer, was strengthened in 1990 and again in 1992 when 1996 was established as the cut-off date for CFC production in industrialized countries. D., 1993, Environmental tracers for age-dating young ground water: Alley, W. Measurements of dissolved methane are useful in recognizing environments where all three CFCs can be degraded. Measurements of dissolved nitrogen and argon can be used to help determine recharge temperature and excess air and to recognize environments undergoing denitrification.
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.
H), and other chemical and isotopic substances in ground water, can be used to trace the flow of young water (water recharged within the past 50 years) and to determine the time elapsed since recharge.
Information about the age of ground water can be used to define recharge rates, refine hydrologic models of ground-water systems, predict contamination potential, and estimate the time needed to flush contaminants from ground-water systems.
Because of the effect of these factors on CFC concentration, collection of additional data is often needed to determine the apparent age of ground water.
For example, measurements of concentrations of dissolved gases, such as dissolved oxygen, help to define the potential for microbial degradation.