Dating ground com
Prior to the late 1980s, however, there were no reliable means of dating ground water recharged during this time scale and, until recently, none of those methods were considered practical for use in establishing regional patterns.
In the early 1990s, USGS scientists (Busenberg and Plummer, 1992) developed a method to date ground water on the basis of chlorofluorocarbon (CFC) content of the water that is practical, cost-effective, and applicable to most shallow ground-water systems.
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.
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.
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.
Shallow ground-water systems are commonly used for drinking water sources and they make up a large part of the baseflow in rivers and lakes.