Groundwater age dating using tritium
Sampling for tracers The feasibility of using CFCs as tracers of recent recharge and indicators of ground-water age was first recognized in the 1970s (see Plummer and Busenberg, 1997 and references therein).
CFCs have been increasingly used in oceanic studies since the late 1970s as tracers of oceanic circulation, ventilation, and mixing processes.
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.
In the atmosphere, these substances have mixed and spread worldwide.
These include (1) uncertainty of the temperature at the water table during recharge, (2) entrapment of excess air during recharge, (3) uncertainty in recharge elevation, (4) thickness of the unsaturated zone, (5) effect of urban air where CFC values may exceed regional values, (6) contamination from a specific local source, (7) microbial degradation (in anaerobic environments), (8) sorption onto organic and mineral surfaces, and (9) mixing of younger and older water in a well. A., 1998, Chemistry of unsaturated zone gases sampled in open boreholes at the crest of Yucca Mountain, Nevada: Data and basic concepts of chemical and physical processes in the mountain: Water Resources Research, v. For additional information, see or contact: Niel Plummer (email: [email protected]) U. 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.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.