Groundwater age dating tritium
Locating the position of the mids bomb peak is difficult due to the required high density of vertical sampling and, therefore, is often an impractical means of obtaining ground-water age information. CFC age dating tritium is best suited for ground water in relatively rural environments where localized contamination, such as that from septic systems, sewage effluent, landfills, or urban runoff, can contain CFC concentrations in excess of that of atmospheric sources.
Such a plot typically provides fairly good clues with respect to the origin of the terrigenic helium. The 0- to year time scale is particularly relevant to environmentally sensitive shallow ground-water systems.
The CFC age of the young fraction is then computed by comparing air concentrations that would be in equilibrium with the CFC concentration in the young fraction with historical air concentrations Busenberg and Plummer, If the tritium delivery as a function of time can be reconstructed, this penetration process can be used for quantitative studies of water movement through identification of the bomb peak in certain ground water bodies.
Additional comments and case studies. This document is also available in pdf format: BarringerZoltan SzaboPamela A. See research for more information.
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Geological Survey Open-File Report72 p. Although SF 6 is almost entirely of human origin, there is likely a natural, igneous source of SF 6 that will complicate dating in some environments.
Additional He sources, terrigenic He, may be present in aquifers where the rocks are enriched in U or Th, or in ground-water samples in which young water has mixed with relatively old water containing terrigenic He. CFCs have been increasingly used in oceanic studies since the late s as tracers of oceanic circulation, ventilation, and mixing processes.
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By continuing to browse this site you agree to us using cookies as described in About Cookies. The dating indicates that few domestic and municipal supply wells produce river-water fractions that are younger than 5 years, and most river water sampled was likely recharged through the sinkholes during the past 20 to 30 years.
One TU Tritium Unit means a tritium to hydrogen ratio of 10 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 environments8 sorption onto organic and mineral surfaces, and 9 mixing of younger and older water in a well.
Flow patterns, age of groundwater, and influence of lakewater leakage: It is independent of the initial tritium concentration of the water sample which is one of the advantages of the method because it eliminates the necessity to establish the exact time- dependent tritium delivery to the aquifer. StotlerShaun K.
This unexpected finding indicated that shallow ground washington dating scene was being mixed with deep, older water during pumping.
References Cited Bayer, R.
Go to old article view Get access. Shallow ground-water systems are commonly used for age dating tritium water sources and they make up a large part of the baseflow in rivers and lakes.
Alternatives to using CFCs in dating ground water will clearly be needed as atmospheric CFC concentrations continue to fall. The most practical approach to determining R terr is to measure it in groundwater samples from the same aquifer that are free of tritium. Le Gal La SalleH.