Usgs groundwater age dating

16-Dec-2017 03:02

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However, there are natural limits to this method because tritium decay and dispersion make it increasingly difficult to identify the bomb peak in groundwater.These problems can be overcome by using tritium in combination with its decay product He age formally calculated from equation (1) is an apparent age.Not the date of rock formation, but dating how long a rock has sat at the Earth's surface and been exposed to cosmogenic radiation.The bombardment of the earth by particles causes spallation within the atmosphere, and within the upper centimeters of terrestrial surfaces.He ages increase with depth in individual piezometer clusters and with distance down flowpaths.However, the age gradient with depth (Δt/ΔDz) is smaller in the plume than that above or below the plume in the regional waters, due to the intense loading near the infiltration beds.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.A potential problem for the quantitative interpretation of the tritium/He age is typically biased toward the age of the water component with the higher tritium concentration.

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Thanks to precise, global, atmospheric CFC measurements beginning in 1978 as a part of the Atmospheric Lifetime Experiment, there is an excellent recorded record of global CFC concentrations.Tritium decays to the stable isotope helium-3, which stays dissolved in the water below the water table.With precise measurement, the parent and daughter isotopes can be quantified, providing a means to accurately date when water was recharged.Longer exposures to radiation increases amounts of helium-3 among other cosmogenic isotopes.

He method is used along with extensive sampling to determine not only average aquifer properties, but also detailed characteristics of a regional flow system and a sewage plume over a distance of four kilometers in a sand and gravel aquifer located on Otis Air Base in Falmouth, Massachusetts.The Noble Gas lab contains a mass spectrometer system capable of precise isotopic measurements of the noble gases as well as most other common atmospheric gases.

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