Groundwater age-dating simplified

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A new USGS report documents a simple method to classify groundwater age as premodern (recharged before 1953), modern (recharge in 1953 or later), or a mix of the two. The method—tritium-based age classification, or TBAC—requires just a single measurement of tritium, along with knowledge of sample date and location.

Knowing groundwater age can help water-resource managers determine which contaminants are anticipated to be present or absent in groundwater, allowing more cost-effective use of monitoring dollars. For example, if the TBAC method determines that groundwater is premodern, that groundwater is less likely to contain chemicals, such as current-use pesticides, that came into use after 1953. The TBAC method can also be used as a screening tool to determine if other, more complicated age-dating methods are needed to resolve questions of groundwater age.

The TBAC method can be used retroactively to assign age categories to any sample previously analyzed for tritium, provided that the sampling date and location are known. When this was done for existing USGS data, the results using the TBAC method very closely matched those from studies that used multiple tracers of groundwater age and complex analyses.

Citation: Lindsey, B.D., Jurgens, B.C., and Belitz, K., 2019, Tritium as an indicator of modern, mixed, and premodern groundwater age: U.S. Geological Survey Scientific Investigations Report 2019–5090, 18 p., https://doi.org/10.3133/sir20195090.

Access the associated data release here.

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