³H/³He Age Data in Assessing the Susceptibility of Wells to Contamination.

Regulatory agencies are becoming increasingly interested in using young--ground water dating techniques, such as the ³H/³He method, in assessing the susceptibility of public supply wells (PSWs) to contamination. However, recent studies emphasize that ground water samples of mixed age may be the norm, particularly from long-screened PSWs, and tracer-based "apparent" ages can differ substantially from actual mean ages for mixed-age samples. We present age and contaminant data from PSWs in Salt Lake Valley, Utah, that demonstrate the utility of ³H and ³He measurements in evaluating well susceptibility, despite potential age mixing. Initial ³H concentrations (measured ³H + measured tritiogenic ³He) are compared to those expected based on the apparent ³H/³He age and the local precipitation ³H record. This comparison is used to determine the amount of modern water (recharged after ∼1950) vs. prebomb water (recharged before ∼1950) samples might contain. Concentrations of common contaminants were also measured using detection limits generally lower than those used for regulatory purposes. A clear correlation exists between the potential magnitude of the modern water fraction and both the occurrence and concentration of contaminants. For samples containing dominantly modern water based on their initial ³H concentrations, potential discrepancies between apparent ³H/³He ages and mean ages are explored using synthetic samples that are random mixtures of different modern waters. Apparent ages can exceed mean ages by up to 13 years for these samples, with an exponential age distribution resulting in the greatest discrepancies. [ABSTRACT FROM AUTHOR]