H/[.sup.3]He 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 [.sup.3]H/[.sup.3]He 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 [.sup.3]H and [.sup.3]He measurements in evaluating well susceptibility, despite potential age mixing. Initial [.sup.3]H concentrations (measured [.sup.3]H + measured tritiogenic [.sup.3]He) are compared to those expected based on the apparent [.sup.3]H/[.sup.3]He age and the local precipitation [.sup.3]H 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 [.sup.3]H concentrations, potential discrepancies between apparent [.sup.3]H/[.sup.3]He 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.
Introduction There is growing interest among regulatory agencies in using ground water age data in wellhead protection planning. Tracers such as tritium ([.sup.3]H) and radiogenic carbon ([.sup.14]C) have been widely [...]