Tritium Isotope Can Be Used To Track Source Of Well Water

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Editor:

Analytical tests just received by the Payson Water Department's hydrogeologist, Mike Ploughe, P.G., confirm that the Tower Well is drawing water from a substantially different and deeper source than all the other Star Valley wells.

Tritium is one of three isotopes of hydrogen. It is radioactive and therefore easily detected at very low concentrations. Prior to the advent of nuclear weapons testing in the 1940s, tritium's existence in the atmosphere and associated rainwater was essentially nil. Throughout the ‘50s and ‘60s, its concentration increased as a result of weapons testing by a number of countries around the world: Britain, China, France, Russia and the United States.

When above ground testing was banned, tritium concentrations in the atmosphere and rainwater began to decrease. Half of it disappears every 12.5 years through decay.

By measuring the amount of tritium in groundwater, the age of different water sources can be compared.

For example, if you collect rainwater, its tritium concentration is in equilibrium with the tritium currently in the air. A recent sample of water from the Blue Ridge Reservoir had a concentration of 10.2 tritium units.

As soon as rain and snowmelt seep into the ground, the tritium concentration begins to decline. The tritium in the water decays and vanishes, but it is no longer replenished by tritium from the air.

So, the longer water remains isolated underground, the lower the tritium concentration. Samples from the more than 40 shallow wells and springs in and around Payson and Star Valley show a range of tritium values between about three and six tritium units.

These values are typical of groundwater that has been locally recharged by rain and snow within the past 10 years.

Mr. Ploughe collected samples from the Tower Well on two separate occasions and submitted them to the University of Arizona Isotope Laboratory for analysis. In contrast to other wells in the area, samples from the Tower Well show concentrations of only 0.4 to 0.6 tritium units, indicating that the water fell as rain more than 50 years ago and, in the interim, it has not been mixing with local recharge.

The soon-to-be-released Star Valley "safe yield" study considers only local rainfall and snowfall. It ignores the deeper regional groundwater flowing beneath the area. The Tower Well water clearly comes from this deeper source. As a result, the safe yield study, whatever the final value, is likely to significantly underestimate the water available in the Star Valley area.

Lynn Godfrey, Payson Water Task Force

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