Abstract

The principal aquifer supplies about one-third of the water used for public supply in Salt Lake Valley, Utah. Environmental tracers were used to determine the sources of recharge to the aquifer and were correlated to anthropogenic compounds detected in the water from many public-supply wells sampled in 2001 as part of the U.S. Geological Survey’s National Water-Quality Assessment Program.

Oxygen-18 values and recharge temperatures for ground water were used to differentiate between mountain and valley recharge. Water that enters the ground-water system in the valley has a greater likelihood of containing anthropogenic compounds than does mountain-block recharge. Together, the recharge temperature and stable-isotope data define two zones of the valley’s principal aquifer that contain relatively large fractions of valley recharge on the east side of the valley.

Apparent tritium/helium-3 ages of well water range from 3 to more than 50 years. Both the apparent age and the fraction of pre-bomb water generally increase with distance from the mountain front on the east side of the valley with the oldest water being in the discharge area.

The presence of anthropogenic compounds at concentrations greater than laboratory reporting levels and elevated nitrate concentrations in the principal aquifer are well correlated with the distribution of interpreted-age categories. Anthropogenic compounds were not detected in water with an apparent age of more than 50 years, except for water from one well. All of the samples composed mostly of modern water contained at least one anthropogenic compound.