CHEMICAL CHARACTERIZATION OF GROUND WATER IN CEDAR VALLEY, UTAH COUNTY, NORTH-CENTRAL UTAH

Cedar Valley is a basin-and-range-type valley located 40 miles southeast of Great Salt Lake; recent urbanization here has increased demand for ground-water resources. Important aquifers are the Quaternary to Tertiary basin-fill sediments, including valley-center lacustine sediments and valley-margin alluvial fans, and the Paleozoic carbonate and clastic bedrock in the surrounding mountains.

Ground-water chemistry evolves along a west-to-east flowpath. Ground water in or near the Oquirrh Mountains on the west is high-quality calcium-bicarbonate type. As water flows eastward, its total dissolved solids (TDS) concentration increases up to three fold as it dissolves other major ions and trace elements. The increased TDS is likely due to reactions between ground water and clay-rich, evaporite- and sulfide-bearing sediments and shale.

Stable isotope (oxygen and hydrogen) and radioactive isotope (¹⁴C and tritium) data show the center of the basin-fill aquifer contains isotopically depleted and older water compared to ground water at the valley margins. These data imply a flow system of precipitation falling on the highest mountains and infiltrating bedrock at high or moderate elevations, then following a deep flowpath and recharging the basin-fill aquifer at depth. Younger and isotopically heavier water at the valley margins is recharged at lower elevations and has a shorter flowpath. Water from one well has a strongly evaporative isotopic signature, revealing some recharge to the basin-fill aquifer infiltrates through thick lacustine clays underlying the center of the valley, where water ponds and evaporates.

Drillers' logs and water levels suggest the existence of a shallow aquifer in unconsolidated sediment overlying bedrock in the Cedar Pass area in the northeast part of the valley. Geochemical data strongly support the existence of the perched aquifer and implicate a distinct local recharge source in the Traverse Mountains. All waters tested from this unit were calcium-chloride type with elevated TDS, which contrasts sharply with the chemistry of surrounding ground waters.

Utah Geological Survey is using the insight into ground-water flow patterns provided by these geochemical data to develop a digital ground-water flow model of the basin-fill aquifer in Cedar Valley.