THE RELATIONSHIP OF EXISTING AND POTENTIAL NONPOINT-SOURCE CONTAMINATION TO RECHARGE AND DISCHARGE AREAS FOR SEVERAL VALLEY-FILL AQUIFERS IN UTAH

Unconsolidated valley-fill aquifers are the most important sources of drinking water in Cedar Valley (Iron County), Sanpete Valley (Sanpete County), and Tooele Valley (Tooele County). We mapped ground-water quality, potential contaminant sources, and primary recharge, secondary recharge, and discharge areas for these aquifers to aid in managing potential contaminant sources. Ground-water quality was determined from chemical analyses of well water. Potential contaminant sources were inventoried from windshield surveys. Recharge and discharge areas were delineated primarily by the presence or absence of thick (20 feet or greater) clay and/or silt confining layers and ground-water gradients. Primary recharge areas have no thick confining layers and the water table is below the ground surface. Secondary recharge areas have thick confining layers and a downward ground-water gradient through the confining layer. Discharge areas either have thick confining layers and an upward gradient through the confining layer, or for unconfined aquifers, are areas where the water table intersects the ground surface.

In Cedar Valley, the main potential nonpoint sources of ground-water contamination are animal feed lots, septic tanks, and sediments eroded from nitrogen-bearing rocks. In Sanpete Valley, the main potential sources are animal feed lots and septic tanks. Additionally, natural contamination from ground water entering the valley-fill aquifer from adjacent salt-rich bedrock is responsible for elevated total-dissolved-solids concentrations in some areas of Sanpete Valley. In Tooele Valley, the main potential sources are chemical storage and disposal facilities, accumulations of assorted junk (old cars, tires, and appliances) on private land, and feed lots. Additionally, we used fecal coliform bacteria in wells to link high nitrate concentrations to septic tanks in one area of Tooele Valley. Although preliminary data indicate overall water quality in all three valleys is good, many of the potential contamination sources we identified are located in primary recharge areas where they pose the greatest threat for degrading ground-water quality.