GROUND-WATER QUALITY MAPPING FOR THE VALLEY-FILL AQUIFERS IN THE SOUTHERN SANPETE AND CENTRAL SEVIER VALLEYS, CENTRAL UTAH

Southern Sanpete and central Sevier Valleys are rural areas characterized by extensive agricultural activity and increasing population. The unconsolidated valley-fill aquifers in both valleys are an important source of drinking water. In cooperation with the U.S. EPA, I mapped water quality in the valley-fill aquifers with emphasis on nitrate and total-dissolved-solids (TDS) concentrations. Water-well samples were collected and analyzed during spring/summer 2007. I selected 70 water wells and 6 springs, without bias to land-use practice, to represent a widespread valley-wide distribution. Most of the sampled water wells are less than 200 feet deep. Water samples from all wells were analyzed for nutrients (nitrate, nitrite, ammonia, and phosphate), general chemistry, and dissolved metals. Of those, 25 were tested for nitrogen and oxygen isotopes, tritium, and chlorofluorocarbons (CFC); 20 wells were analyzed for carbon-14. All of the wells sampled for isotopes have relatively high (>5 mg/L) nitrate concentration. Although results for most of the sampled wells are available, the remaining samples are being processed by the Utah Epidemiology and Lab Services and the U.S. EPA, except for the isotope samples (various labs).

Nitrate concentrations for 55 water wells and 6 springs in the study area range from less than 0.1 mg/L to 39 mg/L, with an average concentration of 7.6 mg/L, and a median of 6.4 mg/L. Thirty-nine percent of these wells and springs yielded values less than 5 mg/L, and 20 percent showed nitrate values that exceed the Utah and EPA standard of 10 mg/L. Preliminary data indicate most high-nitrate-concentration areas are widespread. Possible sources of nitrate include fertilizer, feed lots, septic-tank systems, and natural sources. Nitrogen and oxygen isotope data will help determine the sources of nitrate. Carbon-14, tritium, and CFC data will be used to determine the relative age of the ground water in the area. TDS concentrations for water wells and springs in the valley range from 360 to 2632 mg/L (average 1009 mg/L and a 728 mg/L median). TDS concentrations for 82 percent of these wells and springs are greater than 500 mg/L. Elevated TDS concentrations are likely associated with wells situated near the evaporite-rich Arapien Shale or downgradient from irrigation canals.