Analysis of Environmental Tracer Data to Help Determine Sources of High Nitrate Ground Water in Sanpete and Central Sevier Valleys, Central Utah

Southern Sanpete and central Sevier Valleys are rural areas where most residential development and agricultural activities are on unconsolidated deposits, the principal drinking-water aquifers for the area. The area has pervasive high nitrate in ground water as documented in this study and supported by historical water-quality data. I sampled 68 water wells and 6 springs during 2007, and used data from 76 water wells and 19 springs, without bias to land-use practice, to represent a widespread distribution. All water samples were analyzed for nitrate. Of those, 24 were tested for nitrogen and oxygen isotopes, 23 for tritium, 17 for chlorofluorocarbons (CFC), and 21 wells were analyzed for carbon isotopes. All samples for isotopes have high nitrate concentrations (average of 11 mg/L, median of 8.5 mg/L).

Nitrate concentrations for water samples from 76 water wells and 19 springs in the study area range from less than 0.1 mg/L to 39 mg/L, with an average of 6.7 mg/L, and a median of 5.2 mg/L; 19% of these samples yield nitrate values exceeding the EPA standard of 10 mg/L. Sixty-two percent of samples have nitrate concentrations that exceed 3 mg/L; ground water having values exceeding 3 mg/L is typically associated with human- or animal-derived sources. Possible sources of nitrate include fertilizer, feed lots, septic tanks, and natural sources. Most nitrogen and oxygen isotope data plot in overlapping fields characteristic of septic tank/manure, soil, and ammonia fertilizer/rain. Tritium ages plot as pre-“bomb”(1952), bomb water, post bomb, and mixed water. CFC data ages range from 1943 to 2000. Carbon isotope data ages range from modern to 19,000 years. Environmental tracer data show three samples are categorized as modern-age water (<1981) with remaining samples showing mixed-age ground water. Based on the environmental tracer data, nitrate contamination is from multiple sources in mixed ground-water systems.