ANALYSIS OF ENVIRONMENTAL TRACER DATA TO HELP DETERMINE SOURCES OF HIGH-NITRATE GROUND WATER IN THE BOTHWELL POCKET, NORTHERN UTAH

The Bothwell Pocket in northwestern Utah is a rural area dominated by agricultural activity but is also experiencing an increase in residential development. The unconsolidated basin-fill aquifer is an important source of drinking water for this small Box Elder County community. The area has sporadic high nitrate in ground water as documented in this study. I selected 31 water-sampling sites, including wells, springs, canals, and sloughs/lakes, without bias to land-use practice, to represent a valley-wide distribution of water-quality data. Most of the sampled wells are less than 200 feet (60 m) deep. Water samples from all wells were analyzed for nitrate, general ion chemistry, and dissolved metals. Samples having relatively high (greater than 5 mg/L) nitrate concentrations (10 wells) were analyzed for nitrogen and oxygen isotopes in nitrate. Nineteen samples were analyzed for the environmental tracers oxygen and deuterium isotopes in water, tritium, chlorofluorocarbons (CFCs), and carbon isotopes. Water samples analyzed for isotopes have an average nitrate concentration of 6.5 mg/L and a median of 2.6 mg/L.

Nitrate concentrations for 31 water samples in the study area range from less than 0.1 mg/L to 40.8 mg/L. Forty-two percent of the wells and springs yielded values greater than 5 mg/L, and 23% showed nitrate values that exceed the Utah and EPA primary drinking-water-quality standard of 10 mg/L. 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 analysis of ground water from wells indicates that contaminated ground water was recharged pre-, post-, and during above-ground nuclear testing when tritium concentrations in the atmosphere were at their low, medium, and peak levels, respectively. CFC data ages range from 1943 to 1988. Carbon isotope data ages range from modern to 13,200 years. Environmental tracer data show two samples are categorized as modern-age water (post 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.