EVALUATION OF NITRATE CONTAMINATION IN SANPETE VALLEY, CENTRAL UTAH

Sanpete Valley is a rural area where residential development and agricultural activities are on unconsolidated valley-fill deposits, the principal drinking-water aquifer. Septic-tank effluent, agricultural fertilizer, and feed-lot waste are potential sources of nitrate, the principal ground-water contaminant identified in previous studies; many wells have yielded water exceeding 10 mg/L, the drinking-water standard. To help accomplish our primary objectives to identify sources of nitrate contamination and evaluate transport and fate of nitrate, we used ground-water modeling and particle tracking of contamination to determine ground-water flow paths and calculate travel times. We analyzed ground water from 436 wells for nitrate; 3 high-nitrate wells (>10 mg/L) were also tested for tritium to constrain age.

Average nitrate concentration is 3.3 mg/L. The majority of wells yielded water having values less than 5 mg/L, with 3.5 percent exceeding 10 mg/L. Tritium analysis indicates nitrate-contaminated ground water was recharged during the 1960s. Simulated reverse-particle modeling shows ground-water flow rates near contaminated wells range from 7 to 200 feet per year, indicating contamination sources are within about 1.5 miles of high-nitrate wells.

We examined the relationships between nitrate concentration and well depth, total-dissolved-solids (TDS) concentrations, and proximity to alfalfa fields, irrigated lands, and feed lots. No correlation exists between nitrate concentration and well depth, though most high-nitrate wells are less than 150 feet deep. Effluent from septic tanks can have high TDS, but we attribute relatively high TDS to rock types in this valley; no correlation exists between high nitrate and elevated TDS. Calculations show no correlation between high-nitrate wells and land-use practice, except for wells within 0.25 miles of animal-feeding operations, which have slightly higher average nitrate concentrations. At least half of the high-nitrate wells are isolated single-well contaminations. We believe multiple sources contribute nitrate to ground water in Sanpete Valley; these sources vary from well to well and include poor well construction, septic-tank systems, agricultural fertilizer, and animal-waste products.