BASELINE WATER-QUALITY DATA FOR PART OF NORTHERN MILLARD COUNTY, CENTRAL UTAH, WITH EMPHASIS ON NITRATE CONCENTRATIONS IN THE SHALLOW UNCONFINED AQUIFER

In 1998, Millard County, Utah, approved a conditional use permit for a new poultry farm, to ultimately house up to 3 million hens. Much of the chicken manure produced by the birds is applied as fertilizer on adjacent farmlands on unconsolidated basin-fill deposits. An upper and lower confined aquifer in the basin-fill deposits are the principal sources of drinking water for the area; a shallow unconfined aquifer overlies the upper confined aquifer. Although ground water in the confined aquifers is generally of good quality, the potentially large amount of manure associated with the poultry farm and its potential impact on the quality of water resources caused concern among local government officials.

Our study’s purpose is to establish baseline ground-water quality in the area prior to the farm becoming fully operational so any adverse impacts from the fertilizer application can be documented. To accomplish this, we installed 26 PVC monitoring wells in the shallow unconfined aquifer, and sampled water from these wells and from seven seeps/land-drains for five sampling periods between April 2000 and November 2001; the samples were analyzed for total-dissolved-solids (TDS) and nutrient concentrations. We also sampled 19 wells in the area completed in the upper and lower confined aquifers in June 2003, and analyzed the water for general chemistry, nutrients, and TDS. For the combined data from the shallow unconfined aquifer, TDS concentrations range from 1,204 to 63,974 mg/L (average 13,293 mg/L) and nitrate concentrations range from <0.1 to 236 mg/L (average 6.34 mg/L). For the upper and lower confined aquifers, TDS concentrations range from 240 to 372 mg/L (average 294 mg/L), and nitrate concentrations range from <0.1 to 0.51 mg/L (average 0.33 mg/L). The shallow unconfined aquifer shows no indication of impact by the initial fertilizer applications. The poor-quality water in the shallow unconfined aquifer, likely partitioned from water in the confined aquifer, is probably due to flood irrigation using high-TDS Sevier River water.