GROUND-WATER-QUALITY MONITOR-WELL SAMPLING FOR THE VALLEY-FILL AQUIFER IN CASTLE VALLEY, GRAND COUNTY, SOUTHEASTERN UTAH

Castle Valley in southeastern Utah is experiencing an increase in residential development, all of which uses septic tank soil-absorption systems for wastewater disposal. Most of this development is on unconsolidated deposits of the unconfined valley-fill aquifer, the primary source of drinking water. The purpose of our study is to install ground-water monitoring wells in order to: (1) examine the ground-water quality of the principal aquifer, especially in southern Castle Valley where a paucity of wells exist, and (2) establish some wells within the Town of Castle Valley downgradient from septic-tank systems, to identify current nitrate concentration at the water table.

To accomplish this, we installed six 2-inch diameter PVC monitoring wells in the unconfined aquifer. The wells ranged in depth from 80 to 260 feet; well construction included factory slotted PVC from 5 feet above the water table to 20 feet below the water table. We sampled water from these six wells and from one spring for three sampling periods between December 2005 and June 2006; the samples were analyzed for general chemistry (including total dissolved solids [TDS]), dissolved metals, and nutrient (nitrate, nitrite, ammonia, and total phosphorous) concentrations.

Data from the three sampling periods (winter, spring, and summer) for TDS concentrations range from 196 to 1324 mg/L (average 551 mg/L) and nitrate concentrations range from 0.10 to 1.80 mg/L (average 0.53 mg/L). The spring, located downgradient from the majority of all water wells in the valley, had a TDS concentration of 952 mg/L and a nitrate concentration of 0.8 mg/L. The unconfined aquifer shows no indication of impact by contamination associated with septic-tank effluent based on our data. In addition, no seasonal variation in water quality for both TDS and nitrate concentrations for the wells exists. Continued sampling of these wells will provide the valley residents future water-quality information to detect any changes in water quality, and thus help manage proposed new development.