Recharge area and water quality of the valley-fill aquifer, Castle Valley, Grand County, Utah

Abstract All culinary water in Castle Valley is from wells. Increased residential development using individual waste-water-disposal systems has raised concerns for the long-term quality of ground water in the valley-fill aquifer. In this study, ground-water recharge and discharge areas, potentiometric surface elevation, and specific conductance were mapped to serve as tools for protecting ground-water quality and managing potential contaminant sources in Castle Valley. Castle Valley is one of several northwest-trending salt anticline valleys on the Colorado Plateau in southeastern Utah. The unconsolidated valley fill is the principal aquifer in the valley and consists of coarse alluvial-fan deposits and stream alluvium, with minor clay. Some recharge to the valley-fill aquifer comes from underflow from bedrock aquifers, but most is from La Sal Mountains runoff via Castle Creek and Placer Creek. Because of the absence of protective, low-permeability confining layers, the valley-fill aquifer is unconfined, and most of the valley is classified as primary recharge area. The only discharge area is along lower Castle Creek. Water quality in the valley-fill aquifer is generally high in upper Castle Valley, but declines in the lower valley, perhaps due to recharge from saline ground water in bedrock aquifers in contact with Paradox Formation evaporites. Wells tapping the Cutler Formation aquifer beneath the valley fill also yield poor-quality water. The coarse-grained, unconfined valley-fill aquifer is highly susceptible to contamination from surface recharge.