Hydrogeologic and geochemical characterization of groundwater resources in Pine and Wah Wah Valleys, Iron, Beaver, and Millard Counties, Utah

Abstract

Pine and Wah Wah Valleys are neighboring structural basins that encompass about 1,330 square miles in Beaver, Iron, and Millard Counties in Utah, approximately 50 miles northwest of Cedar City, Utah, and 50 miles southeast of Baker, Nevada. Perennial streamflow is limited and only exists in higher-altitude reaches of small mountain streams in both basins. Groundwater is in unconsolidated basin-fill aquifers and bedrock mountain aquifers. Groundwater in Pine and Wah Wah Valleys is being targeted for large-scale groundwater extraction and export to provide municipal supply to the growing population in Iron County, Utah. Concern about declining groundwater levels and spring flows from proposed groundwater withdrawals has increased interest in an improved understanding of the groundwater system. Previous studies have indicated that an average of 28,000 acre-feet per year of recharge occurs mostly as infiltration of precipitation in high-altitude regions in the two basins. Groundwater discharge in the mountain hydrologic systems was estimated to average 8,500 acre-feet per year and is assumed to be consumed before subsequently recharging the valley basin-fill aquifers. Subsurface groundwater outflow moves from basin-fill aquifers in Pine and Wah Wah Valleys northward to adjacent regional basins and was estimated to average 19,500 acre-feet per year. An updated water-level map for the basin-fill aquifers in Pine and Wah Wah Valleys indicates that groundwater moves northward along the lengths of both valleys toward adjacent basins. Measured depths to water range from about 210 to 750 feet below land surface in Wah Wah Valley, and from about 300 to 620 feet below land surface in Pine Valley. Long-term water levels at seven wells completed in the basin-fill aquifers of Pine and Wah Wah Valleys with records spanning more than 40 years are generally stable with observed fluctuations of less than 5 feet. Observed discharge from two springs monitored between 2013 and 2016 also is generally stable. Groundwater leaving Pine and Wah Wah Valleys through the subsurface moves northward, converges with regional groundwater flow, and discharges by evapotranspiration at regional groundwater discharge areas, likely Tule Valley, Utah. In this study, basin-scale groundwater discharge was estimated by (1) mapping the groundwater discharge areas in each valley; (2) evaluating the 2005–11 summer multispectral satellite images against the Basin and Range carbonate-rock aquifer system study evapotranspiration measurements to select scenes broadly representative of average conditions in the study area and partitioning the groundwater discharge areas into evapotranspiration units using the selected satellite images and field reconnaissance; and (3) scaling evapotranspiration to the evapotranspiration units using evapotranspiration-rate estimates from several studies in the Great Basin. The resulting updated estimates of average annual groundwater evapotranspiration in the Tule Valley and Sevier Lake groundwater discharge areas were 35,000 and 10,500 acre-feet per year, respectively, with a likely uncertainty of plus or minus 35 percent. Groundwater samples from 13 sites in Pine Valley and 11 sites in Wah Wah Valley were analyzed for major ions and nutrients, to characterize geochemistry and water quality. Groundwater samples also were analyzed for the stable isotopes of oxygen, hydrogen, and carbon, the radioactive isotopes of carbon and hydrogen, and dissolved noble gases including helium-3, helium-4, neon, argon, krypton and xenon. Groundwater sampling sites included 12 wells and 12 springs. Carbon-14 and tritium/helium groundwater age dating indicate that groundwater in the basin-fill aquifers is typically thousands to tens of thousands of years older than groundwater in the shallow mountain aquifers. Dissolved-solids concentrations are lower and noble-gas temperatures are warmer in the valley wells compared to almost all groundwater sampled from wells and springs in the surrounding mountains. These results indicate a hydraulic discontinuity between the mountain and valley aquifers throughout much of the study area, and that much of the valley recharge is not derived from direct infiltration of precipitation in the mountains.