WETLANDS IN TOOELE VALLEY, UTAH--AN EVALUATION OF THREATS POSED BY GROUND-WATER DEVELOPMENT AND DROUGHT

We evaluated the wetlands in Tooele Valley, Utah, to assess the threats posed by increased ground-water development and drought. The Tooele Valley wetlands are located in ground-water discharge areas adjacent to Great Salt Lake. Tooele Valley is a mostly rural area at the south end of Great Salt Lake experiencing a rapid increase in residential development, resulting in less agricultural land use. The change from agriculture to domestic water use could significantly affect the amount of ground water discharged from the confined aquifer system, where most wells are completed, to the shallow unconfined aquifer system, which provides water to springs and wetlands. Additionally, drought conditions over the past six years have reduced the amount of recharge to aquifers across the state, also impacting Tooele County's wetlands, and have caused the surface elevation of Great Salt Lake to decline to near its historical lowstand record set in 1963.

We documented the current status of the wetlands by performing a functional assessment and by installing wells in three wetland areas. The functional assessment results suggest that the wetland hydrology has been impacted the most by the numerous roads, canals, and ditches in the area. The magnitude and direction of the hydraulic gradient is similar to what has been documented previously. Water sample analyses indicate that the ground water in the wetland areas is supersaturated with numerous mineral phases and that there are no down-gradient improvements in water quality.

To determine the potential impacts posed by increased ground-water development and continued drought, we used regional, three-dimensional, steady-state and transient MODFLOW models of Tooele Valley to estimate the water budgets for the wetland areas. The modeling results indicate that subsurface inflow into the wetland areas would be most affected by increased ground-water withdrawals, and that discharge from springs that feed the wetlands would be most affected by continued drought conditions. The worst-case scenario for the wetlands would be a combination of both conditions. These results will be useful in guiding land-use and development decisions in Tooele Valley.