WETLANDS IN NORTHERN SALT LAKE VALLEY, SALT LAKE COUNTY, UTAH : AN EVALUATION OF THREATS POSED BY GROUND-WATER DEVELOPMENT AND DROUGHT

Salt Lake Valley, Salt Lake County, Utah, is in a formerly rural area that is now largely urban and continues to undergo population growth. Most of the development in Salt Lake Valley uses municipal water sources, principally wells completed in the basin-fill aquifer system. The population growth and concomitant increase in municipal ground-water pumping could significantly decrease the amount of ground water discharged from the principal aquifer system (where most wells are completed) to the shallow unconfined aquifer system.

The shallow unconfined aquifer overlies confining beds above the principal aquifer system in the central and northern parts of the valley, and provides water to springs and approximately 58,000 acres of wetlands in ground-water discharge areas. Decreased recharge to the shallow unconfined aquifer from the principal aquifer due to increased ground-water pumping could reduce water supply to these springs and wetlands. Also, water supply to the springs and wetlands is affected by climatic conditions and Great Salt Lake level. Drought conditions during 1999-2004 reduced the amount of recharge to ground-water aquifers across the state, negatively impacting the Salt Lake Valley wetlands.

To evaluate the potential impacts of drought and increased development on the Salt Lake Valley wetlands, we used existing data to estimate a water budget for the wetlands area. To determine the potential impacts posed by increased ground-water development and further drought, we used regional, three-dimensional, steady-state and transient MODFLOW models for Salt Lake Valley to evaluate water-budget changes for the wetland areas. The modeling suggests that subsurface inflow into the wetland areas would be most affected by decreased subsurface inflow due to long-term (20-year) drought conditions, but subsurface inflow would also decrease due to increased municipal and industrial well withdrawals over the same time period. Therefore, the worst-case scenario for the wetlands would be a combination of both conditions. As a conservative goal, the Salt Lake Valley wetlands area should be managed to maintain its current budget of water, which is estimated to include at least 52,410 acre-feet per year of recharge, of which 27,800 acre-feet per year is from subsurface inflow.