Tooele Valley Wetlands – A Valuable but Potentially Endangered Resource
By Neil Burk, Charles Bishop, and Mike Lowe
Wetlands are one of the most important ecosystems on Earth. They have numerous functions and are a valuable resource to communities. Wetland functions include wastewater treatment or water filtration, flood-water control and storage, wildlife habitat (about 45% of the species listed as threatened or endangered under the Endangered Species Act use wetland habitats), biologic productivity, and food-chain support.
Approximately 80% of the wetlands in Utah (about 400,000 acres) surround Great Salt Lake. An estimated 30% of Utah’s wetlands have been lost since the mid-1800s, mostly due to construction of land drains and changes from agricultural to residential land uses.
The Utah Geological Survey (UGS) recently completed an evaluation of the potential impacts of residential development and climatic conditions on wetlands in Tooele Valley, Tooele County, Utah. The U.S. Environmental Protection Agency’s Wetland Protection Program partly funded the project. This is the first in a series of proposed and ongoing similar UGS studies of the wetlands surrounding Great Salt Lake.
The wetlands in Tooele Valley are in the northern part of the valley near Great Salt Lake and occupy about 79,000 acres, or almost 50% of the valley- floor area. Tooele Valley is mostly rural, but a rapid increase in residential development is resulting in less agricultural land use. The change from agricultural to domestic water use and concomitant increase in water-well pumping could significantly decrease 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 in groundwater discharge areas. Additionally, drought conditions over the past six years have reduced the amount of recharge to these aquifer systems.
As part of our evaluation, we documented the current status of the Tooele Valley wetlands by performing a functional assessment of three discrete wetland areas, and by installing shallow monitoring wells in these areas. We also used a regional, threedimensional, steady-state MODFLOW ground-water flow model created by the U.S. Geological Survey (USGS) for Tooele Valley to determine the amount of ground water presently available to these wetland areas. Our findings suggest that the wetland hydrology has been impacted mostly by the numerous roads, canals, and ditches in the area, and that agricultural land use is more beneficial to wetland health and functionality than industrial or urban land use.
To determine the potential impacts of increased ground-water development and continued drought, we modified and used a regional, three-dimensional, transient MODFLOW groundwater flow model, also created by the USGS for Tooele Valley, to estimate potential changes in the water available to the wetland areas. The evaluation suggests that increasing water-well withdrawals in Tooele Valley from 11,200 acrefeet per year to 16,800 acre-feet per year reduces the modeled subsurface flow through the wetland areas, but does not significantly reduce the modeled spring discharge. However, continued drought conditions would reduce the discharge from springs in the wetland region. Wetland health depends on maintaining both subsurface inflow and spring discharge; therefore, the worst-case scenario for the wetlands would be a combination of both increased water-well withdrawals and prolonged drought conditions. Flow volumes currently available to the wetlands, necessary to maintain the current health of the wetlands, are estimated in the steady-state groundwater flow model to be 98,000 acrefeet per year as subsurface inflow, and 6600 acre-feet per year as discharge from springs in the wetland region.
Wetlands in Tooele Valley are potentially endangered from drought and increased development, which could reduce the amount of water they receive. However, land-use planning can be used to mitigate the detrimental impacts of increased residential development. To balance development with wetland conservation, Tooele County is in the process of creating a Special Area Management Plan for the wetlands in Tooele Valley. Examples of protective land-use planning measures that could be implemented include (1) restricting areas available for development, such as allowing development only in upland environments or placing a non-development buffer around the wetland areas, (2) requiring municipal sewer and water lines in new developments and designing these sewer systems so that, where possible, the treated water is reused or discharged upgradient of the wetlands, and (3) enacting water conservation practices beneficial to the wetlands.
The UGS report, “Wetlands in Tooele Valley, Utah – An Evaluation of Threats Posed by Ground-Water Development and Drought,” can be purchased on CD from the Utah Department of Natural Resources Map & Bookstore (http://mapstore. utah.gov) or viewed on the Internet at http://geology.utah.gov/online/ss/ss -117/ss-117.pdf.
Survey Notes, v. 38 no. 2, May 2006