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Ground-water assessment in northeastern Tooele Valley

Project Number: 9716CNP
Project Chief: Bernard Stolp, USGS Salt Lake City, Utah
Cooperator:
Period of Project: July 2006 to September 2010

PROBLEM

The structure, stratigraphy, and ground-water flow paths in northeastern Tooele Valley are not well understood in the context of the regional ground-water flow system. Water quality in the area varies dramatically. Steep hydraulic gradients in the northeastern part of the valley may indicate the presence of low-permeability barriers to ground-water flow. However, large discharges from regional springs indicate the presence of high-permeability flow paths. The current Tooele Valley regional MODFLOW model simulates total fluxes through the area using a complex distribution of hydraulic conductivities and horizontal-flow barriers. Although the current model representation allows for a reasonable match between computed and observed water levels and discharge to springs, limited understanding of the system framework does not sufficiently constrain the model parameters and boundaries. The area is experiencing rapid suburban development and new municipal water sources will be developed, adding new stresses to the ground-water system. Additional information is needed to improve the accuracy of conceptual models of the ground-water flow system in northeastern Tooele Valley and to increase the utility of computer representations of those concepts as water management tools.

OBJECTIVES

The principal objective of the study is to improve conceptualization and computer model representation of the ground-water flow system in northeastern Tooele Valley. Specific study tasks include (1) refine the distribution of bedrock, flow barriers, and high-permeability flow zones; (2) refine the distribution of hydraulic head and ground-water flow patterns, (3) determine the horizontal and vertical water-quality distribution in the basin-fill aquifer and relate it to geologic structures and stratigraphy and saline ground water associated with the Great Salt Lake; and (4) incorporate new information from 1, 2, and 3 above into improved regional ground-water flow model and simulate the potential range of effects of increased ground water withdrawals.

RELEVANCE AND BENEFITS

The mission of the USGS Water Resources Discipline is to provide hydrologic information and understanding needed to achieve the best use and management of the Nation's waters. This study will provide information and assessment tools needed by local water managers to make informed decisions on water management in Tooele Valley. The proposed study will improve the understanding of the Tooele Valley flow system and improve water-managers’ ability to assess the vulnerability of current and future ground-water supplies to the effects of migrating poor-quality water.

APPROACH

The study approach will include compiling existing water-quality and well-log data, collecting additional water-quality and hydraulic data from wells, and using surface geophysical methods to improve the understanding of the stratigraphy and geometry of the basin fill and underlying consolidated rock. Water-quality data, including stable isotopes and environmental tracers, will be collected to define the distribution of water types and chemistries and to determine ground-water ages and flow velocities. The study will collect water levels from new wells in northeastern Tooele Valley to improve the resolution of the potentiometric surface and refine areas with steep hydraulic gradients. The conceptual flow model will be refined and changes incorporated into the Tooele Valley regional MODFLOW model. The refined model will be used to simulate changes in water levels and discharge to springs from new stresses.

PRODUCTS

Results of the proposed study will be documented in a USGS Scientific Investigations Report. The report will contain a description of the new conceptual ground-water flow model of the area including updated descriptions of aquifer system framework, potentiometric surfaces, and water-quality distributions. Changes to the regional Tooele Valley MODFLOW model and predictive simulations to assess the effects of increased withdrawals also will be documented in this report.