Predictions of the Effects of Groundwater Pumping in the Colorado Regional Groundwater Flow System, Southeastern Nevada

Tetra Tech Inc.

With the projected population growth in southeastern Nevada and the increase in industrial activities and generation of electricity, there is an increasing demand for water. During the recent several decades, the Office of the Nevada State Engineer has received many applications for water-rights permits for groundwater production within the Colorado River Groundwater Flow System in southeastern Nevada. This report provides the results of modeling simulations of the groundwater system in selected basins of the Colorado River Groundwater Flow System in southeastern Nevada and in parts of Utah and Arizona. The simulations were performed using a recently updated groundwater flow model of the area developed by Tetra Tech, Inc. Seven different scenarios were evaluated. The first two scenarios were developed within the framework of existing permits. Scenario 1 evaluates the effects of existing pumping, while Scenario 2 simulates pumping the full amount of all existing permitted groundwater rights, continued into the future. Scenarios 3 through 7 simulate pumping the full amount of all existing permitted groundwater rights, plus all large (>1,000 af/yr) pending groundwater applications before the Nevada State Engineer’s Office through 2009, in five different steps. Results are presented at simulated pumping times of 10, 50, 100, 500, and 1000 years. The predicted drawdowns are presented in a series of maps for the uppermost model layer, which represents the water table. Temporal plots present the changes in simulated discharge at selected springs in the Muddy River Springs Area, at Rogers and Blue Point Springs (combined), and at selected locations along the Muddy River and the Virgin River. The simulation results indicate that impacts of pumping on spring discharge and stream flow will increase as time passes, and as the rates of pumping increase. Beginning with a continuation of the current rates of pumping (Scenario 1), the model predicts that a new equilibrium may be established after more than 1,000 years, and the impacts on most springs will be less than a 35% reduction in discharge. If pumping were to increase to a rate equal to the total of all existing permitted groundwater rights (Scenario 2), the Muddy River Springs will completely dry up in approximately 1,100 years. The model further predicts that the discharge from Rogers and Blue Point Springs will decline approximately 40% in 1,000 years. Flows in the Muddy River at Glendale are predicted to decrease to less than 40% of the existing flow within about 500 years, while flows in the Virgin River basin near Overton will be reduced by approximately 13% over this period. The model is predicting that the groundwater system will not be able to supply the existing groundwater rights without impacting surface-rights holders. There will also be impacts at areas where there are sensitive habitats. With the addition of simulated pumping of all pending applications up to and through 1989, as represented by Scenario 3, impacts become greater and occur more quickly. All discharge from the Muddy River Springs ceases and the flow in the Muddy River at Moapa ceases in about 160 years. Pumping near the Virgin River will totally deplete the baseflow in the lower reaches. The combined discharge from Rogers and Blue Point Springs is predicted to decrease by 65% in 500 years. The pumping simulated in Scenarios 4 through 7 is predicted to cause greater and faster impacts to the groundwater and surface-water resources. In some areas, the aquifers may not be able to sustain the projected pumping, regardless of effects elsewhere. The effects of drawdown will cause impacts outside the modeled area, and capture flow from adjoining basins, including those in Utah and Arizona. While there is uncertainty in these projections that needs to be evaluated further, it is unlikely that the general conclusions will be altered substantially.