Effects of Groundwater Withdrawals, Tracy Segment
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The USGS Nevada Water Science Center began an investigation in 2010 to quantify hydrologic effects of groundwater withdrawals on Truckee River streamflow in the Tracy Segment hydrographic area, Storey, Washoe and Lyon Counties, Nevada. Groundwater gradients have been monitored at sites near pumping wells and the river. Water-level fluctuations resulting from local pumping are interpreted with flow models to estimate the transmissivity of basin fill and bedrock aquifers and quantify the volume of Truckee River water potentially captured by groundwater pumpage.
Recent and proposed changes in land-use and related water-supply requirements within the Truckee River Basin have prompted concern about water availability and allocation. Surface-water resources from the Truckee River and groundwater resources from basin-fill and bedrock aquifers in the Tracy Segment Hydrographic Area (HA) of western Nevada are water supplies for the Reno-Sparks and Fernley metropolitan areas and for the Pyramid Lake Paiute Tribe, as well as for the Department of the Interior’s Newlands Reclamation project. The Pyramid Lake Paiute Tribe, Bureau of Reclamation, Bureau of Indian Affairs, and Department of Justice need estimates of the effects of withdrawing groundwater from basin-fill and bedrock aquifers on discharge to the Truckee River. Understanding the effects of all changes in groundwater withdrawals in the Tracy Segment HA is important because groundwater discharge to the river might be decreased enough to jeopardize efforts to protect threatened (Lahontan cutthroat trout) and endangered (Cui ui sucker) species, the limnology of Pyramid Lake, and downstream surface water rights. This study will measure changes in annual groundwater gradients and estimate groundwater discharge to selected reaches of the Truckee River in the Tracy Segment HA.
To quantify hydrologic effects of groundwater withdrawals from basin-fill and bedrock aquifers in the Tracy Segment hydrographic area on discharge to the Truckee River, we have:
- Instrumented sites along flow paths between areas of groundwater withdrawal and the Truckee River to continuously measure groundwater levels and gradients for at least 5 years.
- Obtained records of groundwater supply well pumping from water purveyors.
- Estimated withdrawals by riparian phreatophytes using thermal dissipation probes to measure rates of sap flow by representative phreatophytes and distributed to the riparian forest based on leaf area index estimated from LiDAR dataset flown in 2010.
- Quantified distribution and abundance of riparian species using remotely sensed imagery and ground-truthed mapping.
- Monitored depth to groundwater to estimate daily water use, groundwater gradient responses, and potential effects on transpiration rates.
- Analyzed measured transient responses to known pumping and estimated phreatophyte transpiration using both analytic and numerical simulations to quantify flow gradients and refine physical and hydrologic properties associated with the pumped aquifers.
The Pyramid Lake Paiute Tribe Reservation in northern Nevada relies on the Tracy Segment hydrographic area of the Truckee River for a large portion of its water supply. About 2,500 acres of the Tracy Segment HA are decreed for the Reservation. Rapid growth of the neighboring Reno/Sparks metropolitan area and development of the Tahoe Reno Industrial Center (TRIC) within Storey County have resulted in changes in land-use and water-supply requirements in the Tracy Segment HA. In addition, increased consumptive water use by evapotranspiration may be an unintended result of efforts to restore the Truckee River riparian ecosystem and its associated flood-attenuation capacity. Groundwater use has increased with new development, but the effects of increased groundwater withdrawal on discharge to the Truckee River and on appropriated surface-water rights have not been quantified. Historically, the Office of the Nevada State Engineer has ruled that applications to appropriate underground water from the State cannot exceed the volume of groundwater that has been estimated to come from precipitation within the hydrographic area. Surface-water rights are treated independently of groundwater rights. However, accurate quantification of groundwater recharge is confounded in the Tracy Segment HA by the sparse distribution of long-term precipitation stations, complexities of estimating infiltration and recharge, and estimates of groundwater discharge from the HA are less than the uncertainty associated with measuring the volume of flow in the Truckee River. Estimates of groundwater recharge in Nevada are assumed to equal pre-development groundwater discharge to surface water that flows out of an HA and by evapotranspiration. In the Tracy Segment HA, the volume of Truckee River water that leaves the HA is two orders of magnitude greater than empirically estimated groundwater discharge and uncertainty associated with measurement of surface water is 5 times the estimated groundwater discharge.
Perennial “safe” yield of groundwater in the Tracy Segment HA was first estimated to be 6,000 acre-ft/yr and supplementally adjusted to 11,500 acre-ft/yr (Nevada State Engineer Ruling 5747; June 27, 2007). Applications to withdraw an additional 22,211 acre-ft/yr of underground water are pending and either ready for action or under protest. TRIC General Improvement District treated about 200,000 gallons per day (about 250 acre feet per year) of wastewater from industrial operations during 2009, only a portion of the total volume permitted to withdraw. Treated wastewater is stored in a small reservoir where it both infiltrates and evaporates. In addition, flood control and river ecosystem restoration efforts are in progress to re-establish about 330 acres of riparian vegetation with most of the Truckee River floodplain in the Tracy Segment HA identified for future ecosystem restoration. The restoration efforts should attenuate peak flood waves by re-establishing meanders, widening the river channel, and establishing riparian vegetation along the floodplain. Although the vegetation will improve the ecosystem by providing shade and bank stability, typical semiarid riparian vegetation may consume 3 ft/yr (3 acre-ft/yr per acre), depending of the depth to water, vapor gradients, and stand density and composition.
Assessing hydrologic effects of developing groundwater supplies in the Tracy Segment HA required numerical groundwater-flow models to estimate aquifer hydraulic properties and to simulate the timing and magnitude of streamflow capture from the Truckee River. The hydraulic conductivity of basinfill and transmissivity of volcanic-rock were estimated using aquifer test data from 7 wells and property distributions were refined by calibrating a steady state, three-dimensional, MODFLOW model of the subsurface to predevelopment conditions. A transient, three-dimensional, MODFLOW model was calibrated simultaneously to refine estimates of specific storage. Groundwater development scenarios were investigated with a capture distribution approach to produce a continuous distribution of groundwater capture at specified time intervals. Truckee River capture is proportional to the pumping rate, largely independent of the pumping rate and tied to proximity to the river.