In March 2007 the Utah State Legislature requested the UGS to establish a groundwater-monitoring network in Snake Valley and adjacent areas, in response to concerns over potential drawdown of the water table and reduced spring flow from proposed large-scale groundwater development projects in east-central Nevada and west-central Utah. The UGS groundwater-monitoring network was completed in spring 2009, and includes 67 newly constructed wells, 11 previously existing wells, and surface-flow gages at six springs.
The objectives of the UGS groundwater-monitoring network are to (1) determine baseline temporal and spatial groundwater-level and chemical trends, (2) establish groundwater monitoring near the planned pumping wells and areas of current groundwater use to assess their impacts on groundwater resources, (3) monitor discharge from selected springs that host environmentally sensitive aquatic species, (4) conduct aquifer tests to measure the hydraulic properties of, and hydraulic connectivity between, the basin-fill and carbonate-rock aquifers, and (5) evaluate groundwater-flow systems within Snake Valley and from Snake Valley to Fish Springs National Wildlife Refuge.
This article focuses on our preliminary findings from monitoring groundwater levels and spring flow. The UGS monitoring wells have five to seven years of high-frequency (hourly) data. The U.S. Geological Survey (USGS) has collected waterlevel data annually for some of these wells for at least 30 years, providing a long-term context. Groundwater-level trends can be compared to precipitation data from the Great Basin National Park climate station, in a principal area of groundwater recharge for the Snake Valley basin-fill and carbonate-rock aquifers.
Near the Eskdale agricultural area in southern Snake Valley, data from UGS sites 9 and 36 show that groundwater levels vary seasonally due to local pumping for crop irrigation, andthat this cyclical variation is superposed on a trend of steadily declining groundwater levels of 3.1 to 3.5 inches per year in the basin-fill and bedrock aquifers from 2007 to 2013. Long-term data from the USGS show that groundwater levels in the basin-fill aquifer have declined by about 4.2 inches per year since 1987. Data from UGS site 6, seven miles northeast of Eskdale, may help distinguish groundwater-level trends related to climate from those related to pumping. Groundwater levels at site 6 lack the cyclical fluctuations characteristic of groundwater pumping, so long-term groundwater-level trends here likely show little or no effect from pumping and result primarily from overall decreased annual precipitation after a period of far greater-than-normal precipitation (and, presumably, recharge to the aquifers) during the early 1980s. Groundwater levels at site 6 have declined by 2.2 inches per year since 1989, which can be interpreted as the long-term climatic signal in the Eskdale area. Groundwater pumping, therefore, may account for at least 1 inch per year of the decline at sites 9 and 36 observed in the UGS data, and for about 2 inches per year of the decline observed in the long-term data.
Groundwater levels in UGS monitor wells near other agricultural sites in Snake Valley also respond cyclically due to groundwater pumping, and show similar or greater rates of long-term decline. Groundwater levels in wells far from current agricultural pumping have declined at substantially lower rates than those near agricultural areas. Collectively the data show that in Snake Valley agricultural areas, groundwater is currently being withdrawn at rates slightly greater than it can flow to the discharge areas. Proposed large-scale groundwater-development projects would greatly expand the area and magnitude of groundwater withdrawal and discharge capture, resulting in lower groundwater levels and reduced spring flow throughout southern and central Snake Valley.
Snake Valley has numerous springs that support agriculture and provide habitat for environmentally sensitive species. At the Bishop Springs area in east-central Snake Valley, the average discharge from Foote Reservoir Spring is 5.9 ± 0.1 acre-feet per day. Utah Division of Wildlife personnel removed Russian olive (Eleagnus angustifolia) trees around the reservoir and outlet stream in early 2012. Prior to tree removal, the spring discharge shows a slight downward trend, whereas the discharge after the trees were removed trends up. Non-native Russian olive and saltcedar trees have been implicated in increased water loss from riparian systems through evapotranspiration in the West since the 1980s. Our data suggest more water is flowing in the stream channel since the removal of the trees. If verified, this result could provide an approach for the Division of Wildlife Resources to preserve in-stream flow that provides habitat for several aquatic species of concern in west desert spring-fed wetlands, in support of a conservation agreement that decreases the likelihood that the species will be listed by the U.S. Fish and Wildlife Service as threatened.
In the southern part of the Snake Valley hydrographic area, flow from Dearden Springs supports much of the in-stream flow and shallow groundwater (i.e., underflow) in Lake Creek, which extends from the springs 9 miles downstream to Pruess Lake. The Dearden Springs–Lake Creek reach, together with the upstream Big Springs–Big Springs Creek reach in Nevada, forms a 17-mile-long surface water-groundwater system, the largest and most continuous in Snake Valley. The creek and associated wet meadows are used for grazing and provide wildlife habitat. Recent groundwater-flow models constructed by the Southern Nevada Water Authority (SNWA) and the USGS suggest that groundwater pumping along the eastern flank of the southern Snake Range west of Lake Creek would severely reduce groundwater levels and spring flow in the Big Springs–Dearden Springs–Lake Creek–Pruess Lake hydrologic/ecologic system. UGS monitoring of spring flow at Dearden Springs and Clay Spring, and groundwater levels in many wells in this area, will help quantify the impact if development occurs.
UGS flow data for Dearden Springs are an essential part of an in progress study of surface flow and groundwater-surface water interaction along Big Springs Creek–Lake Creek, associated with a monitoring agreement between the Department of the Interior and SNWA. The Dearden Springs data are also part of a monitoring and mitigation plan in the Nevada State Engineer’s decision to grant water rights to SNWA in Spring Valley, and will likely be incorporated into any future monitoring and mitigation plan associated with large-scale groundwater development in southern Snake Valley. Separating climatic and groundwater pumping signals in the data will be the main use, and challenge, of the UGS monitoring.