USGS Home
Contact USGS
Search USGS

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5261

Back to Table of Contents

Ground-Water Conditions

By Lari A. Knochenmus¹, Randell J. Laczniak¹, Michael T. Moreo¹, Donald S. Sweetkind¹, J.W. Wilson¹, James M. Thomas², Leigh Justet¹, Ronald L. Hershey², Sam Earman², Brad F. Lyles², and Kevin W. Lundmark²

¹U.S. Geological Survey
²Desert Research Institute

The ground-water flow system in the study area is influenced by a combination of topography, climate, and geology. Driven by the hydraulic gradient, ground water moves through permeable zones from areas of recharge to areas of discharge. The ground-water flow system includes flow paths of three distinct scales—local, intermediate, and regional (fig. 16). These terms are adapted from Toth (1963) and Freeze and Cherry (1979), and were defined by the depth of ground-water circulation and length of the flow path. Local flow systems are characterized by relatively shallow and localized flow paths that terminate at upland springs. These springs are low volume, tend to have temperatures similar to annual average ambient atmospheric conditions and have discharge that fluctuates according to the local precipitation. Intermediate flow systems include flow from upland recharge areas to discharge areas along the floor of the intermontane valley. Within intermediate-flow systems, ground-water discharge from springs typically occurs near the intersection of the alluvial fan and the valley floor near the range front and on the adjacent valley floor. Intermediate-flow system springs often are of moderate volume and tend to have less variable flow relative to local springs. Regional ground-water flow is driven by hydraulic gradients that continue over long distances (tens to hundreds of miles). Deep regional flow through basin-fill or consolidated bedrock aquifers is less constrained by local topographic or drainage features. Under pre-development conditions, recharge to the regional ground-water flow system primarily originates in mountains and may travel beneath several basins and mountain ranges before reaching its ultimate discharge area. Discharge from these regional flow systems manifests as large springs and, in some areas, extensive wetlands (Mendenhall, 1909).

Under steady-state conditions, ground-water inputs and ground-water outputs are equal and storage is constant. Ground-water input to a basin includes recharge from precipitation and infiltration from hydraulically connected lakes and streams. Ground-water output from a basin includes discharge from springs and hydraulically connected lakes and streams, and evapotranspiration (ET). Early on numerous scientists recognized that many individual basins, particularly in the Great Basin Province were not closed systems and that subsurface inflow and outflow to basins must be considered (Meinzer, 1911; Eakin, 1966; Harrill and others, 1988; Prudic and others, 1995; Harrill and Prudic, 1998; Nichols, 2000). Excess recharge relative to discharge for individual basins was an important factor in recognizing the existence of flow across basin boundaries. Additionally large volume springs in the study area could not be supported entirely by the local recharge from the adjacent mountain ranges, and therefore must be supplied in part from subsurface ground-water flow originating outside the basin. Based on chemistry, temperature, and other criteria, Mifflin (1968) identified selected springs that likely are discharge points from the regional aquifer system.

Typically ground-water pumping initially removes water from storage. This transition from steady-state to transient conditions is recognized by lowering of water levels in wells, declines in spring flow, and, where the ground-water system is hydraulically connected to surface-water bodies, can lead to increased recharge from streams or loss of baseflow. To better characterize the aquifers in White Pine County, water in storage was estimated for a representative volume of aquifer, and water-quality data were compiled and collected to assess the quality of ground water relative to primary and secondary drinking-water standards.

Ground-Water Flow

Ground-water flow patterns in the basin-fill and carbonate-rock aquifer systems can be inferred from the water-table and potentiometric-surface maps, respectively. A spatially interpolated contour map of the ground-water potential (potentiometric surface) is a visual representation of a surface connecting points of equal altitude to which water will rise in tightly cased wells that tap a confined aquifer system (Lohman, 1979). The water table is a particular potentiometric surface; the pressure is atmospheric (Lohman and others, 1970). Water-table and potentiometric-surface maps were constructed to exemplify aquifer system scales, hydraulic barriers, and gradients that control the direction and relative rates of ground-water flow. Ground water generally flows from areas of recharge (high heads) to areas of discharge (low heads) in a direction perpendicular to the water-level contours. The potentiometric-surface map was used to evaluate the permissible locations for flow between HAs and provided hydraulic gradient information needed to assess the volume of ground water flowing across basin boundaries.

The water-table and potentiometric-surface maps primarily were based on measured ground-water levels in wells. Data used to construct the water-table and potentiometric-surface maps shown on plates 2 and 3, respectively, are summarized in Wilson (2007, appendix A). In areas where few control points were available, published water-table and potentiometric-surface maps were used to guide map construction (Mifflin, 1968; Hess and Mifflin, 1978; Garside and Schilling, 1979; Johnson, 1980; Pupacko and others, 1989; Thomas and others, 1986; and Bedinger and Harrill, 2005). Geologic information and delineated recharge and discharge areas also aided in map construction.

Ground water in the basin-fill aquifer generally flows from recharge areas (high heads) at the intersection of the mountain front with the valley margin to discharge areas (lower heads) on the valley floors. Internally drained HAs, where water is lost by evaporative discharge, have closed, or nearly closed contours on the valley floors on plate 2. Ground water can exit a basin as subsurface flow to downgradient basins where hydraulic continuity in the basin fill exists between HAs or where ground-water in the basin-fill aquifer flows downward into the underlying carbonate (Thomas and others, 1986). Hydraulic continuity among several basins is depicted by open water-level contours on the water-table map. The water-table map was constructed by contouring the water-level data from 299 wells completed in the basin-fill aquifer at 100-ft intervals (pl. 2). Water-level altitudes ranged from slightly more than 6,800 ft to slightly less than 4,400 ft above sea level in southern Steptoe Valley and in northern Snake Valley, respectively.

The potentiometric-surface map was constructed from water levels in wells completed in basin fill and underlying carbonate rock. In many places, basin fill and carbonate rocks are hydraulically connected resulting in a single continuous ground-water flow system (Thomas and others, 1986). The following general guidelines used for identifying regional head for mapping regional potential are from Bedinger and Harrill (2005). Regional hydraulic head can be represented by shallow water levels in large areas of low topographic relief and virtually no recharge. Regional hydraulic head is at or above shallow water levels in areas of local, intermediate, and terminal discharge by ET in basins, at regional spring heads, and areas where ground water is discharging to major surface-water bodies. Regional hydraulic head is below the altitude of non-discharging dry playas, lower than the water table in areas of recharge, and lower than local spring heads. Using these guidelines, it was considered acceptable to map selected water-level data from wells completed in the basin fill where suitable data from the carbonate aquifer are scarce or lacking. Due to the scarcity of wells completed in the carbonate rocks, the control points used to construct the map are less precise and a large contour interval (500 ft) was selected for representing the potentiometric surface of the carbonate-rock aquifer. In locations where multiple wells are completed at differing depths, such as at MX well sites, the vertical gradients generally are less than 200 ft (Tumbusch and Schaefer, 1996, tables 1-2). The potentiometric-surface map was constructed by contouring water-level data from 119 wells, 76 basin fill wells, and 43 carbonate-rock and other consolidated-rock wells (pl. 3). Water-level altitudes ranged from slightly more than 6,500 ft to slightly less than 4,500 ft above sea level in Steptoe Valley and in northern Snake Valley, respectively.

The regional ground-water recharge area for the carbonate-rock aquifer is a relatively large recharge mound over Steptoe, Butte, Long, and Jakes Valleys; small, high mounds are centered on the Schell Creek, and Egan Ranges (pl. 3). This large recharge mound comprises the headwaters of four regional flow systems—Great Salt Lake Desert, Goshute Valley, Colorado, and Newark Valley (fig. 1). Ground water in west-central Steptoe Valley flows into Jakes and White River Valleys. Ground-water flow is toward the south in Long, Jakes, White River, and Cave Valleys and is part of the Colorado regional flow system. Ground water in southern Steptoe Valley flows into Lake Valley and then moves east into Spring and Snake Valleys as part of the Great Salt Lake Desert regional flow system. Flow generally is toward the north-northeast in northern Steptoe, Tippett, and Snake Valleys. Although Butte Valley is considered part of the Goshute Valley regional flow system (Harrill and others, 1988), ground-water likely exits this valley to the north as part of the Ruby Valley flow system. Some regional ground water moves upward into overlying basin-fill sediments, such as in southern White River Valley and south-central Spring Valley, or is discharged from valley floor springs.

Volume of Water Stored in Aquifers

Water stored within unconfined and confined aquifers becomes available as ground water is pumped and water levels decline. When pumping ceases, water levels will not recover to previous levels if the amount of water removed is not replaced by an equal amount or if the declines altered the hydraulic or physical properties of the aquifer. The magnitude of water-level decline or recovery depends, in part, on the storage properties of the aquifer; that is, on whether ground water is unconfined (a water-table aquifer) or confined. Water is stored within the pore spaces of saturated unconsolidated sediment or rock in a water-table aquifer and becomes available as the water table is lowered and the sediment drains. Under water-table conditions, storage is the product of the area of sediment or rock drained, the magnitude of the water-level decline in the drained area, and the specific yield of the drained sediment. Specific yield is limited by the porosity of the saturated sediment, but usually is less than the sediment porosity because some stored water is tightly bound to the sediment grains or the rock, preventing complete drainage of the pore water. Water stored within confined aquifers becomes available as hydraulic head in the aquifer decreases, water expands, and sediment or rock material compresses. Under confined conditions, storage is the product of the area of confined aquifer where hydraulic heads are lowered, the magnitude of the hydraulic-head decline in the affected area, and the storage coefficient of the confined aquifer. In confined aquifers, the storage coefficient typically is between two to four orders of magnitude less than the specific yield.

The volume of water stored in unconfined and confined aquifers was computed using the extent of basin-fill deposits, a water-level decline of 100 ft, and a storage term (the specific yield of the basin-fill aquifer is 0.15; the storage coefficient of the carbonate-rock aquifer is 0.001). A water-level decline of 100 ft was arbitrarily selected, but likely is a reasonable limit for widespread lowering of the ground-water surface for a valley. The area used to calculate storage is the region where the thickness of the basin fill is equal to or greater than 100 ft. This area is assumed to reasonably approximate the acreage of saturated basin fill. In calculating the unconfined storage, the saturated basin-fill area was reduced by removing the acreage of fine-grained playa deposits (fig. 17 and appendix A). The subsurface extent of fine-grained playa deposits is assumed to be equivalent to the fine-grained marsh, playa, and alluvial-flat deposits shown on the geologic map (pl. 1). This small area is assumed to reasonably approximate the acreage of the drainable basin fill. The estimated acreage of drainable basin fill ranges from less than 100,000 acres for Cave, Jakes, Lake, Long, or Tippett Valleys to more than 350,000 acres for Snake, Steptoe, or White River Valleys. Snake Valley has the largest estimated acreage of drainable basin fill at nearly 600,000 acres (appendix A).

Ground-water storage is estimated for each HA and is the contribution of unconfined and confined storage (fig. 18 and appendix A). Estimates range from less than 1 million acre-ft for Cave, Jakes, or Tippett Valleys to more than 5 million acre-ft for Snake, Steptoe, or White River Valleys. Storage estimates for the remaining HAs range from more than 1 to less than 4 million acre-ft. Snake Valley has the largest estimated storage at nearly 9 million acre-ft. For equivalent volumes of aquifer material, the capacity of the basin-fill aquifer to store water is significantly greater than that of the carbonate-rock aquifer. About 36 million acre-ft of water is stored in a 100 ft of saturated basin-fill aquifer beneath all valley floors. In contrast, only about 300,000 acre-ft of water is stored in a 100 ft of saturated carbonate-rock aquifer for a slightly larger area, or about 2 orders of magnitude less than the basin-fill aquifer. Confined storage contributes less than 100,000 acre-ft to the total storage of any HA. Estimates of storage do not consider the effects of any limiting geologic, hydrologic, or cultural factors, such as impermeable or low permeability lithologies, recharge to basin fill or carbonate-rock aquifers, declining water levels in wells, decreasing spring flow, diminished water quality, or loss of native vegetation.

Ground-Water Quality Relative to Drinking-Water Standards

Existing ground-water quality data were compiled from a number of sources for the study. These sources include the USGS National Water Information System (NWIS; http://waterdata.usgs.gov/nwis), Desert Research Institute databases, and published reports (Bateman, 1976; Kirk and Campana, 1988; Pupacko and others, 1989). Additionally, geochemical samples were collected as part of the study from wells and springs in a number of HAs. Based on a subset of chemical constituents having National primary and secondary drinking-water standards (U.S. Environmental Protection Agency, 2004), ground water in the study area generally is of good quality (table 4). Primary standards regulate constituents that are believed to pose a risk to human health if consumed above a certain threshold. Secondary standards regulate water-quality parameters that are not believed to pose a risk to human health, but can have undesirable aesthetic, cosmetic, or technical effects (Hershey and others, 2007). For chemical constituents with available analyses from more than 25 sampling sites, only arsenic and fluoride exceeded their primary standards at more than 1 site. Secondary drinking-water standards were exceeded more often than the primary standards but exceedances were not common. Values of pH were outside of the acceptable range of 6.5–8.5 (secondary standard) at 21 of 179 sites. Chloride concentrations exceeded their secondary standard at 6 of 179 sites. Sulfate concentrations exceeded their secondary standard at 4 of 177 sites.

Only a small number of ground-water samples from the study area have been analyzed for anthropogenic organic compounds. Schaefer and others (2005) discuss the results of a broad range of organic constituents, including volatile compounds, and pesticides and their metabolites, in samples from wells located in the study area. The study by Schaefer and others (2005) reports low concentrations of pesticides or their metabolites, and no volatile organic compounds were detected.

Back to Table of Contents