Scientific Investigations Report 2006-5088

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5088

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Hydrologic Conditions

The Snake River Plain aquifer is one of the most productive aquifers in the United States (U.S. Geological Survey, 1985, p. 193). The aquifer consists of a thick sequence of basalts and sedimentary interbeds filling a large, arcuate, structural basin in southeastern Idaho (fig. 1).

Recharge to the Snake River Plain aquifer is principally from infiltration of applied irrigation water, infiltration of streamflow, ground-water inflow from adjoining mountain drainage basins, and infiltration of precipitation.

Surface Water

The Big Lost River drains more than 1,400 mi2 of mountainous area that includes parts of the Lost River Range and Pioneer Mountains west of the INL (fig. 1). Flow in the Big Lost River infiltrates to the Snake River Plain aquifer along its channel and at sinks and playas at the river’s terminus. To avoid flooding at the INL facilities, excess runoff has been diverted since 1965 to spreading areas in the southwestern part of the INL (Bennett, 1990, p. 15), where much of the water rapidly infiltrates to the aquifer. Other surface drainages that provide recharge to the Snake River Plain aquifer at the INL include Birch Creek, Little Lost River, and Camas Creek (fig. 1).

The average streamflow at gaging station 13127000, Big Lost River below Mackay Reservoir (fig. 1) for complete water years from 1904 to 2001 was 223,800 acre-ft/yr (O’Dell and others, 2002, p. 193) (fig. 8). Streamflow at surface-water gaging stations at and downstream of gaging station 13127000 (fig. 1) for water years 1999–2001 are given in table 2 and figure 8.

Recharge to the Snake River Plain aquifer downstream of Arco is substantial during wet years because of infiltration of streamflow from the Big Lost River channel, diversion areas, sinks, and playas. For example, measured infiltration losses at various discharges measured during 1951–85 ranged from 1 (ft3/s)/mi in the river channel to 28 (ft3/s)/mi in the sinks (Bennett, 1990, p. 24-26). Bennett (1990) considered streamflow losses to evapotranspiration minor compared with infiltration losses. However, infiltration can be zero in years when there is little or no flow in the Big Lost River channel as was the case in 2001 at and downstream of gaging station 13132500 (table 2).

Ground Water 

Water in the Snake River Plain aquifer moves principally through interflow and fracture zones in the basalt. A significant proportion of the ground water moves through the upper 200 to 800 ft of basaltic rocks (Mann, 1986, p. 21). Ackerman (1991, p. 30) and Bartholomay and others (2000, p. 15) reported a range of transmissivity of basalt in the upper part of the aquifer of 1.1 to 760,000 ft2/d. Anderson and others (1999) reported a range of hydraulic conductivity at the INL of 0.01 to 32,000 ft/d. The hydraulic conductivity of rocks underlying the aquifer is from 0.002 to 0.03 ft/d, several orders of magnitude smaller (Mann, 1986, p. 21). The effective base of the Snake River Plain aquifer probably ranges from about 815 to 1,710 ft below land surface in the western half of the INL (Anderson and others, 1996, table 3).

Depth to water in wells completed in the Snake River Plain aquifer ranges from about 200 ft in the northern part of the INL to more than 900 ft in the southeastern part. During March–May 2001, the altitude of the water table was about 4,600 ft in the northern part of the INL (fig.  9) and about 4,420 ft in the southwestern part. Water flowed southward and southwestward beneath the INL (fig. 9) at an average hydraulic gradient of about 4 ft/mi.

From March–May 1998 to March–May 2001, water levels in wells in the northern and west-central part of the INL generally rose; rises ranged from about 1 to 3 ft (fig. 10). These rises may be attributed to lag time for water infiltrating the Big Lost River channel and sinks during 2000–01.

During the same period, water-level declines in wells in the southwestern part of the INL near the spreading areas ranged from about 1 to 4 ft (fig. 10). These declines are attributed to lack of infiltration in the spreading areas during 2000 and 2001.

Water levels monitored in wells USGS 12, USGS 17, and USGS 23 (fig. 2), and USGS 20 (fig. 3) show long-term water-level changes in the Snake River Plain aquifer at different locations at the INL in response to infiltration of streamflow (fig. 11). Long-term water-level fluctuations have ranged from about 11 ft in well USGS 20 to about 27 ft in well USGS 12. Water levels in these wells declined from 1999 to 2001 because of decreased infiltration of streamflow in the Big Lost River, and an overall decrease in recharge to the Snake River Plain aquifer.

Ground water moves southwestward from the INL and eventually is discharged to springs along the Snake River near Twin Falls, 100 mi southwest of the INL. Discharge from the springs estimated by methods given by Kjelstrom (1995) was about 4.26 million acre-ft/yr for the 2001 water year (Tom Brennan, U.S. Geological Survey, written commun., 2002). Historically, the discharge to these springs has ranged from 2.97 million acre-ft/yr in 1904 to 4.94 million acre-ft/yr in 1951 (Daniel J. Ackerman, U.S. Geological Survey, written commun., 2005).

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