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Scientific Investigations Report 2008–5089

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5089

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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 primarily is 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 terminus of the river. 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 2005 was 220,100 acre-ft/yr (Brennan and others, 2005, p. 269) (fig. 10). Streamflow at gaging stations at and downstream of gaging station 13127000 (fig. 1) for water years 2002–05 are shown in table 4 and figure 10.

Recharge to the Snake River Plain aquifer downstream of Arco is substantial during wet years because of streamflow infiltration 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 little or no flow is in the Big Lost River channel as during 2002–04 at and downstream of gaging station 13132500 (table 4).

Ground Water 

Water in the Snake River Plain aquifer primarily moves through interflow and fracture zones in the basalt. A large proportion of 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 one-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 2005, the altitude of the water table was about 4,570 ft in the northern part of the INL (fig. 11) and about 4,400 ft in the southwestern part. Water flowed southward and southwestward beneath the INL (fig. 11) at an average hydraulic gradient of about 4 ft/mi.

Water levels in wells declined in the INL area from March–May 2001 to March–May 2005 (fig. 12). The declines ranged from about 3 to 8 ft in the southwestern part of the INL, about 10 to 15 ft in the west central part of the INL, and about 6 to 11 ft in the northern part of the INL (fig. 12). Water levels in perched water wells also declined, as evidenced by lack of any water in many wells during 2002–05. These declines may be attributed to lack of infiltration to the spreading areas, lack of infiltration of water in the Big Lost River channel (table 4), and a decrease in recharge at the INL during 2002–05.

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. 13). Long-term water-level fluctuations have ranged from about 16 ft in well USGS 20 to about 34 ft in well USGS 12. Water levels in these wells steadily declined from 2002 to 2005 because of lack of streamflow infiltration from 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, Idaho, about 100 mi southwest of the INL. Discharge from the springs estimated by methods given by Kjelstrom (1995) was about 3.54 million acre-ft/yr for the 2005 water year (Tom Brennan, U.S. Geological Survey, written commun., 2006). 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., 2007).

Perched Ground Water 

Radiochemical and chemical constituents in wastewater migrate to the Snake River Plain aquifer through perched ground water beneath wastewater infiltration ponds at the RTC and INTEC. Perched ground water beneath the RWMC formed from infiltration of snowmelt and rain and recharge from the Big Lost River and INL spreading areas. This perched water contains constituents leached from buried radioactive and organic-chemical wastes. Disposal of wastewater to infiltration ponds and infiltration of surface water at waste-burial sites resulted in formation of perched ground water in basalts and in sedimentary interbeds that overlie the Snake River Plain aquifer. Perched ground water is an integral part of the pathway for waste-constituent migration to the aquifer. The extent of this perched ground water is affected by the waste-disposal practices.

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