Scientific Investigations Report 2006–5236

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5236

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Geohydrologic Setting

The eastern Snake River Plain is a northeast oriented structural basin about 200-mi long and 50- to 70-mi wide. The plain consists of surficial alluvial and eolian sediments and basalt outcrops underlain by a layered sequence of basalt flows and sedimentary interbeds. Individual basalt flows are from 10- to 50-ft thick, although the average thickness is from 20 to 25 ft (Mundorff and others, 1964, p. 143). Surficial sediments and sedimentary interbeds consist of sand, silt, clay, and lesser amounts of gravel. Locally, rhyolitic flows and tuffs are exposed at land surface or are present at depth.

The top of the Snake River Plain aquifer is about 450 ft below land surface at the RTC and INTEC and about 600 ft below land surface at the RWMC. The unsaturated zone beneath these facilities consists of alluvial and eolian surficial sediments, basalt flows, and sedimentary interbeds and is typical of the stratigraphy at the INL. Anderson and Lewis (1989), Anderson (1991), Anderson and Bowers (1995), and Anderson and Liszewski (1997) described the stratigraphic sequence of the unsaturated zone and uppermost part of the Snake River Plain aquifer at selected INL facilities and at and near the INL. This sequence was formed by extrusion and cooling of basaltic lava followed by periods of volcanic quiescence and sedimentary deposition (Nace and others, 1975, p. 16; Anderson and others, 1997). Vertical and horizontal fractures developed as lava flows cooled. These fractures and interflow rubble zones are primary conduits through which water is transmitted.

Locally, perched ground water formed in the basalt and in sedimentary interbeds in response to recharge from wastewater infiltration ponds and localized infiltration of snowmelt and rain. Perched ground water also formed from streamflow infiltration of the Big Lost River (fig. 1). Transmissivity estimates from 22 wells completed in perched ground water ranged from 1.0 to 15,000 ft2/d.(Ackerman, 1991, p. 10). Differences in the vertical hydraulic conductivity of basalt layers and sedimentary interbeds in the unsaturated zone provide mechanisms for development of perched ground water (Cecil and others, 1991, p. 17). The vertical hydraulic conductivity of a sedimentary interbed typically is smaller than an overlying fractured basalt layer. Perched ground water is closely associated with sedimentary interbeds beneath the RTC, INTEC, and RWMC. Several perched water zones exist below each facility. A more detailed description of the perched zones is available in Cecil and others (1991). Alterations in the baked zones between two basalt layers may contribute to decreased vertical hydraulic conductivity. Dense, unfractured basalt or sediment and chemical filling of fractures near the upper contact of a basalt layer limit the capability of the basalt to transmit water.

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