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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5169

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Introduction

The subsurface at the Idaho National Laboratory (INL; fig. 1) consists of thick layers of fractured basalt interbedded with thin layers of fluvial, eolian, and lacustrine sediments. These sedimentary interbeds affect vertical and horizontal flow through the saturated and unsaturated zones, although the effect on the regional aquifer flow regime is not well understood. In the current conceptual model of the eastern Snake River Plain (ESRP) aquifer (Ackerman and others, 2006), the overall volume of sediment and basalt is treated as three composite units with effective hydraulic properties that are considered to account for the hydraulic effects of both types of material in combination within each unit. The modeling effort aims to simplify geologic and hydrologic features while retaining those features that are important to water flow and contaminant transport.

Welhan and others (2006) evaluated sediment thickness within the model domain using data from 333 boreholes in and around the INL and determined that within the Big Lost Trough (fig. 1), an area known to have the greatest sediment accumulation, sediment comprises more than 50 percent of the stratigraphic thickness at some locations. The hydrologic significance of this sediment is evident in water-table contours that indicate increased gradients in this area of high sediment accumulation (Joseph Rousseau, U.S. Geological Survey, oral commun., 2007). The hydraulic influence of these units, and hence their importance in the subregional flow model, requires an assessment of the nature of the sedimentary material that would facilitate verification of the model-simulated flow behavior. Vertical gradients observed in the field and those produced by numerical simulation could be verified using knowledge of the hydraulic properties of the sedimentary material. Sediment abundances (Welhan and others, 2006) along with the properties measured in this study provide information useful in model refinement and establish the foundation for an aquifer-specific property-transfer model.

Site Description

The INL was established in 1949 for nuclear-energy research under the U.S. Atomic Energy Commission (now the U.S. Department of Energy). The INL occupies about 2,300 km² of the west-central part of the ESRP (fig. 1). The site hosts several facilities of which at least four have been used to generate, store, or dispose of radioactive, organic, and inorganic wastes. The ESRP is a northeast-trending basin, about 320 km long and 80–110 km wide, that slopes gently to the southwest and is bordered by northwest-trending mountain ranges. The ESRP is underlain by interbedded volcanic and sedimentary layers that extend as much as 3,000 m below land surface. The sedimentary interbeds, which constitute about 15 percent by volume of the unsaturated zone and ESRP aquifer (Anderson and Liszewski, 1997), result from quiet intervals between volcanic eruptions and are of fluvial, eolian, and lacustrine origin. Volcanic units composed primarily of basalt flows, welded-ash flows, and rhyolite, may be vesicular to massive with either horizontal or vertical fracture patterns.

The climate of the ESRP is semiarid and the average annual precipitation is 22 cm. Parts of the ESRP aquifer underlie the INL. The depth to the water table ranges from 60 m in the northern part of the INL to about 200 m in the southern part (Barraclough and others, 1981; Liszewski and Mann, 1992). The predominant direction of ground-water flow is from northeast to southwest. Recharge to the aquifer is primarily from irrigation water diversions from streams, precipitation and snowmelt, underflow from tributary-valley streams, and seepage from surface-water bodies (Hackett and others, 1986). Within the INL boundaries, the Big Lost River (fig. 1) is an intermittent stream that flows from southwest to northeast.

Purpose and Scope

This report describes the nature of the sedimentary material within the ESRP aquifer, which will aid in further development of the subregional ground-water flow model. The measurements of saturated hydraulic conductivity (K_sat) and bulk properties on highly consolidated core samples from the ESRP aquifer and estimates of K_sat based on the site-specific models developed by Winfield (2005) and Pudney (1994) also are presented. In recent years, numerous deep boreholes have been drilled and core samples obtained which give an opportunity to directly measure K_sat and other relevant properties of sedimentary materials, such as particle-size distribution and bulk density (ρ_bulk). This report includes K_sat and bulk properties measured on 10 minimally disturbed core samples, and particle size distributions for 77 samples from 4 boreholes within the INL (fig. 2). K_sat also was estimated for all samples using the site-specific property-transfer model of Winfield (2005) and the simple linear relation between the logarithm of K_sat and ρ_bulk identified by Pudney (1994).

The Winfield model uses bulk physical-property data, including ρ_bulk and particle-size statistics [median particle diameter (d₅₀) and uniformity coefficient (C_u)], to estimate K_sat using a multiple linear-regression equation. Winfield (2005) describes the available data, data selection and processing, multiple linear-regression assumptions and approach, and regression equations. A simple linear relation between K_sat and ρ_bulk, as identified by Pudney (1994), also was tested for comparison to the Winfield model. The Pudney (1994) data set includes samples from depths greater than 250 m; whereas, the Winfield (2005) model includes samples from depths less than 80 m.

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