Scientific Investigations Report 2006-5122

Scientific Investigations Report 2006-5122

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The Idaho National Laboratory (INL), established in 1949, occupies about 890 mi2 of the eastern Snake River Plain (SRP) in southeastern Idaho (fig. 1). The INL was established to build, operate, and test nuclear reactors. The scope of work at the INL increased during the 1950s to include other nuclear-research programs, the reprocessing of spent nuclear fuel, and the storage and disposal of radioactive and chemical waste.

The INL overlies the west-central part of the eastern SRP aquifer, which is a major source of water for agricultural, industrial, and domestic uses in southeastern Idaho. Radioactive and chemical wastes were discharged to the unsaturated zone and aquifer at the INL for over 50 years and several waste contaminants are present in the aquifer (fig. 2) at concentrations that exceed U.S. Environmental Protection Agency maximum contaminant levels (MCLs) for drinking water (table 1). The principal sources of contaminants are wastewater-disposal sites at Test Area North (TAN), the Test Reactor Area (TRA), and the Idaho Nuclear Technology and Engineering Center (INTEC). Wastewater-disposal sites have included lined evaporation ponds, unlined infiltration ponds and ditches, drain fields, and injection wells. Wastes buried in shallow pits and trenches at the Radioactive Waste Management Complex (RWMC) also are sources of contaminants in ground water. Appendix A (at back of report) contains a brief history of wastewater-disposal practices at the INL.

The presence of contaminants in the eastern SRP aquifer has led to public concern about the quality of water in the aquifer and the effect that contaminated ground water may have on the region. To respond to this concern a thorough understanding of the fate and movement of contaminants in the subsurface is needed. The U.S. Geological Survey (USGS) in cooperation with the U.S. Department of Energy (DOE) is developing a three-dimensional numerical model of ground-water flow and contaminant transport in the aquifer at the INL and vicinity. The model will assist the DOE to minimize the effect that subsurface contaminants may have on the region and to plan effectively for remediation. Ground-water flow and contaminant transport in the aquifer beneath the INL were modeled previously by Robertson (1974) and Goode and Konikow (1990b). The new calibrated, numerical flow-and-transport model will build on the work of these earlier models and will be based on a refined conceptual model of flow beneath the INL presented in this report. New information available for developing the current flow-and-transport model includes (1) an increased understanding of the hydrogeology beneath the INL, (2) an increased understanding of the amount and timing of transient recharge to the aquifer beneath the INL (including a near record amount of streamflow infiltration from the Big Lost River in 1983 and 1984 and a period with little or no infiltration between 1987 and 1994), (3) additional information about the amount and timing of wastewater disposal, (4) more than 30 years of additional water-level data for numerical-model calibration, and (5) more than 30 years of additional measurements of contaminant concentrations in the aquifer. This additional information will allow a new flow-and-transport model to be developed using more accurate estimates of hydrogeologic properties, such as hydraulic conductivity, storativity, and effective porosity and to be calibrated over a longer simulated period than that of the previous flow-and-transport models.

Purpose and Scope

This report describes a conceptual model of ground-water flow in the west-central part of the eastern SRP aquifer that will be used in numerical simulations to evaluate contaminant transport at the INL and vicinity. This part of the aquifer contains contaminants that were discharged to the subsurface at facilities located at the INL. The conceptual model was developed to provide a qualitative description of ground-water flow at the INL and vicinity so that hydrogeologic features most strongly affecting contaminant movement could be identified. The model integrates the current understanding of the stratigraphy, hydraulic properties of the rocks, hydrology, and ground-water movement in the model area.

The model area (1,940 mi2) is subregional in scale, and is intermediate in scale between the regional-scale model of the eastern SRP aquifer (10,800 mi2), described in reports of the Regional Aquifer System Analysis (RASA) study of the aquifer, and local-scale models of the aquifer at INL facilities (less than about 10 mi2) (Schafer-Perini, 1993; Magnuson and Sondrup, 1998). The conceptual models for the three scales include the same basic information but at different levels of detail (Appendix B, at back of report). For example, at many INL facilities data are sufficient to represent the heterogeneity and anisotropy of the aquifer at the scale of individual basalt flows or basalt-flow groups. In the current subregional-scale model, however, data are not sufficient to resolve individual basalt flows and basalt-flow groups and these were grouped with other basalt flows having similar hydraulic characteristics into homogenous and anisotropic hydrogeologic units.

Data used for developing the conceptual model were obtained from numerous reports published during the past 50 years, including ground-water modeling studies, reports generated for the eastern SRP regional aquifer system as part of the USGS RASA study, and several recent studies. These data, which include petrologic, stratigraphic, geophysical, geochemical, hydraulic-property, and hydrologic data were acquired primarily from (1) surficial mapping, (2) analyses of numerous outcrops and hundreds of core samples, (3) aquifer tests, (4) borehole geophysical logs and surface geophysical surveys, (5) more than 50 years of water-level measurements and water-chemistry analyses from more than three hundred wells, and (6) long-term records of streamflow (100 years) and wastewater discharge to the aquifer (34 years).

Although a large quantity of data were available for developing the conceptual model, there were several limitations with these data. These data limitations included (1) uneven spatial distribution, both areally and vertically [for example, of the 333 boreholes used in this study to define the hydrogeologic framework, 300 are within the boundaries of the INL, mostly in and near INL facilities (Anderson and others, 1997, figs. 2 to 5, table 1)], (2) scaling-compatibility issues involving the application of small-scale measurements to a large-scale study (for example, the use of core-scale measurements to define large-scale hydraulic properties), (3) uncertainties arising from partial borehole penetrations and different borehole completions that complicate interpretations of water-level, water-chemistry, and hydraulic-conductivity measurements (for example, of the 114 monitoring wells that were used to estimate hydraulic conductivity only 13 of these penetrate more than 300 ft into the aquifer), and (4) discontinuous or nonexistent hydrologic records (for example, streamflow records for Little Lost River and Birch Creek streamflow onto the INL).

Description of Model Area

The model area extends 75 mi from northeast to southwest and 35 mi from northwest to southeast and includes most of the INL (fig. 2). The model area is bounded on the northwest by mountain fronts and valleys tributary to the plain and on the southeast by an inferred regional ground-water flowline. The northeast boundary is defined by a steep increase in hydraulic gradient. The southwest boundary is 25 mi downgradient of the southwestern extent of measured concentrations of INL-derived contaminants in the aquifer (Beasley, 1995, appendix 2). The vertical dimension of the model area is represented by the saturated thickness of the aquifer, which is estimated to range from less than 600 ft to more than 3,000 ft.

The model area is contained in the eastern SRP, which is a relatively flat topographic depression, about 200-mi long and 50- to 70-mi wide, surrounded by mountains on three sides. The altitude of the plain ranges from about 2,900 ft near King Hill to about 6,200 ft near the southwestern extent of the Yellowstone Plateau (fig. 3), and the surrounding mountains reach altitudes of 12,000 ft. The surface of the plain is primarily loess and olivine basalt and contains volcanic landforms, such as cinder and lava cones, shield volcanoes, rhyolite domes that rise as much as 2,500 ft above the plain, sand dunes, and a canyon carved by the Snake River that ranges from 50- to 550-ft deep (Lindholm, 1996, p. 5).

Climate in the model area is semiarid and mean annual precipitation at the INL is about 0.7 ft (Clawson and others, 1989, tables D-1 and D-2; Goodell, 1988, fig. 5). About 0.1 to 0.3 ft of snow usually accumulates at the INL during winter and snowmelt and runoff from the adjacent mountains, typically in late spring and early summer, produce peak streamflow. Mean annual air temperature at the Central Facilities Area (CFA) at the INL between 1950 and 1988 was 30ºF and the coldest and warmest mean monthly temperatures were measured in January and during July-August, respectively (Clawson and others, 1989, table B-3).

Ground water in the eastern SRP aquifer flows to the southwest and discharges principally through large springs and seeps along the Snake River in the Thousand Springs area in the southwestern part of the plain (fig. 3). Recharge to the eastern SRP aquifer is from infiltration of precipitation, underflow from tributary drainages, infiltration of surface water diverted for irrigation, and stream and canal losses (Garabedian, 1992, p. 11). Land irrigated with ground water on the eastern SRP is located along the southeastern and southern margins of the plain, from north of Idaho Falls to west of Twin Falls, and in the Mud Lake area northeast of the INL (fig. 3). In 1980, about 1,760,000 acre-ft of ground water was withdrawn from about 4,000 wells across the plain to irrigate about 930,000 acres (Garabedian, 1992, p. 19-21). Intermittent streamflow onto the plain near the model area is from Camas Creek, Birch Creek, Little Lost River, and Big Lost River (fig. 1).

Regional Geology

The eastern SRP was formed by the migration of the North American Continental Plate westward across a mantle-plume hot spot (Pierce and Morgan, 1992, p. 2). The plain is underlain by a 1,000- to 2,000-ft thick-layered sequence of Quaternary basalt flows and sediment interbeds overlying a several-thousand-feet thick sequence of Quaternary and late Tertiary volcanic rocks (fig. 4) (Whitehead, 1992, pl. 3). Major Quaternary and late Tertiary stratigraphic units in the eastern SRP are the: (1) Snake River Group (Qb), which is the most extensive rock unit in the eastern SRP and contains mainly olivine basalt and sedimentary rocks; (2) Yellowstone Group and Plateau Rhyolite (Qsv), containing silicic volcanic rocks; (3) upper part of the Idaho Group (includes the Bruneau and Glenns Ferry Formations), containing mainly olivine basalt (QTb) and sedimentary rocks (QTs); (4) Walcott Tuff, Starlight Formation, and Salt Lake Formation (QTs), containing mainly sedimentary rocks, basalt, and tuff; (5) lower part of the Idaho Group (Tb, includes the Banbury Basalt), containing mainly flood-type basalts, silicic rocks, and sediment; and (6) Idavada Volcanics (Tsv), containing silicic volcanic rocks and sediment. Sedimentary units include flood plain and glacial deposits (Qa), windblown deposits (Qw), and lake deposits and tuffaceous sedimentary rocks (QTs) (Whitehead, 1992, pl. 1).

Several volcanic landforms define the structure and topography of the eastern Snake River Plain. An axial volcanic high and volcanic rift zones trend parallel and perpendicular to the long axis of the plain, respectively. The rift zones appear to be extensions of the adjacent Basin and Range structures (Kuntz and others, 1992, 1994; Anderson and Liszewski, 1997). Volcanic vents and fissures are concentrated along the axial volcanic high and volcanic rift zones (Anderson and others, 1999, p. 13, fig. 7; Hughes and others, 1999, p. 145), and are major sources of basaltic rocks on the plain. Rhyolite domes also are concentrated within the axial volcanic high. Topographic depressions in areas defined by elevated volcanic topography, such as the axial volcanic high and volcanic rift zones, are areas where abundant sediment accumulated during recent times (Bestland and others, 2002, fig. 1; Geslin and others, 2002, p. 12-13, fig. 1).

Previous Investigations

The most complete and systematic study of the eastern SRP aquifer was conducted as part of the Regional Aquifer System Analysis (RASA) program. In this study, Whitehead (1992) provided an extensive description of the regional hydrogeologic framework, hydrologic properties, and geologic controls on ground-water movement. Kjelstrom (1995) described streamflow gains and losses and compiled regional ground-water budgets. Garabedian (1992) synthesized hydrologic data and constructed a regional ground-water flow model (fig. 5). These studies provide the regional context for describing flow in the model area.

Information from several previous studies was used to construct the conceptual model. Anderson and Liszewski (1997) combined information from geophysical logs, rock cores, and outcrops to describe the stratigraphy of the unsaturated zone and aquifer beneath the INL. This study provided the basis for determining the hydrogeologic units of the eastern SRP aquifer at the INL. Ackerman (1991) and Anderson and others (1999) described the distribution of hydraulic properties in the eastern SRP aquifer and the geologic features that control that distribution. Bennett (1990) estimated the amount, distribution, and timing of recharge to the eastern SRP aquifer at the INL from the infiltration of streamflow from the Big Lost River.

Previous ground-water models also provided information used to construct the conceptual model. Spinazola (1994) simulated ground-water flow in the Mud Lake area upgradient of the INL (fig. 5). Ackerman (1995) developed a regional ground-water flow and advective-transport model based on studies by Garabedian (1992). Pathline simulations from Ackerman’s (1995) model provide a basis for description of ground-water flow near the INL. McCarthy and others (1995) developed a ground-water flow model for the area of the INL to support ground-water remediation for specific INL facilities (fig. 5).

The current modeling effort builds on the ground-water flow and contaminant transport modeling results of Robertson (1974) and Goode and Konikow (1990b). Robertson (1974) simulated two-dimensional steady-state and transient ground-water flow and contaminant transport. The simulations successfully reproduced the unusually wide contaminant plumes present in the upper 200 ft of the aquifer; however, to do so required a larger transverse (αT= 450 ft) than longitudinal (αL = 300 ft) dispersivity. These values for dispersivity were derived using a steady-state flow model and no recharge from the Big Lost River (Goode and Konikow, 1990b, p. 417); no values for dispersivity were reported for the transient flow simulations. The αL/ αT ratio derived from this model, 0.67, is smaller than the ratios of 24 other published pairs of αL/ αT; the other αL/ αT ratios ranged from 1 to 53 with a mean of 8.8 (Gelhar and others, 1992, table 1). This small αL/ αT ratio, and the large simulated values for αL and αT, indicated that some geologic and hydrologic features of the aquifer that were important for modeling contaminant transport were not represented in Robertson’s (1974) model.

Goode and Konikow (1990b) recognized that ground-water flow directions and velocities varied temporally in the shallow flow field near contaminated ground water at the INL, and that the large model-derived αT of Robertson (1974) could have included these fluctuations. If transient variations in the flowfield are ignored or unknown, then larger estimates for αT and smaller αL/ αT ratios may be required to account for the temporally-variable directions of advective flow. Using Robertson’s (1974) steady-state and transient models, but modifying the simulated magnitude, timing, and spatial distribution of recharge from the Big Lost River, Goode and Konikow (1990b) evaluated the effect of transient ground-water flow at the INL on estimates of longitudinal and transverse dispersivity. Although Goode and Konikow’s (1990b) simulations resulted in αT values that were smaller than values for αL, their results were inconclusive and they were unable to determine the effect that transient variations in the flow field had on dispersivities.

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