Scientific Investigations Report 2006-5122

Scientific Investigations Report 2006-5122

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Ground-water flow in the west-central part of the eastern Snake River Plain aquifer is described in a conceptual model that will be used in numerical simulations to evaluate contaminant transport at the Idaho National Laboratory (INL) and vicinity. The model encompasses an area of 1,940 square miles (mi2) and includes most of the 890 mi2 of the INL. A 50-year history of waste disposal associated with research activities at the INL has resulted in measurable concentrations of waste contaminants in the aquifer. A thorough understanding of the fate and movement of these contaminants in the subsurface is needed by the U.S. Department of Energy to minimize the effect that contaminated ground water may have on the region and to plan effectively for remediation.

Three hydrogeologic units were used to represent the complex stratigraphy of the aquifer in the model area. Collectively, these hydrogeologic units include at least 65 basalt-flow groups, 5 andesite-flow groups, and 61 sedimentary interbeds. Three rhyolite domes in the model area extend deep enough to penetrate the aquifer. The rhyolite domes are represented in the conceptual model as low permeability, vertical pluglike masses, and are not included as part of the three primary hydrogeologic units. Broad differences in lithology and large variations in hydraulic properties allowed the heterogeneous, anisotropic basalt-flow groups, andesite-flow groups, and sedimentary interbeds to be grouped into three hydrogeologic units that are conceptually homogeneous and anisotropic. Younger rocks, primarily thin, densely fractured basalt, compose hydrogeologic unit 1; younger rocks, primarily of massive, less densely fractured basalt, compose hydrogeologic unit 2; and intermediate-age rocks, primarily of slightly-to-moderately altered, fractured basalt, compose hydrogeologic unit 3. Differences in hydraulic properties among adjacent hydrogeologic units result in much of the large-scale heterogeneity and anisotropy of the aquifer in the model area, and differences in horizontal and vertical hydraulic conductivity in individual hydrogeologic units result in much of the small-scale heterogeneity and anisotropy of the aquifer in the model area.

The inferred three-dimensional geometry of the aquifer in the model area is very irregular. Its thickness generally increases from north to south and from west to east and is greatest south of the INL. The interpreted distribution of older rocks that underlie the aquifer indicates large changes in saturated thickness across the model area.

The boundaries of the model include physical and artificial boundaries, and ground-water flows across the boundaries may be temporally constant or variable and spatially uniform or nonuniform. Physical boundaries include the water-table boundary, base of the aquifer, and northwest mountain-front boundary. Artificial boundaries include the northeast boundary, southeast-flowline boundary, and southwest boundary. Water flows into the model area as (1) underflow (1,225 cubic feet per second (ft3/s)) from the regional aquifer (northeast boundary—constant and nonuniform), (2) underflow (695 ft3/s) from the tributary valleys and mountain fronts (northwest boundary—constant and nonuniform), (3) precipitation recharge (70 ft3/s) (constant and uniform), streamflow-infiltration recharge (95 ft3/s) (variable and nonuniform), wastewater return flows (6 ft3/s) (variable and nonuniform), and irrigation-infiltration recharge (24 ft3/s) (variable and nonuniform) across the water table (water-table boundary—variable and nonuniform), and (4) upward flow across the base of the aquifer (44 ft3/s) (uniform and constant). The southeast-flowline boundary is represented as a no-flow boundary. Water flows out of the model area as underflow (2,037 ft3/s) to the regional aquifer (southwest boundary—variable and nonuniform) and as ground-water withdrawals (45 ft3/s) (water table boundary—variable and nonuniform).

Ground-water flow increases progressively in a direction downgradient of the northeast boundary. This increased flow is the result of tributary-valley and mountain-front underflows along the northwest boundary and precipitation recharge and streamflow-infiltration recharge across the water-table boundary. Ground water flows in all three hydrogeologic units beneath the INL. South of the INL, the younger rocks, hydrogeologic units 1 and 2, are either not present or are above the water table and all flow occurs through the intermediate-age rocks, hydrogeologic unit 3.

The direction of regional ground-water flow is from northeast to southwest. Flow directions beneath the INL vary locally from southeast to southwest and fluctuate in response to episodic recharge from streamflow infiltration. Water-table gradients immediately upgradient of the northeast boundary are 27 to 60 feet per mile (ft/mi); and southwest of the INL gradients are 4 to 30 ft/mi. Beneath the INL gradients are much flatter, 1 to 8 ft/mi, and precise definition of flow direction is difficult to determine.

Long-term monitoring of contaminant movement in the aquifer at the INL indicates that ground-water velocities in the thin, fractured basalts of hydrogeologic unit 1, the uppermost hydrogeologic unit of the aquifer, range from 4 to 20 feet per day (ft/d) south of the Test Reactor Area and the Idaho Nuclear Technology and Engineering Center. These velocities probably indicate preferential flow along the many interflow zones of the thin, fractured basalt flows composing the uppermost hydrogeologic unit. Hydraulic conductivities (500 to 5,000 ft/d) estimated from velocity measurements were consistent with those derived from aquifer tests conducted in this hydrogeologic unit. Almost two-thirds of the hydraulic conductivities derived from aquifer-test measurements in hydrogeologic unit 1 were larger than 100 ft/d and about one‑third were larger than 1,000 ft/d.

Most contaminant movement beneath the INL probably takes place in the thin, densely fractured, and highly conductive basalts and interbedded sediments of hydrogeologic unit 1, which compose most of the upper 200 ft of the aquifer beneath most of the INL. This hypothesis is based on interpretation of a generalized northeast-to-southwest cross section of ground-water flow across the model area that depicts the effects of the hydrogeologic framework on flow in each of the hydrogeologic units used to represent the aquifer. This interpretation indicates that head decreases and then increases with depth with thickening and thinning of the aquifer in a direction downgradient of the northeast boundary. Beneath the INL, the smaller conductivity of the massive, less densely fractured basalts and interbedded sediments of hydrogeologic unit 2 restricts the downward movement of contaminants from hydrogeologic unit 1. The largest changes in water-table gradients are upgradient of where the massive basalts of hydrogeologic unit 2 are inferred to intersect the water table south of the INL. Water probably flows downward through hydrogeologic unit 2 into hydrogeologic unit 3 at this location, implying deeper circulation of contaminants that migrate offsite.

Features of the conceptual model that most affect interpretations of contaminant transport are (1) implicit representation of infiltration recharge through the unsaturated zone, (2) preferential flow along highly conductive interflow zones, primarily in the thin, densely fractured basalts of hydrogeologic unit 1, implying large horizontal to vertical anisotropy, (3) restricted downward movement of flow and contaminants in hydrogeologic unit 1 into the less conductive basalts of hydrogeologic unit 2 beneath the INL, (4) the inferred downward movement and deeper circulation of water upgradient of where the massive, less densely fractured basalt of hydrogeologic unit 2 intersects the water table southwest of the INL, and (5) enhanced dispersion of contaminants resulting from the spatial and temporal variability of streamflow-infiltration recharge that is in close proximity to contaminated ground water.

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