Scientific Investigations Report 2006-5122
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5122
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Ground-water flow in the west-central part of the eastern Snake River Plain (SRP) 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 conceptual model emphasizes the effects of various geologic and hydrologic controls on the transport of contaminants at the INL.
The conceptual model encompasses an area of 1,940 mi2 and includes most of the 890 mi2 of the INL. The model area extends 75 mi from northeast to southwest and 35 mi from northwest to southeast. The subregional scale of the conceptual model is intermediate in size between that of the Regional Aquifer System Analysis (RASA) of the eastern SRP aquifer (10,800 mi2) and that of a local INL facility-scale model (less than about 10 mi2).
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 and are treated 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 of 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 within individual hydrogeologic units result in much of the small-scale heterogeneity and anisotropy of the aquifer in the model area.
Intermediate-age rocks make up the largest volume of the aquifer. Hydrogeologic units 1 and 2 are not present in the aquifer in the northern and southern parts of the model area; in those areas, hydrogeologic unit 3 is inferred to be present at the water table. An alternative depiction of the hydrogeologic framework includes a large area of the aquifer beneath the northern and central parts of the INL where the combined sediment content of all three hydrogeologic units exceeds 11 percent and, in some cases, accounts for 25 to more than 50 percent of the composite stratigraphic section. The large sediment content of these rocks may require incorporation of a fourth hydrogeologic unit or modification of the existing hydrogeologic units to properly characterize flow and contaminant transport in the aquifer.
The aquifer is underlain by older rocks composed of intensely altered basalt and interbedded sediment. Based on surface-based electrical-resistivity surveys and stratigraphic data from boreholes at and near the INL, depth to the base of the aquifer in the model area was estimated to range from 700 to 4,800 ft below land surface. Although stratigraphic data from 10 deep boreholes indicated that depth to the base of the aquifer ranges from 815 to 1,710 ft, the 700 to 4,800 ft estimate was used to define the base of the aquifer because the surface-based electrical-resistivity surveys provided coverage over the entire model area.
The geometry of the aquifer is an important feature for developing models of flow and contaminant transport. Most interpretations of this geometry, particularly the thickness and distribution of hydrogeologic unit 2, depth to the base of the aquifer, and distribution of the hydrogeologic units south of the INL were based on very limited data and indirect measurements that are subject to considerable uncertainty. These data indicate that the three-dimensional geometry of the aquifer 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 of the aquifer across the model area. Variations in thickness, depth, and attitude of individual hydrogeologic units are attributed primarily to differential subsidence and uplift.
Ground-water flow in hydrogeologic unit 1 occurs preferentially in the interflow zones of the many thin, mainly 14- to 24-ft thick, pahoehoe flows that compose this hydrogeologic unit. Aquifer tests conducted in 67 wells indicated that the hydraulic conductivity of the densely fractured rocks composing hydrogeologic unit 1 varies by more than six orders of magnitude, from 0.01 to 24,000 ft/d. Almost two-thirds of these estimates are greater than 100 ft/d, and about one-third are greater than 1,000 ft/d. The hydraulic conductivity of hydrogeologic unit 2 was estimated based on comparisons with massive basalts of the Columbia Plateau, measurements from flowmeter tests conducted in two wells at the Idaho Nuclear Technology and Engineering Center, and single-well aquifer tests in four wells having perforated intervals only in hydrogeologic unit 2. These estimates ranged from 6.5 to 1,400 ft/d. Individual basalt flows composing hydrogeologic unit 2 are mainly 21- to 37-ft thick, and the hydraulic conductivity of hydrogeologic unit 2 is presumed to be much smaller than that of hydrogeologic unit 1 because of the massive character of the individual basalt flows and the fewer number of interflow zones. Estimates of the hydraulic conductivity of the slightly altered, fractured basalt and interbedded sediment composing hydrogeologic unit 3 were based on single-well aquifer tests in 14 wells. These test results indicated that the hydraulic conductivity of these rocks ranges from about 0.32 to 24,000 ft/d. Although large hydraulic conductivities were measured for hydrogeologic unit 3, the hydraulic conductivity of hydrogeologic unit 3 is presumed to be smaller than that of hydrogeologic unit 1 because hydrogeologic unit 3 probably has fewer interflow zones and is slightly-to-moderately altered.
Estimates of porosity vary greatly because they are dependent on the methods, scales, and locations used to determine them. Core measurements indicated that the porosity of hydrogeologic unit 1 ranges from about 0.05 to 0.27. The porosity of hydrogeologic unit 2 probably is less than that of hydrogeologic unit 1 because of the dense, massive character and fewer interflow zones associated with the rocks that compose hydrogeologic unit 2. For similar reasons, and because of alteration, the porosity of hydrogeologic unit 3 is likely to be smaller than hydrogeologic unit 1.
Three physical and three artificial boundaries define the model area. The physical boundaries are the (1) water-table boundary, (2) base of the aquifer, and (3) northwest mountain-front boundary. The artificial boundaries are the (1) northeast boundary, (2) southeast-flowline boundary, and (3) southwest boundary. In the conceptual model ground water flow across these boundaries is represented as temporally constant or variable with spatially uniform or nonuniform flow distributions.
Flow through the unsaturated zone to the water-table boundary is represented implicitly in the conceptual model as time-averaged net infiltration recharge. Inflow across the water-table boundary, represented in the model as variable and nonuniform, occurs as (1) diffuse areal precipitation recharge (70 ft3/s) (constant and uniform), (2) streamflow-infiltration recharge from the Big Lost River (95 ft3/s) (variable and nonuniform), (3) wastewater return flows (6 ft3/s) (variable and nonuniform), (4) irrigation-infiltration recharge (24 ft3/s), and (5) release of water from storage (80 ft3/s) (variable and uniform). Flux across the water-table boundary from streamflow-infiltration recharge is about 20 (ft3/s)/mi2, or more than two orders of magnitude larger than flux from precipitation recharge. Flux from streamflow-infiltration recharge during years with much larger than average streamflow was estimated to be as large as 170 (ft3/s)/mi2 . Because of its large flux and proximity to known sources and areas of contamination in the aquifer, streamflow-infiltration recharge from the Big Lost River is considered much more significant to contaminant transport than the other components of inflow across the water-table boundary.
The base of the aquifer is represented as a constant-flow, uniform boundary. Hydraulic conductivity of the intensely altered older rocks near the boundary, 0.002 to 0.03 ft/d, was estimated to be three to four orders of magnitude smaller than that of the overlying intermediate-age rocks. Inflow across this boundary, for that part of the aquifer underlying the INL, was previously estimated at 20 ft3/s. This estimate was based on data from one 10,365-ft-deep corehole at the INL. Extrapolation of the estimate to the 1,940 mi 2 area of the conceptual model resulted in an inflow estimate of 44 ft3/s or about 2 percent of the total water-budget inflow estimate. This inflow estimate represents a flux of about 0.02 (ft3/s)/mi2, or about half of the flux for precipitation recharge, 0.04 (ft3/s)/mi2. Because of its low flux and distance from known areas of contamination in the aquifer, the base of the aquifer, for purposes of contaminant transport modeling, can be treated as a no-flow boundary.
The northwest mountain-front boundary is represented as a constant-flow, nonuniform boundary. Inflows to the aquifer across this boundary (695 ft3/s) consist of tributary-valley underflow from the Big Lost River valley (367 ft3/s), the Little Lost River valley (226 ft3/s), and the Birch Creek valley (102 ft3/s).
The northeast boundary of the model area is represented as a nonuniform, constant-flow boundary. Inflow into the aquifer across this boundary consists of underflow from the regional aquifer (1,225 ft3/s) and is the largest inflow component of the budget. Estimates of inflow across this boundary are considered fairly reliable because there is a relative abundance of information available to characterize the hydrologic conditions and hydraulic properties of the aquifer in this area.
The southeast-flowline boundary is represented as a noflow boundary and was defined by a generalized flowline derived from the RASA model and three-dimensional pathline analyses in an advective transport model.
Most of the outflow (1,731–2,343 ft3/s) from the aquifer occurs as underflow across the southwest boundary of the study area, which is represented as a variable-flow, nonuniform boundary. The variable flow across this boundary reflects the character of large, episodic inflows from Big Lost River streamflow-infiltration recharge. Head definition for this part of the aquifer was limited to water-level measurements from six wells that are from 3 to 10 mi from the southwest boundary; consequently, outflow estimates across this boundary are perhaps the least reliable of all the water-budget estimates.
Flow in the aquifer increases progressively in a direction downgradient of the northeast boundary. Increased flow is the result of mountain-front and tributary-valley underflows along the northwest boundary (695 ft3/s), precipitation and streamflow-infiltration recharge (165 ft3/s) across the water-table boundary, and release of water from storage (80 ft3/s). Together these additions account for more than 45 percent of the outflow across the southwest boundary. Ground-water flow beneath the INL occurs in all three hydrogeologic units. In the northern part of the INL and south of the INL, where the younger rocks of hydrogeologic units 1 and 2 are either not present or are above the water table, all flow takes place through the slightly-to-moderately altered intermediate-age rocks of hydrogeologic unit 3.
Hydraulic gradients were defined by measurements of water levels in 201 wells in the model area, 175 of which are within the boundaries of the INL. These gradients indicate that the regional direction of ground-water flow is from northeast to southwest. Hydraulic gradients are largest immediately upgradient of the northeast boundary, 27 to 60 ft/mi, and southwest of the INL, 4 to 30 ft/mi. Beneath the INL, gradients are much smaller, 1 to 8 ft/mi, and precise definition of flow direction is difficult to determine. Flow directions in the aquifer beneath the INL vary locally from southeast to southwest and fluctuate in response to episodic recharge from streamflow infiltration. In 1984, streamflow-infiltration recharge from the Big Lost River accounted for more than 20 percent of the estimated 1980 steady-state water budget and resulted in water-level rises in the aquifer exceeding 10 ft in places. These temporary changes in water levels alter flow directions locally and, in some extreme cases, can cause reversals in flow direction. Lateral spreading of contaminants could be strongly affected by episodic recharge and resulting temporal fluctuations in water-table gradients.
Long-term monitoring of contaminant movement in the aquifer at the INL indicates that ground-water velocities in hydrogeologic unit 1 range from 4 to 20 ft/d in areas affected by contamination south of the Test Reactor Area and the Idaho Nuclear Technology and Engineering Center. These velocities were based on presumed first-arrival times of contaminants at wells downgradient of known contaminant-input locations (injection wells) at various INL facilities. Hydraulic conductivities derived from these velocity estimates range from 500 to 5,000 ft/d for assumed effective porosities of 0.1 and 0.2. These estimates are generally consistent with results of aquifer tests conducted in hydrogeologic unit 1. The ground-water velocity in hydrogeologic unit 1 of from 4 to 20 ft/d probably reflects preferential flow along the many interflow zones of the thin, densely fractured basalts composing hydrogeologic unit 1. However, because of the sometimes uncertain flow directions and gradients that are subject to fluctuations, actual flow paths may be longer and velocities higher than those estimated from first arrivals of contaminants.
Because most wells penetrate only the upper 200 ft of the aquifer, there are few data to evaluate how hydraulic gradients and flow directions vary with depth. Two boreholes at and near the INL are equipped with dedicated piezometers to measure head changes at depth. These two boreholes are near the northeastern boundary of the INL. At one well 2 and 11 ft increases in head at depths of 335 and 545 ft below the water table, respectively, were measured, and at another well 0.4 and 0.3 ft increases in head at depths of 100 and 425 ft below the water table, respectively, were measured. The resulting vertical head differences probably are a reflection of head losses resulting from regional aquifer underflow into the layered basalts and interbedded sediments downgradient of the steep hydraulic gradient, 27 to 60 ft/mi, which defines the northeast boundary of the model area.
A simplified northeast to southwest cross section showing the approximate distribution of hydrogeologic units and ground-water flow paths through the model area indicated that head decreases and then increases with depth with thickening and thinning of the aquifer in a direction downgradient of the northeast boundary. Downward flow is indicated in areas where head decreases with depth, and upward flow in areas where head increases with depth. The largest changes in vertical gradient are depicted as occurring upgradient of where hydrogeologic unit 2 is inferred to intersect the water table south of the INL. Downward movement of water across the less permeable rocks of hydrogeologic unit 2 probably occurs at this location, implying downward movement and deeper circulation of contaminants that may move offsite.
The conceptual model implies that most contaminant movement beneath the INL probably takes place in the highly conductive rocks of hydrogeologic unit 1 that compose most of the upper 200 ft of the aquifer. This hypothesis is based on (1) definition of the hydraulic properties and the inferred distribution of the hydrogeologic units selected to characterize flow in the aquifer, and (2) long-term observations of contaminant movement in the aquifer, the location of contaminant sources, and the history of waste-disposal practices at the INL. This hypothesis is consistent with conclusions reached by earlier investigators who studied waste behavior and aquifer characteristics at the INL and noted that waste plumes “…generally remain as relatively thin lenses within about 250 feet of the water table.”
The features of the flow system represented in the conceptual model that most affect interpretations of contaminant transport at the INL and vicinity are the (1) implicit representation of infiltration recharge through the unsaturated zone, (2) preferential flow along highly conductive interflow zones, 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) 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 south 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.
Uncertainties were associated with all aspects of the current conceptual model. Many aspects of this model are based on interpretations or extrapolations using very limited data or indirect measurements that may be subject to considerable error. To understand contaminant transport, some uncertainties are considered to be more significant than other uncertainties. Sensitivity analysis using numerical models can be used to assess the significance of some of these uncertainties. Model performance depends on how well simulation results compare with historical records of the spatial and temporal distribution of contaminants. Confidence in a well-documented source inventory is a necessary prerequisite. However, three-dimensional field definition of contaminant concentrations in the subsurface at the INL and vicinity is lacking; so, regardless of how well transport simulations match the available field observations, independent verification of those aspects of the conceptual model that rely on interpretations of vertical flow to describe the movement of contaminants will be needed.
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