Development Report on the Idaho National Laboratory Sitewide Three-Dimensional Aquifer Model

Abstract

A sub-regional scale, three-dimensional flow model of the Snake River Plain Aquifer was developed to support remediation decisions for Waste Area Group 10, Operable Unit 10-08 at the Idaho National Laboratory (INL) Site. This model has been calibrated primarily to water levels and secondarily to groundwater velocities interpreted from stable isotope disequilibrium studies and the movement of anthropogenic contaminants in the aquifer from facilities at the INL. The three-dimensional flow model described in this report is one step in the process of constructing a fully three-dimensional groundwater flow and contaminant transport model as prescribed in the Idaho National Engineering and Environmental Laboratory Operable Unit 10-08 Sitewide Groundwater Model Work Plan.An updated three-dimensional hydrogeologic conceptual model is presented along with the geologic basis for the conceptual model. Sediment-dominated three-dimensional volumes were used to represent the geology and constrain groundwater flow as part of the conceptual model. Hydrological, geochemical, and geological data were summarized and evaluated to infer aquifer behavior. A primary observation from development and evaluation of the conceptual model was that relative to flow on a regional scale, the aquifer can be treated with steady-state conditions. Boundary conditions developed for the three-dimensional flow model are presented along with inverse simulations that estimate parameterization of hydraulic conductivity. Inverse simulations were performed using the pilot-point method to estimate permeability distributions. Thermal modeling at the regional aquifer scale and at the sub-regional scale using the inverted permeabilities is presented to corroborate the results of the flow model.The results from the flow model show good agreement with simulated and observed water levels almost always within 1 meter. Simulated velocities show generally good agreement with some discrepancies in an interpreted low-velocity region near the toe of the Arco Hills. This discrepancy persisted in each of the aquifer bottom thickness scenarios that were simulated precluding decisions on which aquifer bottom thickness to use in transport simulations. When joint-calibration was performed using both water levels and velocities assigned as calibration targets, the discrepancy was prevented. This result highlighted the need to consider multiple calibration objectives and not rely solely on calibration to water levels.

SSHAC Level 1 Probabilistic Seismic Hazard Analysis for the Idaho National Laboratory

“…The Big Lost fault in the south central region of INL is identified by a stratigraphic discontinuity. Wood et al (2007) recognized that the basalt stratigraphy on the west side of the Big Lost River is very different from that on the east side of the river (Figure 4-16). One possibility is the presence of the Big Lost fault.…”

“…One possibility is the presence of the Big Lost fault. Wood et al (2007) show correlations of basalt flows between two of the closest boreholes have a down to the east step with a maximum offset of 309 m that occurs in flows with ages from >292 ka to 780 ka. Stratigraphic discontinuities in other nearby wells may support up to a 15 km length and SE-strike for the Big Lost fault (see Section 5.4).…”

Evaluation of Groundwater Impacts to Support the National Environmental Policy Act Environmental Assessment for the INL Remote-Handled Low-Level Waste Disposal Project

“…Default Track 2 dispersivity values of 9 m (longitudinal) and 4 m (transverse) were applied. An aquifer porosity of 0.06 was used, which corresponds to that determined through calibration of the INL-wide groundwater model (DOE-ID 2005;Wood et al 2007).…”

“…Low and high-permeability interbeds have been characterized at INTEC through the CERCLA investigations (DOE-ID 2006a) and are represented by the values shown in Table 8. Properties for Site 5 interbeds were determined by the U.S. Geological Survey for well ICPP-SCI-V-213 (DOE-ID 2003c;DOE-ID 2005;Wood et al 2007;DOE-ID 2008). Unsaturated basalt properties were documented in Wood et al (2007).…”

“…Properties for Site 5 interbeds were determined by the U.S. Geological Survey for well ICPP-SCI-V-213 (DOE-ID 2003c;DOE-ID 2005;Wood et al 2007;DOE-ID 2008). Unsaturated basalt properties were documented in Wood et al (2007).…”