Development, calibration, and predictive results of a simulator for subsurface pathway fate and transport of aqueous- and gaseous-phase contaminants in the Subsurface Disposal Area at the Idaho National Engineering and Environmental Laboratory

Abstract

T IThe first detailed comprehensive simulation study to evaluate fate and transport of low-level, mixed, and transuranic wastes buried in the Subsurface Disposal Area (SDA) at the Idaho National Engineering and Environmental Laboratory (INEEL) has recently been conducted. The study took advantage of pertinent information relating to describing aqueous-and vapor-phase movement of contaminants in the primarily fractured basalt subsurface. The study included spatially and temporally variable infiltration, barometric pressure changes, positive down-hole air pressure during well drilling, vapor-vacuum extraction, and regional hydraulic gradients. Use of the TETRAD simulation code allowed all the pertinent information to be included into a single comprehensive model of the SDA subsurface. An overview of the model implementation and comparisons of calibrated model results to the observed vadose zone water distribution, volatile organic vapor concentrations, and aqueous concentrations of volatile organics and nitrate are presented. Additionally, comparisons between simulated and observed concentrations for other contaminants which were not used for model calibration are made.As part of this modeling exercise, inadequacies in the available data relating to characterization of nonsorbing aqueous-phase transport have been identified. Even with the identified data inadequacies, the comparisons between simulated and observed contaminants along with the calibration results give confidence that the model is a conservative representation of flow and transport in the subsurface at the SDA. The results from this modeling study are being used to guide additional data collection activities at the SDA for purposes of increasing confidence in the appropriateness of model predictions.

Transport Models for Radioactive Carbon Dioxide at RWMC

“…In Table 6, effective diffusion coefficients are calculated for anticipated low and high moisture content conditions in SDA sediments using Equation 9 for two sets of parameters for m and n. The Millington parameters are those proposed by Millington (1959), and the parameters used in risk assessment calculations at the SDA are from Magnuson and Sondrup (1998). Based on the laboratory diffusion experiments in this investigation, alternative parameters are proposed where m and n differ by a value of 2.6.…”

Analogues to features and processes of a high-level radioactive waste repository proposed for Yucca Mountain, Nevada

“…Dissimilarities in fracture patterns, local recharge, the amount of organic matter, and fracture-filling materials that influence the type of fracture flow exist between the hydrogeologic settings at the SDA and Yucca Mountain (Barraclough and others, 1976;Martian, 1995;Magnuson and Sondrup, 1998;Knutson and others, 1990). These dissimilarities need to be taken into account when radionuclide migration in the unsaturated zone at the SDA is used as an analogue for how radionuclides could migrate in the unsaturated zone at Yucca Mountain.…”

“…The numerical model involved determining parameters controlling unsaturated and saturated flow and transport in fractured basalt by using a dual-permeability modeling approach that has been extensively used to simulate flow and transport at Yucca Mountain. Both regional-and local-scale LPIT models were developed, based upon the work of Magnuson (1995) and Magnuson and Sondrup (1998). The sequence of model development was first to use a local-scale LPIT model ( fig.…”

“…Radioactive decay was not included in the simulations. An aqueous diffusion coefficient of 10 -5 cm 2 /s with a tortuosity of 0.294 taken from Magnuson and Sondrup (1998) for the surficial sediment layer was used in the simulations.…”

Summary of Conceptual Models and Data Needs to Support the INL Remote-Handled Low-Level Waste Disposal Facility Performance Assessment and Composite Analysis

“…In these cases, sorption coefficients were calculated based on an estimated fracture surface area. Magnuson and Sondrup (1998) list sorption coefficients for sediments and fractures, and the fracture sorption coefficients are approximately four to six orders of magnitude smaller. Based on this, sorption on basalts will not be considered.…”