[en] This report summarizes the Hanford Site-Wide Groundwater Model and its application to the Immobilized Low-Activity Waste (ILAW) Disposal Facility Performance Assessment (PA). The site-wide model and supporting local-scale models are used to evaluate impacts from the transport of contaminants at a hypothetical well 100 m downgradient of the disposal facilities and to evaluate regional flow conditions and transport from the ILAW disposal facilities to the Columbia River. These models were used to well-intercept factors (WIFs) or dilution factors from a given areal flux of a hypothetical contaminant released to the unconfined aquifer from the ILAW disposal facilities for two waste-disposal options: (1) a remote-handled trench concept and (2) a concrete-vault concept. These WIFs are being used in conjunction with calculations of released contaminant fluxes through the vadose zone to estimate potential impacts from radiological and hazardous chemical contaminants within the ILAW disposal facility at compliance points

[en] From the mid-1940s through the 1980s, large volumes of wastewater were discharged at the Hanford Site in southeastern Washington State, causing a large-scale rise (in excess of 20 m) in the water table. When wastewater discharges ceased in 1988, groundwater mounds began to dissipate. This caused a large number of wells to go dry and has made it difficult to monitor contaminant plume migration. To identify the wells that could potentially go dry, a first order uncertainty analysis was performed using a three-dimensional, finite element code (CFEST) coupled with UCODE, a nonlinear parameter estimation code. The analysis was conducted in four steps. First, key parameter values were identified by calibrating to historical hydraulic head data. Second, the model was tested for linearity, a strict requirement for representing output uncertainty. Third, results from the calibration period were used to verify model predictions by comparing monitoring wells? wet/dry status with field data. In the final step, predictions on the number and locations of dry wells were made through the year 2048. A non-physically based model that extrapolated trends at each individual well was also tested as a predictor of a well?s wet/dry status. Results demonstrated that when uncertainty in both parameter estimates and measurement error was considered, the CFEST-based model successfully predicted the majority of dry wells, outperforming the trend model. Predictions made through the year 2048 identified approximately 50% of the wells in the monitoring well network are likely to go dry, which can aid in decisions for their replacement

[en] The baseline three-dimensional transient inverse model for the estimation of site-wide scale flow parameters, including their uncertainties, using data on the transient behavior of the unconfined aquifer system over the entire historical period of Hanford operations, has been modified to account for the effects of basalt intercommunication between the Hanford unconfined aquifer and the underlying upper basalt confined aquifer. Both the baseline and alternative conceptual models (ACM-1) considered only the groundwater flow component and corresponding observational data in the 3-Dl transient inverse calibration efforts. Subsequent efforts will examine both groundwater flow and transport. Comparisons of goodness of fit measures and parameter estimation results for the ACM-1 transient inverse calibrated model with those from previous site-wide groundwater modeling efforts illustrate that the new 3-D transient inverse model approach will strengthen the technical defensibility of the final model(s) and provide the ability to incorporate uncertainty in predictions related to both conceptual model and parameter uncertainty

[en] This report describes a new initiative to strengthen the technical defensibility of predictions made with the Hanford site-wide groundwater flow and transport model. The focus is on characterizing major uncertainties in the current model. PNNL will develop and implement a calibration approach and methodology that can be used to evaluate alternative conceptual models of the Hanford aquifer system. The calibration process will involve a three-dimensional transient inverse calibration of each numerical model to historical observations of hydraulic and water quality impacts to the unconfined aquifer system from Hanford operations since the mid-1940s

[en] Tritium transport simulations were conducted to model the mechanisms associated with dilution, dispersion, and radioactive decay that attenuate the 618-11 Burial Ground tritium plume and limit the risk associated with exposure to the Columbia River and Energy Northwest water supply wells. A comparison of simulated and observed tritium concentrations at two downgradient monitoring wells indicated that the model was a reasonable representation of the tritium concentrations immediately downgradient of the site (699-13-3A) and near the leading edge of the plume (699-13-0A). This good match increased confidence in the conceptual model, its numeric implementation, and ultimately the validity of predictive simulations of tritium fate and transport. Three release scenarios were investigated to measure the impact of the tritium plume at primary receptor locations under different conditions. The three cases were (1) a pulse release of tritium from the burial ground that was the best fit between observed and simulated tritium concentrations; (2) a continuing, decaying source beneath the burial ground through 2015, the milestone for source removal under the River Corridor Closure Contract; and (3) a pulse release as in the best fit case but at twice the concentration. For the best fit case, the model predicts that the maximum tritium concentration will decline to below the drinking water standard by 2031 For the other two release scenarios, maximum tritium concentrations declined to below the drinking water standard by 2040 and 2037, respectively. Tritium from the 618-11 burial ground is not expected to migrate to the Columbia River or to the Energy Northwest water supply wells at concentrations that would pose a significant risk

[en] Tritium transport simulations were conducted to model the mechanisms associated with dilution, dispersion, and radioactive decay that attenuate the 618-11 tritium plume and limit the risk associated with exposure to the Columbia River and Energy Northwest water supply wells. A comparison of simulated and observed tritium concentrations at two downgradient monitoring wells indicated that the model was a reasonable representation of the tritium concentrations immediately downgradient of the site (699-13-3A) and near the leading edge of the plume (699-13-0A). This good match increased confidence in the conceptual model, its numeric implementation, and ultimately, the validity of predictive simulations of tritium fate and transport. Three release scenarios were investigated to measure the impact of the tritium plume at primary receptor locations under different conditions. The three cases were (1) a pulse release of tritium from the burial ground that was the best fit between observed and simulated tritium concentrations; (2) a continuing, decaying source beneath the burial ground through 2015, the milestone for source removal under the River Corridor Closure Contract; and (3) a pulse release as in the best fit case but at twice the concentration. For the best fit case, the model predicts that the maximum tritium concentration will decline to below the drinking water standard by 2031 For the other two release scenarios, maximum tritium concentrations declined to below the drinking water standard by 2040 and 2037, respectively. Tritium from the 618-11 burial ground is not expected to migrate to the Columbia River or to the Energy Northwest water supply wells at concentrations that would pose a significant risk

[en] Pacific Northwest National Laboratory (PNNL) embarked on a new initiative to strengthen the technical defensibility of the predictions being made with a site-wide groundwater flow and transport model at the U.S. Department of Energy Hanford Site in southeastern Washington State. In FY 2000, the focus of the initiative was on the characterization of major uncertainties in the current conceptual model that would affect model predictions. The long-term goals of the initiative are the development and implementation of an uncertainty estimation methodology in future assessments and analyses using the site-wide model. This report focuses on the development and implementation of an uncertainty analysis framework

[en] This report summarizes efforts to complete an addendum analysis to the first iteration of the Composite Analysis for Low-Level Waste Disposal in the 200 Area Plateau of the Hanford Site (Composite Analysis). This document describes the background and performance objectives of the Composite Analysis and this addendum analysis. The methods used, results, and conclusions for this Addendum analysis are summarized, and recommendations are made for work to be undertaken in anticipation of a second analysis

[en] Evapotranspiration (ET) surface barriers rely on natural processes of water recharge, storage, and release to reduce or eliminate water movement into the underlying contaminant sediments and waste zones. Optimizing the design of an ET surface barrier is often impractical with field experiments because such experiments are usually costly and time-consuming to conduct. Numerical modeling is a very cost-effective and efficient approach to evaluating and understanding the long-term performance of surface barriers of different designs and under different conditions of climate and vegetation. Modified RCRA Subtitle C engineered surface barriers are expected to be used over the waste sites, including areas that will contain landfill-closed single-shell tank farms at the Hanford Site. This modeling investigation assesses the performance of such a surface barrier and evaluates a range of sensitivity analyses that examine key factors affecting on the long-term performance of this type of surface barrier. The factors investigated included those related to weather (i.e., precipitation), vegetation (i.e., vegetation types/root depth) and features of a surficial silt loam layer (i.e., the thickness, porosity, and the saturated hydraulic conductivity of the silt loam) in the barrier that is used to store, evaporate, and transpire water from the barrier system. A base case modeling evaluation of the Modified RCRA Subtitle C surface barrier demonstrated the performance of this type of surface barrier over the 10-year period during which precipitation varied temporally but the mean of the precipitation was about the same as the long-term average. Modeling results from the base case exhibited a clear annual cycle of water content changes that were indicative of the natural processes of water recharge, storage, and release from the barrier system. After each summer, water storage in the surficial silt loam layer within the surface barrier decreased to residual water storage levels within the silt loam. Water content variations with depth were found to be consistent with the presence of capillary breaks between the surficial silt loam and underlying sand layer and between the sand layer and an underlying gravel layer found at depth in the barrier. The resulting 10-year-average annual ET predicted by the modeling was very close to the average precipitation, indicating all the precipitation was released to the atmosphere. The water stored in the surficial silt loam layer never reached its estimated storage capacity and hence water drainage was not predicted to occur from the bottom of the barrier during the 10-year period of model evaluation. Sensitivity analyses showed that variation in precipitation had the largest impact on simulated ET and drainage, while changes in the assumed vegetation types was the second largest factor. However, vegetation types ranked the first based on the impact on barrier water storage and precipitation the second. This modeling evaluation demonstrated that this type of surface barrier needs to have sufficient thickness and porosity of a surficial silt loam layer to store all recharge water without producing drainage. To predict if drainage will happen under a given precipitation condition, the term critical winter-season precipitation is defined as the sum of the winter-season ET and the effective storage capacity of the barrier. When the winter-season precipitation was more than the critical value, drainage tended to occur. The simulation of the performance of a surface barrier and sensitivity analysis of key factors evaluated in the modeling study will be useful as a first step to optimizing future design of engineered surface barriers for waste sites at the Hanford Site. (authors)

[en] DOE and PNNL are working to strengthen the technical defensibility of the groundwater flow and transport model at the Hanford Site and to incorporate uncertainty into the model. One aspect of the initiative is developing and using a three-dimensional transient inverse model to estimate the hydraulic conductivities, specific yields, and other parameters using data from Hanford since 1943. The focus of the alternative conceptual model (ACM-2) inverse modeling initiative documented in this report was to address limitations identified in the ACM-1 model, complete the facies-based approach for representing the hydraulic conductivity distribution in the Hanford and middle Ringold Formations, develop the approach and implementation methodology for generating multiple ACMs based on geostatistical data analysis, and develop an approach for inverse modeling of these stochastic ACMs. The primary modifications to ACM-2 transient inverse model include facies-based zonation of Units 1 (Hanford ) and 5 (middle Ringold); an improved approach for handling run-on recharge from upland areas based on watershed modeling results; an improved approach for representing artificial discharges from site operations; and minor changes to the geologic conceptual model. ACM-2 is the first attempt to fully incorporate the facies-based approach to represent the hydrogeologic structure. Further refinement and additional improvements to overall model fit will be realized during future inverse simulations of groundwater flow and transport. In addition, preliminary work was completed on an approach and implementation for generating an inverse modeling of stochastic ACMs. These techniques were applied to assess the uncertainty in the facies-based zonation of the Hanford formation and the geological structure of Ringold mud units. The geostatistical analysis used a preliminary interpretation of the facies-based zonation that was not consistent with that used in ACM-2. Although the overall objective of this task is to assess uncertainty based on the most current model (ACM-2), this preliminary work provided an effective basis for developing the approach and implementation methodology. A strategy was developed to facilitate inverse calibration analysis of the large number of stochastic ACMs generated. These stochastic ACMs are random selections from a range of possible model structures, all of which are consistent with available observations. However, a single inverse run requires many forward flow model runs, and full inverse analysis of the large number of combinations of stochastic alternative models is not now computationally feasible. Thus, a two-part approach was developed: (1) full inverse modeling of selected realizations combined with limited forward modeling and (2) implementation of the UCODE/CFEST inverse modeling framework to enhance computational capabilities