Mark D. Freshley

Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater–river interaction zone

We examine subsurface uranium (U) plumes at two U.S. Department of Energy sites that are located near large river systems and are influenced by groundwater-river hydrologic interaction. Following surface excavation of contaminated materials, both sites were projected to naturally flush remnant uranium contamination to levels below regulatory limits (e.g., 30 μg/L or 0.126 μmol/L; U.S. EPA drinking water standard), with 10 years projected for the Hanford 300 Area (Columbia River) and 12 years for the Rifle site (Colorado River). The rate of observed uranium decrease was much lower than expected at both sites. While uncertainty remains, a comparison of current understanding suggests that the two sites have common, but also different mechanisms controlling plume persistence. At the Hanford 300 A, the persistent source is adsorbed U(VI) in the vadose zone that is released to the aquifer during spring water table excursions. The release of U(VI) from the vadose zone and its transport within the oxic, coarse-textured aquifer sediments is dominated by kinetically-limited surface complexation. Modeling implies that annual plume discharge volumes to the Columbia River are small (<one pore volume). At the Rifle site, slow oxidation of naturally reduced, contaminant U(IV) in the saturated zone and a continuous influx of U(VI) from natural, up-gradient sources influence plume persistence. Rate-limited mass transfer and surface complexation also control U(VI) migration velocity in the sub-oxic Rifle groundwater. Flux of U(VI) from the vadose zone at the Rifle site may be locally important, but it is not the dominant process that sustains the plume. A wide range in microbiologic functional diversity exists at both sites. Strains of Geobacter and other metal reducing bacteria are present at low natural abundance that are capable of enzymatic U(VI) reduction in localized zones of accumulated detrital organic carbon or after organic carbon amendment. Major differences between the sites include the geochemical nature of residual, contaminant U; the rates of current kinetic processes (both biotic and abiotic) influencing U(VI) solid-liquid distribution; the presence of detrital organic matter and the resulting spatial heterogeneity in microbially-driven redox properties; and the magnitude of groundwater hydrologic dynamics controlled by river-stage fluctuations, geologic structures, and aquifer hydraulic properties. The comparative analysis of these sites provides important guidance to the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by surface water interaction that are common world-wide.

Scientific Opportunities for Monitoring at Environmental Remediation Sites (SOMERS): Integrated Systems-Based Approaches to Monitoring

Through an inter-disciplinary effort, DOE is addressing a need to advance monitoring approaches from sole reliance on cost- and labor-intensive point-source monitoring to integrated systems-based approaches such as flux-based approaches and the use of early indicator parameters. Key objectives include identifying current scientific, technical and implementation opportunities and challenges, prioritizing science and technology strategies to meet current needs within the DOE complex for the most challenging environments, and developing an integrated and risk-informed monitoring framework.

A high-performance workflow system for subsurface simulation

The U.S. Department of Energy (DOE) recently invested in developing a numerical modeling toolset called ASCEM (Advanced Simulation Capability for Environmental Management) to support modeling analyses at legacy waste sites. This investment includes the development of an open-source user environment called Akuna that manages subsurface simulation workflows. Core toolsets accessible through the Akuna user interface include model setup, grid generation, sensitivity analysis, model calibration, and uncertainty quantification. Additional toolsets are used to manage simulation data and visualize results. This new workflow technology is demonstrated by streamlining model setup, calibration, and uncertainty analysis using high performance computation for the BC Cribs Site, a legacy waste area at the Hanford Site in Washington State. For technetium-99 transport, the uncertainty assessment for potential remedial actions (e.g., surface infiltration covers) demonstrates that using multiple realizations of the geologic conceptual model results in greater variation in concentration predictions than when a single model is used. Akuna provides integrated toolset needed for subsurface modeling workflow.Akuna streamlines process of executing multiple simulations in HPC environment.Akuna provides visualization tools for spatial and temporal data.Example application demonstrates risk with remediation impacting infiltration rates.

ADVANCED SIMULATION CAPABILITY FOR ENVIRONMENTAL MANAGEMENT (ASCEM): AN OVERVIEW OF INITIAL RESULTS

ADVANCED SIMULATION CAPABILITY FOR ENVIRONMENTAL MANAGEMENT (ASCEM): AN OVERVIEW OF INITIAL RESULTS Mark Williamson,* Juan Meza,† David Moulton,‡ Ian Gorton,§ Mark Freshley,§ Paul Dixon,‡ Roger Seitz,¶ Carl Steefel,† Stefan Finsterle,† Susan Hubbard,† Ming Zhu,* Kurt Gerdes,* Russ Patterson,# and Yvette T. Collazo* *U.S. Department of Energy, Office of Environmental Management, Washington, DC, USA †Lawrence Berkeley National Laboratory, Berkeley, CA, USA ‡Los Alamos National Laboratory, Los Alamos, NM, USA §Pacific Northwest National Laboratory, Richland, WA, USA ¶Savannah River National Laboratory, Aiken, SC, USA #U.S. Department of Energy, Carlsbad, NM, USA The US Department Energy (DOE) Office of Environmental Management (EM) determined that uni- form application of advanced modeling in the subsurface could potentially help reduce the cost and risk associated with its environmental cleanup mission. In response to this determination, the EM Office of Technology Innovation and Development (OTID), Groundwater and Soil Remediation (GWS Simulation; Model; Groundwater; ASCEM BACKGROUND: INTRODUCTION TO EM NEEDS Fifty years of nuclear weapons production and government-sponsored nuclear energy research in the US during the Cold War generated large amounts of radioactive wastes, spent fuel, excess plutonium and uranium, thousands of contaminated facilities, and contaminated groundwater and soil. During most of that half century, the nation did not have the environmental regulatory structure or nu- clear waste remediation technologies that exist to- day. The result was a legacy of nuclear waste that was stored and disposed of in ways now considered unacceptable (11). At the end of US Government Fiscal Year 2010 (FY10), EM had 18 funded sites. Estimates report these sites to contain 40 million m 3 of contami- nated soil and 6.4 trillion L of contaminated groundwater (7). Current groundwater and soil re- mediation challenges that will continue to be ad- dressed in the next decade include cost-effective characterization, remediation, and monitoring of contaminants in the vadose zone and groundwater.

Elements of complexity in subsurface modeling, exemplified with three case studies

There are complexity elements to consider when applying subsurface flow and transport models to support environmental analyses. Modelers balance the benefits and costs of modeling along the spectrum of complexity, taking into account the attributes of more simple models (e.g., lower cost, faster execution, easier to explain, less mechanistic) and the attributes of more complex models (higher cost, slower execution, harder to explain, more mechanistic and technically defensible). In this report, modeling complexity is examined with respect to considering this balance. The discussion of modeling complexity is organized into three primary elements: (1) modeling approach, (2) description of process, and (3) description of heterogeneity. Three examples are used to examine these complexity elements. Two of the examples use simulations generated from a complex model to develop simpler models for efficient use in model applications. The first example is designed to support performance evaluation of soil-vapor-extraction remediation in terms of groundwater protection. The second example investigates the importance of simulating different categories of geochemical reactions for carbon sequestration and selecting appropriate simplifications for use in evaluating sequestration scenarios. In the third example, the modeling history for a uranium-contaminated site demonstrates that conservative parameter estimates were inadequate surrogates for complex, critical processes and there is discussion on the selection of more appropriate model complexity for this application. All three examples highlight how complexity considerations are essential to create scientifically defensible models that achieve a balance between model simplification and complexity.RésuméIl y a des éléments de complexité à prendre en considération lorsqu’on applique des modèles d’écoulements et de transport souterrains dans le cadre d’analyses environnementales. Les modélisateurs recherchent un équilibre entre les bénéfices et les coûts de la modélisation selon le spectre de la complexité, prenant en considération les caractéristiques de modèles plus simples (c’est-à-dire moins coûteux, exécution plus rapide, plus faciles à expliquer, moins mécanistes) et des caractéristiques des modèles plus complexes (c’est-à-dire plus coûteux, exécution plus lente, plus difficiles à expliquer, plus mécanistes et techniquement fiables). Dans cet article, la complexité en modélisation est étudiée afin d’atteindre cet équilibre. La discussion de la complexité en modélisation s’articule autour de trois principaux éléments : (1) approche de modélisation, (2) description des processus, et (3) description des hétérogénéités. Trois exemples sont utilisés pour étudier ces éléments de complexité. Deux de ces exemples utilisent des simulations résultant d’un modèle complexe afin de développer des modèles plus simples pour une utilisation efficace dans l’application du modèle. Le premier exemple est conçu pour évaluer la performance d’une remédiation par extraction de vapeur du sol pour la protection des eaux souterraines. Le deuxième exemple examine l’importance de différents types de simulation de réactions géochimiques pour le stockage du CO2, en sélectionnant des simplifications appropriées pour évaluer des scénarios de stockage. Dans le troisième exemple, l’histoire de la modélisation pour un site pollué à l’uranium démontre que les estimations des paramètres conservatifs étaient des substitutifs inadéquats pour des processus complexes et critiques et la sélection d’une complexité de modèle plus adaptée est discutée pour cette application. Ces trois exemples mettent en évidence comment la prise en considération de la complexité est essentielle pour réaliser des modèles scientifiquement fiables qui atteignent un équilibre entre simplification et complexité du modèle.ResumenHay elementos de complejidad a considerar cuando se aplican modelos de flujo y transporte en el subsuelo para apoyar los análisis ambientales. Los modelistas equilibran los beneficios y costos del modelado a lo largo del espectro de complejidad, teniendo en cuenta los atributos de los modelos más simples (por ejemplo, menor costo, ejecución más rápida, más fácil de explicar, menos mecanicista) y los atributos de modelos más complejos (costo, ejecución más lenta, más difícil de explicar, más mecanicista y técnicamente defendible). En este trabajo, se examina la complejidad del modelado con respecto a considerar este balance. La discusión de la complejidad de modelado se organiza en tres elementos principales: (1) enfoque del modelado, (2) descripción del proceso y (3) descripción de la heterogeneidad. Se utilizan tres ejemplos para examinar estos elementos de complejidad. Dos de los ejemplos utilizan simulaciones generadas a partir de un modelo complejo para desarrollar modelos más simples para un uso eficiente en aplicaciones de modelos. El primer ejemplo está diseñado para apoyar la evaluación del rendimiento de la remediación para la extracción de vapores del suelo en términos de protección del agua subterránea. El segundo ejemplo investiga la importancia de simular diferentes categorías de reacciones geoquímicas para el secuestro de carbono y seleccionar las simplificaciones apropiadas para su uso en la evaluación de escenarios de secuestro. En el tercer ejemplo, el historial de modelado de un sitio contaminado con uranio demuestra que las estimaciones de parámetros conservadores eran substitutos inadecuados para procesos críticos complejos y hay una discusión sobre la selección de la complejidad de modelo más apropiada para esta aplicación. Los tres ejemplos ponen de relieve cómo las consideraciones de complejidad son esenciales para crear modelos científicamente defendibles que logren un balance entre la simplificación y la complejidad del modelo.摘要应用地表以下水流和运移模型支持环境分析时,有一些复杂性元素需要考虑。建模者在复杂性范围内平衡着建模的效益和成本,要考虑较简单模型的属性(较低成本、较快的实施、易于说明及不那么机械)及较复杂模型的属性(较高的成本、较慢的实施、很难说明及技术上可防御)。在本文中,考虑到这个平衡问题,检查了建模的复杂性。建模复杂性的论述被归纳为三个主要元素:(1)建模方法,(2)过程描述,(3)异质性描述。利用三个例子检查了复杂性元素。为了在模型应用中有效利用模型,从一个复杂模型开发出较简单的模型,其中两个例子使用了所产生的模拟结果。第一个例子被设计为支持地下水保护方面土壤气体萃取修复性能评价。第二个例子调查了针对碳隔离模拟不同类别地球化学反应及选择在隔离方案中使用适当简单化的重要性。在第三个例子中,一个铀污染场地的建模历史说明,保守参数估算不能适当地值代替复杂的、关键过程,应用这个模型在更恰当模型复杂性的选择上有争论。所有三个例子强调了在科学上创建防御性模型取得模型简单化和复杂性之间平衡上,复杂性考量是多么的重要。ResumoExistem elementos de complexidade a se considerar quando se aplica modelos de transporte e fluxo de subsuperficie para auxiliar análises ambientais. Os modeladores ponderam os benefícios e custos da modelagem ao longo do espectro da complexidade, levando em consideração os atributos de modelos mais simples (p.ex. menor custo, execução mais rápida, mais fácil de explicar, menos mecanicista) e os atributos de modelos mais complexos (maior custo, execução mais lenta, mais difícil de explicar, mais mecanicista e tecnicamente defensável). Nesse estudo, a complexidade da modelagem é examinada considerando esse balanço. A discussão da complexidade da modelagem está organizada em três elementos principais: (1) abordagem da modelagem, (2) descrição do processo, e (3) descrição da heterogeneidade. Três exemplos são utilizados para examinar esses elementos de complexidade. Dois dos exemplos utilizam simulações geradas através de um modelo complexo para desenvolver modelos mais simples, para o uso eficiente em aplicações de modelos. O primeiro exemplo é projetado para auxiliar a avaliação do desempenho da remediação por extração de vapor do solo em termos de proteção das águas subterrâneas. O segundo exemplo investiga a importância de simular diferentes categorias de reações geoquímicas para sequestro de carbono e selecionar as simplificações apropriadas para uso na avaliação de cenários de sequestro. No terceiro exemplo, o histórico de modelagem de uma área contaminada com urânio demonstra que as estimativas de parâmetros conservadores foram substitutos inadequados para processos críticos e complexos e há uma discussão sobre a seleção de um modelo com complexidade mais apropriada para essa aplicação. Todos os três exemplos destacam como considerações de complexidade são essenciais para criar modelos cientificamente defensáveis que alcancem um equilíbrio entre simplificação e complexidade do modelo.

Advanced Simulation Capability for Environmental Management (ASCEM): Early Site Demonstration

The U.S. Department of Energy’s Office of Environmental Management (EM), Technology Innovation and Development (EM-32), is supporting development of the Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is a state-of-the-art scientific tool and approach for understanding and predicting contaminant fate and transport in natural and engineered systems. This modular and open-source, high-performance computing tool will facilitate integrated approaches to modeling and site characterization that enable robust and standardized assessments of performance and risk for EM cleanup and closure activities. As part of the initial development process, a series of demonstrations was defined to test ASCEM components and provide feedback to developers, engage end users in applications, and lead to an outcome that would benefit the sites. The demonstration was implemented for a sub-region of the Savannah River Site General Separations Area that includes the F-Area Seepage Basins. The physical domain included the unsaturated and saturated zones in the vicinity of the seepage basins and the Fourmile Branch. An unstructured mesh was used to fit the grid to the hydrostratigraphy and topography of the site. The calculations modeled variably saturated flow, and the resulting flow field was used in simulations of the advection of non-reactive species and the reactivetransport of uranium. As part of the demonstrations, data management, visualization, and uncertainty quantification tools were developed to analyze simulation results and existing site data. These new tools can be used to provide summary statistics, including information on which simulation parameters were most important in predicting uncertainty and visualizing the relationships between model input and output.

Identification and Implementation of End-User Needs During Development of a State-of-the-Art Modeling Toolset

The U.S. Department of Energy (US DOE) Office of Environmental Management, Technology Innovation and Development is supporting a multi-National Laboratory effort to develop the Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is an emerging state-of-the-art scientific approach and software infrastructure for understanding and predicting contaminant fate and transport in natural and engineered systems. These modular and open-source high performance computing tools and user interfaces will facilitate integrated approaches that enable standardized assessments of performance and risk for EM cleanup and closure decisions. The ASCEM team recognized that engaging end-users in the ASCEM development process would lead to enhanced development and implementation of the ASCEM toolsets in the user community. End-user involvement in ASCEM covers a broad spectrum of perspectives, including: performance assessment (PA) and risk assessment practitioners, research scientists, decision-makers, oversight personnel, and regulators engaged in the US DOE cleanup mission. End-users are primarily engaged in ASCEM via the ASCEM User Steering Committee (USC) and the ‘user needs interface’ task. Future plans also include user involvement in demonstrations of the ASCEM tools. This paper will describe the details of how end users have been engaged in the ASCEM program and will demonstrate how this involvement has strengthened both the tool development and community confidence. ASCEM tools requested by end-users specifically target modeling challenges associated with US DOE cleanup activities. The demonstration activities involve application of ASCEM tools and capabilities to representative problems at DOE sites. Selected results from the ASCEM Phase 1 demonstrations are discussed to illustrate how capabilities requested by end-users were implemented in prototype versions of the ASCEM tool.Copyright

Composite Analysis for Low-Level Waste Disposal in the 200 Area Plateau of the Hanford Site, Southeast Washington

A composite analysis of low-level radioactive waste disposal and other radioactive sources was recently completed for the Hanford Site in Southeast Washington State. Impacts from source release and environmental transportwere estimated for a 1 000-year period following Site closure in a multi-step process involving 1) estimation of radiological inventories and releases, 2) assessment of contaminant migration through the vadose zone, groundwater, and atmospheric pathways, 3) and estimation of doses. The analysis showed that most of the radionuclide inventory in past-practice liquid discharge sites and pre-1988 solid waste burial grounds on the 200 Area Plateau will be released in the first several hundred years following Hanford Site closure, well before projected releases from active and planned disposals of solid waste. The maximum predicted agricultural dose was less than 6 mrem/y in 2050 and declined thereafter. The maximum doses for the residential, industrial, and recreational scenarios, were 2.2, 0.7, and 0.04 mrem/y, respectively, and also declined after 2050.

Advanced Simulation Capability for Environmental Management: Current Status and Future Applications

The U.S. Department of Energy (USDOE) Office of Environmental Management (EM), Office of Soil and Groundwater (EM-12), is supporting development of the Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is a state-of-the-art scientific tool and approach that is currently aimed at understanding and predicting contaminant fate and transport in natural and engineered systems. ASCEM is a modular and open source high-performance computing tool. It will be used to facilitate integrated approaches to modeling and site characterization, and provide robust and standardized assessments of performance and risk for EM cleanup and closure activities.The ASCEM project continues to make significant progress in development of capabilities, with current emphasis on integration of capabilities in FY12. Capability development is occurring for both the Platform and Integrated Toolsets and High-Performance Computing (HPC) multiprocess simulator. The Platform capabilities provide the user interface and tools for end-to-end model development, starting with definition of the conceptual model, management of data for model input, model calibration and uncertainty analysis, and processing of model output, including visualization. The HPC capabilities target increased functionality of process model representations, toolsets for interaction with Platform, and verification and model confidence testing. The integration of the Platform and HPC capabilities were tested and evaluated for EM applications in a set of demonstrations as part of Site Applications Thrust Area activities in 2012.The current maturity of the ASCEM computational and analysis capabilities has afforded the opportunity for collaborative efforts to develop decision analysis tools to support and optimize radioactive waste disposal. Recent advances in computerized decision analysis frameworks provide the perfect opportunity to bring this capability into ASCEM. This will allow radioactive waste disposal to be evaluated based on decision needs, such as disposal, closure, and maintenance. Decision models will be used in ASCEM to identify information/data needs, and model refinements that might be necessary to effectively reduce uncertainty in waste disposal decisions. Decision analysis models start with tools for framing the problem, and continue with modeling both the science side of the problem (for example, inventories, source terms, fate and transport, receptors, risk, etc.), and the cost side of the problem, which could include costs of implementation of any action that is chosen (e.g., for disposal or closure), and the values associated with those actions. The cost side of the decision problem covers economic, environmental and societal costs, which correspond to the three pillars of sustainability (economic, social, and environmental). These tools will facilitate stakeholder driven decision analysis to support optimal sustainable solutions in ASCEM.Copyright

Implementation Plan for the Deep Vadose Zone-Applied Field Research Center

The Long-Range Deep Vadose Zone Program Plan was published in October 2010. It summarized the U.S. Department of Energy’s (DOE’s) state-of-knowledge about the contaminant remediation challenges facing the deep vadose zone (DVZ) beneath the Central Plateau of the Hanford Site and their approach to solving those challenges. Developing an implementation plan is the next step to address the knowledge and capabilities required to solve DVZ challenges when needed. This multi-year plan (FY-11 through FY-20) identifies the short to long-term research, management, and execution plans required to solve those problems facing the DVZ-Applied Field Research Center (DVZ-AFRC). The schedule supporting implementation overlies existing activities and milestones from Hanford’s DOE-Environmental Management (EM) end-user projects. Success relies upon multi-project teams focused on coordinated subsurface projects undertaken across the DOE Complex combined with facilitated, problem-focused, research investments implemented through the DVZ-AFRC.