Engineering topics

Freshley, Mark D.

Publications and source records attributed to Freshley, Mark D..

5 records

DOE OSTI2020

Understanding Contaminant Migration within a Dynamic River Corridor through Field Experiments and Reactive Transport Modeling

The behavior of a persistent uranium plume within an extended river corridor at the DOE Hanford site is dominantly controlled by river stage fluctuations in the adjacent Columbia River. The plume behavior is further complicated by substantial heterogeneity in physical and geochemical properties of the host aquifer sediments. Multi-scale field and laboratory experiments and reactive transport modeling were integrated to understand the complex plume behavior influenced by highly variable hydrologic and geochemical conditions in time and space. In this paper, we (1) describe multiple data sets from field-scale uranium adsorption and desorption experiments performed at our experimental well-field, (2) develop a reactive transport model that incorporates hydrologic and geochemical heterogeneities characterized from multi-scale and multi-type datasets and a surface complexation reaction network based on laboratory studies, and (3) compare the modeling and observation results to provide insights on how to refine the conceptual model and reduce prediction uncertainties. The experimental results revealed significant spatial variability in uranium adsorption/desorption behavior, while modeling demonstrated that ambient hydrologic and geochemical conditions and heterogeneities in sediment physical and chemical properties both contributed to complex plume behavior and its persistence. This research underscores the great challenges in adequately characterizing this type of site to model the reactive transport processes over scales of 10 m or more. Our analysis provides important insights into the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by dynamic surface water and groundwater interactions.

Chen, Xingyuan↗

DOE OSTI2017

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 paper, 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.

conceptual models↗

DOE OSTI2016

Performance of a Surface Barrier for Waste Isolation and Flux Reduction at the Hanford Site - 16099

Based on the knowledge gained from a decade of laboratory, field, and numerical studies, the Prototype Hanford Barrier (PHB) was designed and constructed between late 1993 and late 1994 over the 216-B-57 Crib in the 200-BP-1 Operable Unit at the Hanford Site. The PHB has been monitored since 1994 to evaluate the physical, hydrologic, and ecological performance. Two stress tests were carried out in the past: 1) an enhanced (about 3 times the multi-year average of 160 mm/year) precipitation test from water year (WY) 1995 to WY1997, which included simulated 1000-year return, 24-hour rainstorms in March of each year, and 2) a controlled fire test in 2008. The purpose of this article is to present the main findings of the PHB demonstration since 1994. From 1994 to 2013-during which time the barrier experienced 3 years of enhanced precipitation, three 1000-year return, 24-hour simulated rainstorms, and a controlled fire-the PHB limited drainage to well below the 0.5 mm yr{sup -1} design criterion and had minimal erosion. Although the test period represents only 2% of the design life, the observations suggest the PHB is robust enough to control drainage and isolate subsurface contaminants. Future barrier performance will depend on barrier stability and hydrology. Given the 19-year record of successful performance and considering all processes and mechanisms that could degrade barrier stability and hydrology in the future, the results suggest that the PHB is very likely to perform for at least the remainder of its 1000-year design life. This conclusion is based on two assumptions: 1) the exposed sub-grade receives protection against erosion and 2) institutional controls prevent inadvertent human activity on the barrier. The findings at the PHB are useful for the design and monitoring of future surface barriers at Hanford and elsewhere. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2016

Performance of a Surface Barrier for Waste Isolation and Flux Reduction at the Hanford Site

Based on the knowledge gained from a decade of laboratory, field, and numerical studies, the Prototype Hanford Barrier (PHB) was designed and constructed between late 1993 and late 1994 over the 216-B-57 Crib in the 200-BP-1 Operable Unit at the Hanford Site. The PHB has been monitored since 1994 to evaluate the physical, hydrologic, and ecological performance. Two stress tests were carried out in the past: (1) an enhanced (about 3 times the multi-year average of 160 mm/year) precipitation test from water year (WY) 1995 to WY1997, which included a man-made 1000-year return 24-hour rainstorms in March each year, and (2) a controlled fire test in 2008. The purpose of this article is to present the main findings of the PHB demonstration since 1994. From 1994 to present, the PHB has limited drainage of less than 0.2 mm yr-1, which is below the 0.5 mm yr-1 design goal, and minimized erosion. The observations suggest the PHB is robust enough to endure the hydrological stress of three times average precipitation and 1000-year return 24-hour rainstorms. After the controlled fire, far less vegetation grows and grasses are the dominant vegetation (compared to shrubs on the unburned section). Even so, the grasses can remove nearly all the stored water in the burned section, although during a longer period of time than in the unburned section. The findings at the PHB are useful for the design and monitoring of future surface barriers at Hanford and elsewhere.

ET Cover↗

DOE OSTI2014

A high-performance workflow system for subsurface simulation

Subsurface modeling applications typically neglect uncertainty in the conceptual models, past or future scenarios, and attribute most or all uncertainty to errors in model parameters. In this contribution, uncertainty in technetium-99 transport in a heterogeneous, deep vadose zone is explored with respect to the conceptual model using a next generation user environment called Akuna. Akuna provides a range of tools to manage environmental modeling projects, from managing simulation data to visualizing results from high-performance computational simulators. Core toolsets accessible through the user interface include model setup, grid generation, parameter estimation, and uncertainty quantification. The BC Cribs site at Hanford in southeastern Washington State is used to demonstrate Akuna capabilities. At the BC Cribs site, conceptualization of the system is highly uncertain because only sparse information is available for the geologic conceptual model, the physical and chemical properties of the sediments, and the history of waste disposal operations. Using the Akuna toolset to perform an analysis of conservative solute transport, significant prediction uncertainty in simulated concentrations is demonstrated by conceptual model variation. This demonstrates that conceptual model uncertainty is an important consideration in sparse data environments such as BC Cribs. It is also demonstrated that Akuna and the underlying toolset provides an integrated modeling environment that streamlines model setup, parameter optimization, and uncertainty analyses for high-performance computing applications.

Akuna↗