RADIONUCLIDE TRANSPORT IN ARID AND SEMI-ARID ZONES: INTEGRATED SYSTEMS AND PROCESS MODELING TO SUPPORT ENVIRONMENTAL DECISION MAKING

Coupling of hydrogeologic, ecologic, and atmospheric models across multiple spatial and temporal scales is one of the most significant challenges of environmental modeling. Systems-level models, incorporating multiple coupled process-level models and abstractions, are useful for investigating how contaminant transport mechanisms function together and/or influence each other. Here, we integrate systems and process modeling in support of risk assessment efforts at Los Alamos National Laboratory (LANL).

LANL has disposed of radioactive material at Material Disposal Area (MDA) G since the late 1950s, including disposition of low-level waste and transuranic radionuclides. Decisions on whether waste remains at MDA G or is removed to another facility are dependent on analysis of hazards to human health and the environment. To support decision-making, a probabilistic assessment of radiological risk at MDA G is required with a thorough conceptualization of the site, including regulatory context (e.g., performance criteria), engineered systems (e.g., cover design), environmental systems (e.g., factors influencing water and contaminant flux in the thick vadose zone below MDA G), and human exposure scenarios.

Using the GoldSim platform, our model integrates multiple process models (e.g., flow and transport in the vadose and saturated zones simulated using the Finite Element Heat and Mass transfer code, erosion using Landlab components supported by traditional erosion process models such as Hillslope Erosion Model) in a systems-level context. We hypothesize that the processes presenting the largest contribution to human health and environmental risk will be: (i) biotic transport, including chemical-specific uptake by plants and bulk movement of contaminated soil by burrowing animals, and (ii) exposure of waste by erosion. Other migration pathways, including transport through the vadose zone to drinking water aquifers, will present negligible risk for this site. In addition to elucidating the relative contributions of various environmental processes to risk at MDA G, the approach has application to diverse disposal sites within variable regulatory and environmental contexts.