Predicting Contaminant Mobility at the Plume Scale

Synopsis

This challenge calls upon a ‘reactive facies’ organizing principle to integrate laboratory-obtained information about rates and mechanisms with field-based hydrogeological characterization as needed to make reliable and computationally tractable predictions of plume evolution. The challenge includes a formal evaluation of the benefit of increasing complexity on successful predictions of contaminant mobility over stewardship timeframes.

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Predicting Contaminant Mobility Challenge Overview - click for complete image

Research Challenge Contacts

Nic Spycher, NSpycher@lbl.gov, (510) 495-2388

Background

Predictions of plume evolution, migration, and remediation efficacy at the plume scale often fail because of the great simplifications that are typically made in the representation of heterogeneity and the coupled hydrological and biogeochemical processes taking place within the subsurface. Although DOE is steward of many subsurface contaminant plumes and clean-up, closure, and stewardship decisions are made based on plume characteristics, there are few scientific efforts that focus on predicting contaminant mobility at the plume scale as is needed to guide environmental stewardship. Some examples of flow and transport problems relevant to the plume scale include:

The characterization and prediction of plume distribution, key biogeochemical processes, hydrogeological heterogeneity, and key mechanisms of plume evolution over time and as a function of groundwater composition (e.g., pH, Eh, etc.) with sufficient fidelity over large spatial regions. This obstacle is particularly difficult to tackle at plume edges, where large compositional gradients often occur.
Understanding scale-dependent processes, such as the relationship between key biogeochemical processes (which typically operate at pore to grain scale) and plume migration (which is mostly governed by gradients over large scales).
Developing mechanistic reactive transport capabilities that include kinetic models for biodegradation, mineral precipitation and dissolution, and sorption behavior, as needed to predict plume behavior over longer time frames and with more accuracy than is capable with current models, which are mostly empirical.
Quantifying the characterization and modeling complexity that is sufficient but necessary to predict system behavior over long time frames.

Recognizing the wide range of scale-dependent processes at play, the complexities inherent in biogeochemical reaction networks, and the potential impact of heterogeneity and biogeochemical-hydrological feedbacks on flow and transport, definition of the initial model of the Systems Framework must be carefully considered.

We consider in this challenge two constructs that will enable us to develop a predictive understanding of contaminant mobility at the plume scale: geochemical gradients and reactive facies. It is generally recognized that mobile biogeochemical (redox, pH) gradients can develop at the leading and trailing edges of plumes as a result of reactive subsurface transport processes, and that these gradients can lead to transformations that can occur preferentially as a function of the initial hydrogeochemical heterogeneity of the system (e.g., Denham, 2007). It is also recognized that the evolution of these gradients and their propagation rate can dictate the mobility of many contaminants of interest to DOE. However, there is currently an insufficient scientific basis to enable predictions of reactive transport processes with enough accuracy to defend remediation strategies in the presence of strong gradients and over long time-frames (e.g., Steefel et al., 2005). Recognizing this difficulty, we aim this Challenge at predicting contaminant mobility at using a “reactive facies” framework to bridge hydrogeochemical measurements at multiple scales. The reactive facies construct takes advantage of the (non-random) spatial distribution of geological units and the linkage between hydrophysical and geochemical properties that often exists in nature, with the goal of providing sufficient information for reactive transport models in a tractable manner.

This research challenge will be performed at the Savannah River F-Area, where we will investigate U and Sr mobility. Contaminant plumes in groundwater beneath the F-Area of the Savannah River Site were created by the disposal and leakage of low-level radioactive solutions into seepage basins. The plumes extend from the basins approximately 600 meters downgradient to a stream. The plume contains a large number of contaminants; from a risk perspective, the most important contaminants include Sr-90, uranium isotopes, I-129, Tc-99, tritium, and nitrate. We are investigating U and Sr primarily because these contaminants are more strongly sorbed than I-129, Tc-99, or tritium, and therefore will pose longer-term exposure risks to the environment. Another reason for selecting U and Sr for this study is that these contaminants are present in the main plume beneath the F-Area at concentrations exceeding regulatory Maximum Concentration Levels (MCL) and thus quite relevant for evaluating the attenuation capacity of the system.

Hypotheses

The challenge explores the impact of a migrating pH gradient and the power of the novel "reactive facies" concept as an organizing principle to integrate laboratory and field information about properties and mechanisms in order to make reliable and computationally tractable predictions at the plume scale. The challenge includes a formal evaluation of the benefit of increasing complexity on successful predictions of contaminant mobility over stewardship timeframes. Research in this challenge will be performed through testing the following three hypotheses:

Hypothesis 1 - A limited number of sediment groups can be defined that individually display unique U and Sr sorption/precipitation processes and that adequately represent the spectrum of geochemical behavior in the subsurface when considered as an ensemble.
Hypothesis 2 - Reactive transport properties and mechanisms can be represented using a “reactive facies” concept, which capitalizes on the coherent spatial distribution associated with depositional processes and the linkage between hydrological and geochemical properties that often exist in nature. Understanding the geometry of these facies and associated property distributions is expected to enable characterization of properties and mechanisms critical for prediction with sufficient complexity yet over field-relevant scales.
Hypothesis 3 - An optimal level of model complexity for prediction of contaminant mobility at the plume scale can be identified, beyond which model accuracy does not increase significantly.

The research will be carried out through integrated laboratory geochemical, synchrotron, isotopic, geophysical, and reactive transport modeling tasks. In collaboration with EM-supported SRNL scientists (led by Miles Denham), the resulting reactive transport model will be used to assess the natural attenuation capacity of the acidic, Uranium and Strontium plume at the F-Area of the Savannah River Site.

Expected Outcomes

Research in this challenge is expected to lead to several outcomes, such as documentation of:

The ability to characterize sorption/precipitation behavior as a function of sediment classes;
The ability to exploit physical-chemical linkages and geophysical methods in estimating the spatial distribution of reactive facies;
The utility of interpreting isotopic signatures in terms of field scale processes.
The utility of the reactive facies concept to provide a sufficiently complex model parameterization in a tractable manner at the plume scale;
The predictive capability of reactive transport models to capture U and Sr mobility at the local scale through a comparison of predicted versus push-pull field measurements;
The level of model and heterogeneity complexity needed to accurately predict contaminant mobility at F-Area.

LBNL Research Team & Collaborators

Research in this challenge will be carried out by the following LBNL multidisciplinary research team with expertise in experimental geochemistry, reactive transport modeling, isotope geochemistry, synchrotron science, and subsurface characterization using disparate, multi-scale datasets.

This LBNL SFA Challenge research will be performed in collaboration with a parallel and significant effort being led by Miles Denham (SRNL) and funded by the EM-20 Soil and Groundwater Program, entitled “Attenuation-based Remedies for Metal and Radionuclide Contaminated Groundwater.” The LBNL scientific team will closely collaborate with and leverage on this EM-funded effort, which involves scientists from SRNL as well as other labs and universities, site personnel, and regulators, who are striving to identify steps that can be taken to advance the use of current scientific understanding for guiding MNA.