Pore and Flowpath Evolution Challenge

Synopsis

This challenge focuses on development of a predictive understanding of how reaction processes modify the pore scale chemical composition and connectivity of a porous medium; how those changes are manifested at the field scale; and the impact of the system alterations on sustainable remediation.

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Background
Hypotheses
Expected Outcomes
Research Team & Collaborators
Publications
Team Sharepoint

Pore and Flowpath Evolution Challenge Overview - click for complete image

Research Challenge Contacts

Susan Hubbard, SSHubbard@lbl.gov, (510) 486-5266

Background

Although in situ strategies are frequently considered for environmental remediation, the impact of feedbacks between induced biogeochemical transformations and hydrological characteristics on remediation efficacy is not well understood. In situ remediation approaches that strongly perturb subsurface systems (such as chemical oxidation, pH manipulation, redox manipulation or biostimulation) typically lead to biogeochemical end-products. These treatments may, for example, lead to: the dissolution and precipitation of minerals, evolution of gases, changes in soil water and oxygen levels, sorption, attachment/detachment, and biofilm generation. These biogeochemical processes occur at grain-fluid boundaries, within pore spaces, and across pore throats. One key question arises for environmental remediation sciences: “Is the cumulative impact of these changes significant enough to impact field scale flowpaths, thereby rendering it challenging to subsequently introduce amendments and/or alter the hydrobiogeochemical conditions favorable for sustained remediation?’

Although well documented at the laboratory scale, only a few field studies have been performed that indicate the impact of remediation-induced end-products on subsurface flow characteristics. Recent research conducted by LBNL in conjunction with the Rifle Integrated Field Research (IFRC) Site in Colorado further suggests that remediation-induced transformations may impact flow at the field scale. At this site, IFRC investigators have repeatedly demonstrated the ability to rapidly remove uranium from tailings-contaminated groundwater through bioremediation using long-term (~100-day) acetate amendment into the unsaturated aquifer at the site, which is composed of Colorado River floodplain deposits (e.g., Anderson et al., 2003; Vrionis et al., 2005). The stimulation of iron- and sulfate-reducing microorganisms accompanying acetate injection has been shown to promote uranium removal, presumably as insoluble mineral precipitates. LBNL’s analysis of time-lapse tracer test datasets, collected in association with two biostimulation experiments conducted by the IFRC team during 2002 and 2003 in the same Rifle flowcell, has suggested that flowpaths were altered in response to field-scale biostimulation tests. LBNL’s recent reactive transport modeling of Rifle biostimulation column experiments suggests that the predominant mineral phases precipitated during extended periods of acetate injection at the Rifle IFRC include calcite (CaCO3) and iron sulfide (FeS). Finally, recent LBNL research at the Rifle Site has illustrated the promise that synchrotron, geophysical, and isotopic methods hold for monitoring some of the complex subsurface processes from the grain to the local field scale.

Hypotheses

This challenge focuses on development of a predictive understanding of how remediation-induced biogeochemical transformations modify the connectivity of the pore structure and how those changes are manifested at the field scale as is needed to design, execute, and interpret sustainable in situ treatments. Although the recent LBNL research at the Rifle site described above suggests that feedbacks between flow characteristics and biogeochemical transformations occur in response to biostimulation, a systematic study is warranted to document the extent of these feedbacks over space and time and their impact on sustained remediation. We are tackling this challenge through addressing the following three hypotheses that comprise the initial model of the systems framework:

Hypothesis 1 - Remediation treatments can induce significant enough biogeochemical perturbations to alter pore structures.
Hypothesis 2 - The cumulative impact of remediation-induced biogeochemical transformations will be significant enough to impact flowpaths at the field scale.
Hypothesis 3 - The dynamic nature of field scale flowpaths will impact the efficacy and sustainability of in-situ remediation treatments.

Three hypotheses associated with this challenge will be carried out via integrated tasks using synchrotron, isotopic, geophysical, and reactive transport modeling approaches, which will be tested at the Uranium contaminated Rifle, CO site.

Expected Outcomes

Research in this challenge focuses on development of a predictive understanding of how reaction processes modify the flow properties of the porous medium and the impact of dynamic flowpaths on remediation sustainability as needed to improve the design, execution, and interpretation of sustainable in situ remediation approaches. In particular, this challenge will:

Develop approaches for quantifying amounts of carbonate and sulfide precipitates formed during biostimulation experiments using isotopic and geophysical approaches;
Develop approaches to incorporate isotopic and geophysical constraints into parameter identification and reactive transport modeling.
Develop a mechanistic understanding of the pore structure and flowpath evolution across scales, and identify important factors that would most significantly lead to pore clogging;
Develop predictive understanding of cumulative impact of remediation induced biogeochemical processes on field scale flowpaths.

The insights developed in this ‘Evolution’ challenge are expected to have impact on many different types of in-situ remediation and the developed approaches are expected to be transferable to a wide range of environmental stewardship applications. Additionally, the challenge-based research is also expected to be relevant for many other applications that involve the manipulation of subsurface conditions for improved management of natural resources, such as carbon sequestration, aquifer storage and recovery, and enhanced hydrocarbon recovery methods. The systems understanding developed through the proposed challenge research should provide the foundation needed to improve the design of in situ strategies aimed at engineering subsurface conditions, with a goal of rendering the treatment more effective and sustainable.

LBNL Research Team & Collaborators

Research within this challenge will be performed in collaboration between the Rifle IFC team.