Evolution of Pore Structure and Flowpath Challenge
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?"
Addressing this question requires a better understanding of feedbacks between biogeochemical transformation and flow; of critical parameters that control overall system behavior; and of diagnostic signatures of critical system transitions or behavior. Although previously published laboratory studies have illustrated that pore clogging can occur because of an induced perturbation, there have been few systematic efforts to investigate the macroscopic influence of remediation-induced biogeochmecal transformations on aquifer flow and the resulting impact on contaminant dynamics.
This overarching objective of the Evolution Challenge is to quantify reaction induced dynamic porosity -permeability relationships in the subsurface, to assess their impact on elemental fluxes and contaminant mobility, and to identify diagnotistic signatures of key transformations and feedbacks. Rather than examining the extent to which naturally occurring variations in pore-size distribution impact permeability, the proposed research focuses predominantly on dynamic changes in hydrological properties that accompany biogeochemical processes---both naturally occurring and those stimulated through amendment with limiting substrates, such as organic carbon. This Challenge, which is alligned with and leverages on the field research conducted at the Rifle IFRC is being explored through tackling the following hypotheses:
- Hypothesis 1 - The cumulative impact of biogeochemical transformations will lead to changes in flow characteristics at the field-scale that (a) impede delivery of limiting substrates, such as labile forms of organic carbon and (b) enhance the stability of immobilized, redox-sensitive contaminants, such as uranium.
- Hypothesis 2 - Biogeochemical reaction rates will exert a key constraint on the spatial extent of hydrological impacts. Kinetically rapid rates will yield pronounced but localized impacts on flow, whereas slower rates will generate spatially extensive, albeit less significant, impacts on flow.
- Hypothesis 3 - A limited number of critical parameters can be identified that enable accurate modeling of an aquifer's response to a perturbation and its long-tem evolution. Integrative diagnostic signatures (biological, geochemical, and geophysical) can be identified that indicate the onset and distribution of these system transitions over field-relevant scales and over a range of timescales.
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 methods;
- 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 as well as system behavior.
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 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.
- Jonathan Ajo-Franklin
- Eoin Brody
- Jinsong Chen
- Michael Commer
- Mark Conrad
- Don DePaolo
- Jenny Druhan
- Benjamin Gilbert
- Hoi-Ying Holman
- Susan Hubbard
- Mike Kowalsky
- Sumit Mukhopadhyay
- Sergi Molins-Rafa
- Peter Nico
- Dmitriy Silin
- Eric Sonnenthal
- Carl Steefel
- Ken Williams
- Li Yang
- Yuxin Wu
- Li Li (Penn State)
- Phil Long (Rifle IFRC Team)
- John Bargar (SLAC)
- William Moses and LBNL SFA Radioimaging team
- Lee Slater (Rutgers)
- Bob Smith (UI)
Research within this challenge will be performed in collaboration between the Rifle IFC team.