**Status Report on NABIR FRC Modeling Efforts and Proposed Activities**

Jack C. Parker

Oak Ridge National Laboratory

November 7, 2003

The Field Research Center (FRC) at Oak Ridge is a focal point for NABIR
program field research on natural and stimulated biologically-mediated
attenuation of metals and radionuclides. Information from these studies
on the interactions among biological, chemical and physical processes in
a real-world setting with complex hydrogeology and contaminant
characteristics promises to significantly improve DOE's ability to
effectively manage legacy waste sites. Computer models are critical
tools for interpreting field and laboratory data from this complex
system to understand the nature of process interactions and to allow
predictions of plume-scale behavior in response to various
remediation/management strategies.

During the last half of FY03 a Modeling Working Group was established to
implement FRC strategic plan elements associated with conceptual and
mathematical modeling studies. The Working Group has been tasked to
coordinate current and proposed modeling efforts on NABIR/FRC projects
with each other and with related field and laboratory studies. The focus
of efforts in FY03 was to:

-   review current and planned modeling efforts associated with existing
    NABIR projects,

-   identify what computer codes are being used or planned to be used by
    NABIR investigators and evaluate their strengths and weaknesses, and

-   determine what site-wide efforts should be undertaken to minimize
    redundant efforts across projects and provide greater leverage with
    available resources.

[Review of NABIR Modeling Efforts.]{.underline} A survey of current
NABIR PIs was conducted to identify current and planned modeling
efforts. A summary of the survey results is presented in Table 1. Models
are being used by NABIR PIs on the front-end of projects to help design
laboratory and field experiments to allow work to be conducted more
efficiently and to obtain more useful information. On the back-end,
models are being used as key tools to interpret lab and field data by
mathematically separating effects of complex interacting physical,
chemical and biological processes. For some projects, this is an
iterative process as experiments and models incrementally increase in
complexity. At the present time, no efforts are specifically targeted at
applying models to scale-up experimental results from the laboratory and
pilot scales to larger field scales, although the success of currently
planned efforts would certainly open the door to such analyses.

A number of factors must be considered in selecting computer codes to
analyze data from lab and field experiments. Key among these are:

-   Dimensionality. Batch or one-dimensional models are suitable for
    analyzing batch or column experiments respectively. In general,
    field experiments require 3-D models to capture the flow geometry
    (particularly for well injection/extraction experiments), although
    1-D models may be utilized to analyze transport along defined
    streamlines subject to certain limitations.

-   Reaction Models. Since NABIR focuses on biologically-mediated
    reactions affecting metals and radionuclides, capability to model
    complex geochemical and microbiological processes is a critical
    model need. Since many key reactions are kinetically impeded,
    capability to model nonequilibrium reactions is also critical.

-   Complex Hydrology. Most groundwater flow at the FRC occurs through a
    small fraction of the total porosity that is fractured and/or
    preferentially weathered parallel to bedding planes in the bedrock.
    Although the groundwater flow system may be modeled as an effective
    porous medium at a sufficiently large scale (ca. tens of meters),
    flow anisotropy will result that is unlikely to be stationary with
    respect to or even orient locally with a simple Cartesian coordinate
    system. At any reasonable modeling scale, physical nonequilibrium
    due to diffusional mass transfer limitations will be a critical
    process controlling transport.

PHREEQC, which is being used on two NABIR projects, is a geochemical
model that is capable of simulating equilibrium and kinetic reactions
for batch or 1-D flow systems. As such, it is best suited for analyzing
lab data, although application along streamtubes, as planned in the
Criddle project, allows the flow geometry limitation to be overridden.
The model does not explicitly consider biological reactions or
fracture-controlled transport, but may be modified to do so.

At the other of extreme of complexity in flow geometry delineation is
FRAC3DVS, which simulates flow and transport in discrete fracture
networks within otherwise impermeable matrix material. Although the
standard code does not consider geochemical reactions, the version to be
used on the FRC has been modified to include this capability. Explicit
fracture flow models require specification of the orientation, spacing
and dimensions of individual fractures, which represents a daunting
model calibration problem. Commercially available pre- and
post-processors are available for the standard version of FRAC3DVS.

Two projects plan to utilize HBGC123D, which is a 3-D transport model
that considers equilibrium or kinetically-controlled geochemical and
biological reactions with microbial growth kinetics and physical
nonequilibrium (mobile-immobile zone mass transfer). Groundwater flow
can be simulated by the companion code, FEMWATER, that considers fully
anisotropic flow. FEMWATER is supported by the DOE and DoD-sponsored
Groundwater Modeling System (GMS), which provides basic pre- and
post-processing capability. RAFT, which will be used on one project, is
a 3-D flow and transport model that has a generic capability to simulate
complex biogeochemical reactions via user-programmed functions. Other
models that do not have geochemical modeling capability will be used for
specific purposes in various NABIR projects (e.g., flow field
delineation, nonreactive tracer test interpretation).

[FRC Modeling Efforts.]{.underline} The overall goal of the Modeling
Working Group is to achieve maximum leverage on investments in FRC field
and laboratory studies by effective utilization of computer models to
meet FRC strategic goals. To achieve this end, we propose to identify
modeling tasks and needs that are common across projects and develop
mechanisms to minimize redundant efforts by sharing modeling tools,
input files, databases and other key resources. Specific coordination
tasks that have been identified are:

1.  [Implement Site-wide FRC Model]{.underline}. A calibrated flow and
    transport model at the scale of the entire FRC would serve as a
    starting point for project-specific modeling efforts and would
    provide a base for integrating results from diverse laboratory and
    field studies and evaluating larger scale impacts. Such a model
    would also serve as a tool to evaluate critical data gaps and
    research needs to help guide future studies. A site-wide model would
    provide a means to define boundary conditions for smaller-scale
    project-specific models and would reduce the effort required to
    calibrate the latter models to the same level of reliability. The
    net results should be improved project efficiency (i.e., greater
    productivity per unit cost).

> Previous modeling efforts associated with remedial investigations in
> the Bear Creek Valley were performed primarily using MODFLOW and
> FRAC3D. While these studies provide a great deal of valuable
> information, we consider MODFLOW's inability to simulate anisotropy in
> the direction of dipping beds to be a critical limitation. FRAC3D is
> theoretically capable of overcoming this limitation by explicitly
> modeling fractures, but the resulting calibration problem is ill-posed
> and impracticable to resolve. Previous modeling efforts did not
> address geochemical or biological reactions or physical nonequilibrium
> due to fracture-matrix interactions. We will utilize FEMWATER to model
> groundwater flow and HBGC123D for model transport and biogeochemical
> reactions. These codes simulate all physical, geochemical and
> biological processes expected to be of concern at the FRC, are
> computationally efficient and robust. Pre- and post-processing
> capabilities are available for FEMWATER.
>
> Implementation of a site-wide groundwater flow model will be performed
> first. Concurrently, a simplified small-scale transport model will be
> implemented to test biogeochemical model formulations. After the flow
> model calibration is completed, a transport model will be implemented
> on the same grid and calibrated using available lab and field data.

2.  [Develop Model Database]{.underline}. The ready availability of
    information on model formulations and relevant parameter values from
    prior and concurrent FRC projects as well as from the open
    literature is a critical need to enable FRC-based NABIR projects to
    efficiently tackle the research issues central to their individual
    projects. Modeling data requirements may be classified as
    hydrologic, transport and biogeochemical in nature. Hydrologic
    information includes topographic, geologic, rainfall, streamflow and
    water level data as well as measurements or estimates of
    conductivity, transmissivity and aquifer storage parameters.
    Transport data includes information on mobile and immobile (or less
    mobile) region porosities, geometries and mass transfer
    characteristics, hydrodynamic dispersion, and data pertinent to
    defining initial and boundary conditions. Biogeochemical data
    encompasses the enumeration and formulation of reaction equations
    and their relevant equilibrium and rate coefficients in addition to
    data on aqueous and solid phase compositions.

> We propose to develop databases for the foregoing information that
> will be accessible through the FRC web site and will be directly
> linked to pre-processor modules. Due to the complexity of the
> biogeochemical processes, we anticipate the database for the latter
> will require the greatest effort to develop. Databases exist with
> equilibrium constants for a wide range of geochemical reactions.
> Reaction kinetics are more problematic, since a variety of rate
> formulations may be derived depending on assumptions made regarding
> the reaction mechanisms. This situation is particularly problematic
> for biologically mediated reactions, which often require introduction
> of empirical coefficients to adequately quantify observed behavior
> (e.g., inhibition coefficients, metabolic lag coefficients, etc.).
> Where applicable, we plan to tabulate multiple mathematical model
> formulations for various reactions as well as estimates of
> coefficients available from different studies and the relevant
> conditions in the studies. This task will involve close coordination
> between the Rates and Mechanisms Working Group and the Modeling
> Working Group. We believe that in addition to facilitating
> implementation of models to analyze data from the FRC, the
> biogeochemical database will serve as a key repository of research
> results on the biogeochemistry of metals and radionuclides that will
> accelerate advances in this field.

3.  [Develop Pre-processor for HBGC123D]{.underline}. In addition to the
    site-wide FRC model, two projects are currently utilizing HBGC123D.
    Availability of a calibrated site-wide model and parameter sets for
    HBGC123D will reduce efforts required of individual projects to
    perform sophisticated analyses of laboratory and field data. The
    availability pre- and post-processing tools to simplify setup and
    execution of complex site models will further enhance the efficiency
    of modeling efforts associated with individual projects as well as
    with implementation of the site-wide model.

> We propose to develop pre- and post-processor modules for HBGC123D
> that can be used in conjunction with GMS. This will enable the
> relevant reactions, associated parameters (e.g., equilibrium constants
> and rate function coefficients) and model data files to be created
> efficiently. The pre-processor will be linked to databases discussed
> below to further facilitate and simplify model setup. This task will
> be coordinated with Dr. George Yeh, who is the developer of HBGC1234D.

+-----------------+-----------------+-----------------+-----------------+
| **Table 1.      |                 |                 |                 |
| Summary of      |                 |                 |                 |
| NABIR Modeling  |                 |                 |                 |
| Efforts**       |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Project Title   | Modeling        | Codes           | Status/Comments |
| and PIs         | Objectives      |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| In situ         | Basis for       | HBGC123D -- 3-D | Simulations in  |
| immobilization  | iterative       | transport with  | progress for    |
| of uranium in   | design and      | geochemical and | experimental    |
| structures      | interpretation  | bio reactions   | design          |
| media via       | of field        |                 |                 |
| bio             | experiments     | FRAC3DVS -- 3-D | Summary of      |
| mineralization, |                 | fracture        | model           |
|                 | Identify        | transport with  | parameters from |
| *Schiebe,       | mechanisms and  | geochemical rx  | site            |
| Zachara, Brooks | reaction        |                 | investigations  |
| and Rodin*      | parameters from | PHREEQC2 --     | and literature  |
|                 | lab and field   | Geochemical     | being compiled  |
|                 | data            | reaction model  |                 |
+-----------------+-----------------+-----------------+-----------------+
| Evaluation of   | Determine       | ComCZAR/SRG3D   | Work in         |
| biostimulation  | optimal design  | -- Flow field   | progress to     |
| for remediation | of multi-well   | prediction and  | estimate model  |
| of uranium      | extraction      | capture zone    | parameters from |
| contaminated    | systems         | modeling        | in situ         |
| groundwater,    |                 |                 | flowmeter,      |
|                 | Predict         | UPHREEQC --     | seismic and     |
| *Criddle,       | field-scale     | Variant of      | tracer tests,   |
| Kitanidis, et   | flow and        | geochemical     | column studies  |
| al.*            | reactive        | model PHREEQC   | and literature. |
|                 | transport       | with uranium    |                 |
|                 |                 | geochemistry;   |                 |
|                 | Identify        | to be applied   |                 |
|                 | reaction        | along           |                 |
|                 | mechanisms and  | streamtubes     |                 |
|                 | parameters      |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Factors         | Identify and    | PHREEQC2 --     | Modeling is not |
| controlling in  | quantify        | equilibrium or  | a major thrust  |
| situ uranium    | biogeochemical  | kinetic         | in this project |
| and technicium  | reactions       | geochemical     | but             |
| bioreduction    | controlling     | reactions for   | coordination is |
| and             | uranium and     | batch or 1-D    | planned with    |
| reoxidation,    | technicium      | flow geometry   | Schiebe project |
|                 | during          |                 | to utilize data |
| *Istok,         | push-pull       |                 | from field      |
| Krumholtz and   | testing in      |                 | experiments for |
| McKinley*       | field           |                 | parameter       |
|                 |                 |                 | estimation      |
|                 |                 |                 | and/or          |
|                 |                 |                 | validation      |
|                 |                 |                 | using HBGC123D  |
+-----------------+-----------------+-----------------+-----------------+
| Ion reduction   | Determine       | HBGC123D -- 3-D | Batch and       |
| and             | kinetic rate    | multispecies    | column          |
| radionuclide    | equations that  | transport with  | experiments are |
| immobilization: | are consistent  | biogeochemical  | being analyzed  |
| kinetic,        | across a wide   | reactions       | with model to   |
| thermodynamic,  | range of        |                 | ev              |
| hydraulic       | conditions      |                 | aluate/validate |
| constants and   |                 |                 | reaction forms  |
| reaction-based  | Determine       |                 | and to          |
| modeling,       | thermodynamic   |                 | determine       |
|                 | and kinetic     |                 | applicable      |
| *Burgos and     | coefficients    |                 | parameters      |
| Yeh*            | for             |                 |                 |
|                 | biogeochemical  |                 |                 |
|                 | reactions       |                 |                 |
+-----------------+-----------------+-----------------+-----------------+
| Influence of    | Numerical       | CXTFIT -- 1-D   | Lab experiments |
| reactive        | inversion of    | analytical      | being analyzed  |
| transport on    | lab experiments | model for       | to invert for   |
| reduction of    | to estimate     | estimating      | model           |
| U(IV) in the    | physical and    | physical        | parameters      |
| presence of     | biogeochemical  | parameters      |                 |
| Fe(III) and     | parameters      |                 |                 |
| nitrate,        |                 | RAFT -- 3-D     |                 |
|                 |                 | transport model |                 |
| *Wood,          |                 | with            |                 |
| Szecsody, Liu   |                 | user-prescribed |                 |
| and Zachara*    |                 | equilibrium     |                 |
|                 |                 | and/or kinetic  |                 |
|                 |                 | reactions       |                 |
+-----------------+-----------------+-----------------+-----------------+