Proposed LBNL-SRNL Collaborative Project:

**Scientific-Basis of Monitored Natural Attenuation (MNA)**

**at the Savannah River Site F-Area**

**BACKGROUND**

Groundwater plumes within the F-Area of the Savannah River Site were
created by the disposal and leakage of low-level radioactive solutions
into seepage basins. The plume extends 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.
Disposal of the acidic solutions (average influent pH of 2.9) into the
basins began in 1955 and continued until 1989. Groundwater remains
acidic, with pH values as low as 3.2 near the basins. As a result, the
Atlantic Coastal Plain aquifer sediments that underlie the F-Area have
been bathed with acidic solutions for about 40 years. The basins were
closed and capped in 1991, and in 1997 a pump-and-treat remediation
system began operation. In 2004, the pump-and-treat system was replaced
by a hybrid funnel-and-gate system that was installed about 300m from
the stream, where alkaline solutions are injected into the gates in an
attempt to neutralize the acidic groundwater. There is a wealth of
geological, geophysical, and geochemical data that has already been
collected at this site, including archived core and data from 109
groundwater monitoring wells that are sampled regularly (some since the
1980s). A conceptual cross section through the F-Area site is shown
below.

![](media/image1.wmf){width="6.625in" height="3.390277777777778in"}

It is recognized that mobile biogeochemical (pH, redox) gradients can
develop at the leading and trailing edges of acidic plumes as a result
of reactive subsurface transport processes, and that the gradients can
lead to transformations (such as solid phase changes) that can occur
preferentially as a function of the initial hydrogeochemical
heterogeneity of the system. Although it is also recognized that the
evolution of these gradients and their propagation rate dictate the
mobility of many contaminants of interest to DOE, there is currently an
insufficient scientific basis to enable prediction with enough accuracy
to defend monitored natural attenuation (MNA) as an acceptable remedial
strategy.

**PROPOSED PROJECT and COLLABORATIONS**

Understanding the spatio-temporal evolution and migration of plume
gradients is a key to understanding if MNA is a viable remediation
strategy at the F-Area (Denham et al, 2007). As part of the LBNL
Subsurface Science SFA program, we propose to focus on the development
of a scientific basis for assessing the natural attenuation capacity of
the SRS F-Area aquifer (with a particular emphasis on Sr90, I-129, and
U) and on development of characterization and modeling approaches that
can incorporate critical information about linked hydrobiogeochemical
properties and rates over field relevant scales. We will explore the
following ***key hypothesis***, which are divided among our three
primary research areas:

1)  Biogeochemical Processes:

-   Development of a site-specific, multi-component equilibrium/kinetic
    surface complexation model, which quantifies the rates of key metal
    and radionuclide constituents as a function of pH and major mineral
    phases, is expected to be the key for understanding if MNA is a
    viable strategy for the F-Area.

2)  Characterization:

-   A reactive facies framework can be developed that will facilitate
    the characterization of critical hydrobiogeochemical processes and
    rates across scales. Here, "reaction facies" is used to describe
    subsurface units that have similar characteristics relevant to
    reactive transport, such as distribution of surface reactive
    minerals, effective surface area, mineralogy, hydrological
    properties, and microbial communities. If valid for the SRS site, it
    offers an approach to balance the complexity and tractability as
    needed to apply scientific understanding of MNA to plume-scale
    problems;

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-   Isotopic signatures can be used to indicate the state of key
    contaminants and significant biogeochemical processes at the field
    scale.

3)  Reactive Transport Modeling:

-   Through incorporation of reactive-facies and sorption-based
    paradigms, mechanistic reactive transport modeling approaches can be
    used to simulate plume evolution and migration with sufficient
    accuracy over field relevant scales.

Although geared toward the F-Area, the development of an understanding
and capabilities for accurately predicting the spatiotemporal evolution
of geochemical gradients and its impact on metal and radionuclide
mobility is critical for determining whether monitored natural
attenuation (MNA) is a viable stand-alone remediation strategy for many
DOE sites.

The proposed effort is being developed in parallel with an EM-20 funded
technical working group effort that is being led by Dr. Miles Denham
(SRNL) entitled 'Attenuation-based Remedies for Metal and Radionuclide
Contaminated Groundwater'. The proposed collaboration between the Office
of Science funded LBNL group and the EM-funded technical working group
represents a joint basic-applied science effort that is expected to
guide regulators as the F-Area transitions from active remediation to
natural attenuation. The LBNL team members include: Drs. John
Christensen, Susan Hubbard, Peter Nico, Nic Spycher, and Jiamin Wan.

**APPROACH**

The three main project components and associated hypotheses are
described below, followed by an overall project milestone matrix.
Successful completion of many of these tasks and milestones is dependent
on support from both Office of Science and EM, and on close
collaboration between the LBNL SFA group and the SRNL technical working
group and SRNL scientists.

1\) **Biogeochemical Processes.** We propose to use laboratory batch and
flow through column experiments coupled with reactive transport modeling
and synchrotron characterization methods to address the hypothesis
described above.

-   Determine rates and extents of sorption and desorption for U, Sr,
    and I on bulk and the major individual reactive facies for the
    acid-altered and pristine sediments.

-   Explore U, Sr, and I speciation in plume pore liquids and associated
    mineral surfaces, including associations with colloidal phases.

-   Quantify the changes in sediment mineralogy and surface sorptivities
    after long-term acid leaching.

-   Perform reactive transport modeling in conjunction with column
    experiments to guide experimental design and to provide insights on
    key (bio)geochemical and hydrological processes and their coupled
    effects on natural attenuation.

-   Obtain desorption rates and extents of the key contaminants from
    contaminated vadose zone sediments under the vadose zone flow
    conditions. This understanding is essential to insure that capping
    of the contaminant source zone indeed effectively immobilized the
    source.

-   Participate in performing and assessing field-scale tracer tests
    within local-scale study sites, for example, using an injectable
    tracer cocktail that has similar geochemical (sorption, pH)
    characteristics to the plume (i.e., cocktail with stable ^87^Sr,
    depleted uranium, and bromide tracers) and/or to high pH solutions
    used for neutralization.

2\) **Characterization.** Using statistical, hydrogeophysical, isotopic,
geochemical, XRD, and synchrotron methods, we propose to:

-   Establish if linkages exist between in-situ physical and geochemical
    properties of the F-Area using existing and newly collected field
    data from outside of the plume area.

-   Develop stochastic approach for integrating multi-scale
    hydrobiogeochemical property and rate information as a function of
    reactive facies;

-   In conjunction with biogeochemical batch and column experiments,
    develop an understanding about the relationship between isotopic
    signatures of key contaminants and significant (bio)geochemical
    processes;

-   Perform hydrobiogeochemical-geophysical characterization of three
    local-scale sites and along a transect that connects contaminated,
    local-scale sites. (See approximate locations in figure).

-   Assess utility of isotopic data for characterizing system behavior
    at field scale.

-   Assess use of geophysical datasets and reactive facies framework for
    integrating hydrobiogeochemical information at local scale sites and
    along transect.

![](media/image2.wmf){width="3.6277777777777778in" height="2.61875in"}

Figure 1 Schematic locations of three local-scale study sites. Two of
the sites are located along the plume axis, while one of the sites is
located outside of the contaminated region.

**3) Reactive Transport Modeling**. We propose to:

-   Simulate responses to pH change at the manipulation site to better
    understand soil buffering capacity, sorption behavior, and
    contaminant distribution at the local field scale.

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-   Develop preliminary (mechanistic) 'site' reactive transport model
    along one or more key transects, based on existing site data,
    hydrofacies model, and assumptions regarding linkages between
    hydrogeochemical properties, with focus on U, Sr, and I.

-   Develop mechanism to parameterize site model using reactive facies
    concept and to incorporate isotopic data.

-   Test the model reliability by predicting the system responses to
    reactive tracer tests at local scale;

-   Refine site model as results become available from biogeochemical
    laboratory experiments and reactive facies field characterization in
    local scale areas and along key transect.

-   Test the model reliability at field scale by comparing results of
    simulations performed along transect with historical water
    composition trends.

**REFERENCE**

Denham, M. and SRNL/LBNL working group. Impact of Plume-Induced
Transformations on Natural Attenuation Strategies for Metal and
Radionuclide-Contaminated Groundwater, Savannah River National
Laboratory Document WSRC-MS-2007-00199.

**Major Tasks and Milestones.** Completion of many of these tasks and
milestones is dependent on funding from both the SC and EM, and on the
close collaboration between the LBNL SFA group, SRNL scientists, and the
SRNL technical working group.

+---+--------------------+-----------------------+---------------------+
|   | Biogeochemical     | Characterization      | Reactive Transport  |
|   | Processes          |                       | Modeling            |
+---+--------------------+-----------------------+---------------------+
| Y | -   Quantify the   | -   Work with SRNL to | -   Simulate        |
| 1 |     changes in     |     investigate       |     responses to pH |
|   |     sediment       |     linkage between   |     manipulation to |
|   |     mineralogy and |     hydrogeological   |     provide         |
|   |     surface        |     and               |     insights on     |
|   |     properties of  |     biogeochemical    |     soil buffering  |
|   |     acid altered   |     properties in F   |     capacity and    |
|   |     sediments      |     Area using        |     sorption        |
|   |                    |     existing data     |     behavior at     |
|   | -   Perform batch  |                       |     local field     |
|   |     and column     | -   Survey of U, Sr,  |     scale.          |
|   |     experiments    |     and I isotopic    |                     |
|   |     coupled with   |     variations within | **MILESTONE**:      |
|   |     isotopic       |     and outside       | Develop preliminary |
|   |     measurements   |     groundwater plume | (mechanistic) 'site |
|   |     and reactive   |                       | scale' model along  |
|   |     transport      |                       | a key transect      |
|   |     modeling.      |                       | using existing      |
|   |                    |                       | hydrofacies model   |
|   |                    |                       | and data, with      |
|   |                    |                       | focus on U, Sr, and |
|   |                    |                       | I.                  |
+---+--------------------+-----------------------+---------------------+
| Y | -   Experimentally | -   Develop reactive  | **MILESTONE**:      |
| 2 |     and            |     facies framework  | Evaluate            |
|   |     numerically    |     for guiding       | preliminary         |
|   |     explore        |     characterizing    | reactive facies     |
|   |     s              |     effort and        | framework on the    |
|   | orption/desorption |     integrating       | field scale.        |
|   |     of U, ^90^Sr,  |     multi-scale       |                     |
|   |     ^129^I to bulk |     property and      | **MILESTONE:**      |
|   |     and to major   |     process           | Incorporation of    |
|   |     individual     |     information.      | preliminary surface |
|   |     mineral phases |                       | complexation data   |
|   |     of acid        | -   Work with SRNL to | and isotopic        |
|   |     altered        |     develop and       | effects in RT       |
|   |     sediments      |     acquire data      | modeling            |
|   |                    |     within            |                     |
|   | **MILESTONE**:     |     local-scale sites |                     |
|   | Develop            |                       |                     |
|   | preliminary        | **MILESTONE:**        |                     |
|   | surface            | Establish if the      |                     |
|   | complexation/ion   | reactive facies       |                     |
|   | exchange model for | concept is valid for  |                     |
|   | contaminants U,    | the F-Area            |                     |
|   | Sr, and I on both  |                       |                     |
|   | acid-leached and   |                       |                     |
|   | clean saturated    |                       |                     |
|   | sediment as a      |                       |                     |
|   | function of        |                       |                     |
|   | facies.            |                       |                     |
|   |                    |                       |                     |
|   | **MILESTONE:**     |                       |                     |
|   | Develop reactive   |                       |                     |
|   | transport models   |                       |                     |
|   | that accurately    |                       |                     |
|   | predict            |                       |                     |
|   | experimental       |                       |                     |
|   | results at         |                       |                     |
|   | laboratory scale.  |                       |                     |
+---+--------------------+-----------------------+---------------------+
| Y | -   Work with SRNL | -   Perform analysis  | -   Implement       |
| 3 |     and others to  |     of local scale    |     reactive facies |
|   |     design         |     hydrobioge        |     modeling        |
|   |     field-scale    | ochemical-geophysical |     approach at     |
|   |     tracer tests   |     characterization  |     local scale     |
|   |     at local-scale |     datasets at local |     clean site--    |
|   |     study site C.  |     scale sites.      |     predict results |
|   |                    |                       |     of local field  |
|   | -   Continue the   | -   Work with SRNL    |     scale reactive  |
|   |     s              |     and others to     |     tracer tests    |
|   | orption/desorption |     perform           |                     |
|   |     experiments of |     characterization  | -   Refine site     |
|   |     Y2.            |     along key         |     model as        |
|   |                    |     transect          |     results become  |
|   | -   Further        |                       |     available from  |
|   |     explore U,     | -   Apply isotopic    |     biogeochemical  |
|   |     ^90^Sr, ^129^I |     shift model to    |     laboratory      |
|   |     speciation in  |     field-scale       |     experiments and |
|   |     plume pore     |     dataset.          |     reactive facies |
|   |     liquids and    |                       |     field           |
|   |     association    | **MILESTONE:**        |                     |
|   |     with solid     | Develop a model to    |    characterization |
|   |     surfaces by    | describe U, Sr, and I |     in local scale  |
|   |     comparing      | isotopic signatures   |     areas and along |
|   |     acid-leached   | associated with       |     key transect.   |
|   |     and clean      | significant           |                     |
|   |     sediment;      | biogeochemical        |                     |
|   |                    | processes at the      |                     |
|   | -   Work with SRNL | column scale          |                     |
|   |     and others to  |                       |                     |
|   |     perform        | **MILESTONE:** Assess |                     |
|   |     field-scale    | reactive facies       |                     |
|   |     tracer tests   | framework for         |                     |
|   |     in             | integrating           |                     |
|   |     uncontaminated | hydrobiogeochemical   |                     |
|   |     local-scale    | datasets at local     |                     |
|   |     study site C   | scale                 |                     |
|   |     and to analyze |                       |                     |
|   |     results        |                       |                     |
+---+--------------------+-----------------------+---------------------+
| Y | -   Extend         | -   Integrate surface | **MILESTONE:**      |
| 4 |     experiments    |     and borehole      | Assess predictive   |
|   |     and analysis   |     geophysical,      | capability to       |
|   |     to include     |     isotope,          | capture key         |
|   |     unsaturated    |     hydrological,     | contaminant         |
|   |     sediments.     |     etc. data to      | mobility at local   |
|   |                    |     characterize      | scale through       |
|   |                    |     reactive facies   | analysis of Site C  |
|   |                    |     concept along     | prediction and      |
|   |                    |     transect.         | tracer test data.   |
+---+--------------------+-----------------------+---------------------+
| Y | -   Experimentally | **MILESTONE:** Assess | **MILESTONE**:      |
| 5 |     and            | utility of            | Assess predictive   |
|   |     numerically    | interpreting isotopic | capability to       |
|   |     explore        | signatures in terms   | capture key         |
|   |     desorption     | of field scale        | contaminant         |
|   |     rates of U,    | processes.            | mobility and at     |
|   |     Sr, and I from |                       | 'plume scale' along |
|   |     contaminated   | **MILESTONE:** Assess | transect through    |
|   |     vadose zone.   | reactive facies       | comparison of       |
|   |                    | approach for          | simulations with    |
|   | **MILESTONE:**     | integrating           | historical data     |
|   | Finalize sorption  | hydrobiogeochemical   |                     |
|   | based paradigm to  | datasets at plume     |                     |
|   | quantify processes | scale.                |                     |
|   | and rates as       |                       |                     |
|   | function of        |                       |                     |
|   | saturation, pH,    |                       |                     |
|   | species, and       |                       |                     |
|   | facies.            |                       |                     |
+---+--------------------+-----------------------+---------------------+