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OCR for page 51
4
System Assessment Capability
As discussed in Chapter 3, one of the primary functions of the
Groundwater/\/adose Zone Integration Project is to predict current and
future impacts on humans and the environment resulting from the release
of contaminants at the Hanford Site. The Integration Project is developing
what it calls the System Assessment Capability, or SAC, to estimate
these current and future impacts. The SAC will comprise a set of models
and parameter databases that can be used to obtain quantitative
estimates of cumulative impacts of contaminant releases on water
resources, biological (including human) systems, cultures, and economies
in the region around the Hanford Site extending over hundreds of years.
Although the SAC is not formally part of the Integration Project's
science and technology (S&T) program, it is a potentially important end
user of S&T products. These products include mass balances of
inventories and contaminant releases' to be provided by the Inventory
Technical Element (Chapter 5~; conceptual models, numerical models,
and parameter databases for contaminant fate and transport to be
provided by the Vadose Zone, Grounclwater, and River Technical
Elements (Chapters 6-8~; and human, ecological, economic, and cultural
impact data to be provicled by the Risk Technical Element (Chapter 9~.
Given the importance of the S&T program to the SAC, the
committee provides a short review and assessment of the SAC in this
chapter to set the stage for more detailed assessments of the S&T
technical elements later in this report. The primary purpose of this
assessment is to identify knowledge gaps that, if addressed through
additional S&T work, could improve the usefulness of the SAC as a
predictive tool.
The term inventory is used by the Integration Project to describe the
quantities of radionuclides and chemicals that have been placed in storage and
disposal facilities at the Hanford Site for example, high-level radioactive waste
placed in underground tanks or transuranic waste disposed in near-surface
trenches. As noted in Chapter 2, some of this waste has migrated out of these
disposal facilities and into the vadose zone or groundwater. The committee uses
the term contaminant release to describe these releases, whether accidental or
intentional.
51
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Representative terms from entire chapter:
hanford site
System Assessment Capability
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54
Science and Technology for Environmental Cleanup
simplified manner to demonstrate that an assessment can be conducted
at the site-scale level. SAC Rev. 1 is intended to be a production
implementation that is sufficiently developed to support site decisions. It is
planned to be used, for example, to conduct Comprehensive
Environmental Response, Compensation, and Liability Act of 1980
(CERCLA)3 reviews, analyze impacts of changes to cleanup baselines,
and develop closure plans for the 200 Area tank farms (see Chapter 2~. At
least two additional versions of SAC (i.e., Rev. 2 and 3) are planned in the
Integration Project baseline through 2005 (DOE, 2000a). These versions
will continue to add capabilities, complexity, and refinements to Rev. 1.
The Integration Project was in the process of completing SAC
Rev. 0 during the committee's information-gathering meetings, and the
committee received documents (e.g., BHI, 1999; Kincaid et al., 2000) and
briefings (Appendix B) on the details of this version. A summary of the
main design elements of Rev. 0 is provided in Table 4.1. As shown in
Table 4.1, SAC Rev. 0 is designed around a set of simplifying and limiting
assumptions about contaminant transport and long-term site conditions:
Contaminants will be released from a limited number of
"representative" locations at the site. The Waste Information Database
System (see Chapter 5) lists more than 2,600 release locations across
the site.4
· Transport through the vadose zone is modeled as a one-
dimensional, homogeneous, isotropic continuum, and transport through
the saturated zone as a two-dimensional phenomenon. Actual
contaminant transport in the vadose zone and saturated zone occurs in
three dimensions, and potentially important subsurface transport features
(such as elastic dikes) cannot be modeled in the Rev. 0 configuration.
Site conditions, including climate, river discharges, and
economic and sociocultural conditions, will be unchanged from the
present through the year 3050. Rev. 0 also ignores the potential impacts
of extreme events such as large fires, floods, and seismic events.
3Most of the cleanup work at the Hanford Site is proceeding under CERCLA,
which requires periodic reviews to ensure that cleanup objectives continue to be
met.
Initially, the Integration Project planned to use eight different representations
of the vadose zone to model these releases: one each in the 100 and 300 Areas
and six representations for the Central Plateau in and around the 200 Area. The
number was later increased to 13 to add more representations for the 100 Area.
The 2,600+ waste sites at the Hanford Site are to be aggregated into 50-100
representative release sites in the Rev. O assessment. This number will probably
increase to 500 before this assessment is completed (Mark Freshley, Pacific
Northwest National Laboratory, written communication, April 12, 2001~.
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62
Science and Technology for Environmental Cleanup
· Impacts are estimated only for the first 1,000 years following
site closure. It is not clear that the time to peak risk will occur in the first
1,000 years, however, especially in light of the long half-lives of some key
radionuclides and expected long travel times to receptors of interest. For
example, technetium-99, iodine-129, and uranium-238 have half-lives of
0.2 million, 16 million, and 4.5 billion years, respectively. If peak risk
occurs beyond 1,000 years, then other model assumptions, particularly
the assumption that climate remains unchanged, may not be realistic.
SCHEDULE AND BUDGET
Work began on SAC Rev. 0 in fiscal year 1999 and is planned to
be completed by the end of fiscal year 2001, with the initial model runs
scheduled to be completed by the end of July 2001. Model run times are
very long, however, and the Integration Project has had to cut the planned
number of realizations to 10 to keep the project close to schedule (see
comments in Table 4.1~. It is not clear what will be learned from the small
number of planned realizations, except to demonstrate that the model can
produce a numerical "answer." The small number of realizations seems
inadequate to capture the behavior of the system.
Work on SAC Rev. 1 was initially scheduled to begin around the
end of fiscal year 2001. However, in the latest update issued by the
Integration Project, the schedule for Rev. 1 has slipped to fiscal year 2002
(DOE, 2000c, p. 20~. According to the Integration Project Roadmap,
subsequent revisions to SAC are scheduled to be produced at 1 8-month
intervals (DOE, 2000a, Figure 4-1 ) culminating in the release of Rev. 3 in
2005.
The Integration Project is now considering an alternative
schedule that would involve enhancing Rev. 0 and using it to perform
several alternative assessments during fiscal year 2002. The Integration
Project would then solicit feedback from the Department of Energy (DOE),
regulators, and stakeholders about how the capability should evolve and,
if appropriate, will produce a Rev. 1 before 2005.
Funding for the SAC is provided through the DOE-Richland Office
budget. SAC received $1.9 million in fiscal year 1999 and $2.85 million in
fiscal year 2000. The fiscal year 2001 budget request was $2.0 million,
but only $1.7 million was allocated.5
An response to a $300,000 cut in the SAC budget for fiscal year 2001, the
Integration Project plans to delay completion of the documentation of Rev. O
results until fiscal year 2002.
System Assessment Capability
63
DISCUSSION
As a proof-of-principle implementation, SAC Rev. 0 is not
intended to make specific predictions of contaminant transport and its
impacts at the Hanford Site. That capability is planned for subsequent
revisions. Moreover, it is not clear how "proof of principle" will be
demonstrated, given that both the model results (see last column of Table
4.1 ) and the historical release data to which it will be compared have high
degrees of uncertainty.
For SAC to achieve a reliable, predictive capability, however,
additional S&T will be needed to close several important knowledge
"gaps." The gaps that the committee judges to be most important are
described briefly in the following paragraphs. More details are provided in
subsequent chapters.
1. A lack of data on the three-dimensional distributions of
contaminants in the vadose and saturated zones at the Hanford Site
will greatly limit the ability to calibrate or validate the SAC as a
reliable risk assessment tool. Relatively few data sets are available on
contaminant distributions, concentrations, and speciation in the
unsaturated zone deeper than 20 to 30 meters (Mark Freshley, Pacific
Northwest National Laboratory, written communication, September 6-8,
2000; Myers and Gee, 2000~. In addition, the lateral extent of
contaminants beyond the "footprints" of waste disposal sites, such as
tanks, cribs, and trenches, is poorly known. Absent knowledge of the
current state of contamination in the subsurface, it will be difficult if not
impossible to assess the reliability of contaminant migration and impact
predictions derived from the SAC.
Equally important, it will be necessary to characterize the
uncertainty associated with both the model predictions and the measured
distributions of contaminants in the subsurface to which those predictions
are to be compared. To this end, new procedures will have to be
developed or adapted to characterize uncertainties and perform these
comparisons in a manner that allows one to determine the degree of
"success."
2. A lack of understanding of the three-dimensional nature of
contaminant transport will limit the ability of SAC to provide
accurate estimates of residence times for contaminants in the
vadose and saturated zones. As discussed in Chapter 6, field
experiments at Hanford have demonstrated clearly that fluid transport in
the vadose zone is fully three dimensional. The three-dimensional nature
of contaminant transport in the vadose zone is also illustrated in the
document Preliminary System Assessment Capability Concepts for
64
Science and Technology for Environmental Cleanup
Architecture, Platform, and Data Management (BHI, 1999~. Similarly,
groundwater measurements have shown that large vertical gradients can
exist in contaminant concentrations in the saturated zone.
A separate but related problem is that hydraulic and transport
parameters used in the transport models are derived from laboratory
measurements on centimeter-scale core samples and are extrapolated to
scales relevant to field transport. The scientific basis of an "upscaling"
algorithm to calculate "effective" parameters for a large block of
heterogeneous sediments from highly variable measurements on small
samples has not been demonstrated. This problem is discussed in more
detail in Chapter 6 and Appendix C.
3. A lack of understanding of the effects of extreme (high-
magnitude and low~frequency) events such as large fires, floods,
and earthquakes will limit the ability of SAC to provide accurate
estimates of contaminant movement over time scales during which
wastes will remain hazardous. Although the probability of occurrence of
such extreme events in a single year is low, the consequences of these
events could be much higher than those predicted solely by the
advective-dispersive transport mechanisms considered in the SAC,
especially over the time scales during which wastes will remain
hazardous- typically on the order of 103 to 105 years.
The Hanford Site is a fire-prone ecosystem,6 as evidenced by
range fires in 1984 and 2000 each of which burned about half of the area
,
of the site.' Fire represents a potentially important agent for mobilizing
contaminants contained in vegetation and near-surface soils.
Radionuclides contained in the burned vegetation can be released directly
into the atmosphere, and near-surface contaminants could be mobilized
by increased infiltration or surface erosion accompanying the loss of
vegetation. The effects of fire on vegetation removal may be magnified
during periods of severe drought, when a lack of precipitation would
inhibit the recovery of burned areas.
The Hanford Site is also vulnerable to different types of flooding
events, ranging from failures of pressurized water mains to catastrophic
flooding. The latter has occurred repeatedly during glacial periods in the
last 100,000 years. Even under current (interglacial) climatic conditions,
intense rainfall occasionally saturates the land surface and generates
intense runoff events with attendant sediment transport. Such flooding
61n fact, the ecosystem is structured by fire.
71t can be argued that such range fires have return periods on the order of
decades and are not extreme events when measured against time scales of
waste hazards at the site. Nevertheless, this does not diminish their potential
importance as a contaminant transport agent.
System Assessment Capability
events could potentially result in the erosion and transport contaminants
from near-surface soils and waste burial sites.
These extreme processes will have to be better understood and
incorporated into later revisions of the SAC if it is to provide reliable long-
term estimates of contaminant transport at the site. Understanding the
potential impacts of such events is essential for making informed and
durable site cleanup decisions and siting new waste disposal facilities.
This issue will be discussed in more detail in Chapter 9 see especially
Sidebar 9.1.
4. Exposure pathways other than through groundwater may
exist at the Hanford Sit~these are not considered in the current
version of the SAC. For example, exposures to surface contamination
65
may occur as a result of burrowing animals or erosion, or such
contamination may exist in previously undetected locations. Humans
could be exposed through dust inhalation, soil contact, or consumption of
contaminated animals. Depending on what assumptions are made about
groundwater use at Hanford, the soil contamination pathway could be
significant from a risk perspective. Although additional exposure pathways
may be included in later versions of the SAC, the Integration Project S&T
program does not appear to be designed to support such additions. This
issue is discussed in Chapter 9.
A common theme that cuts across these knowledge gaps is
uncertainty. Characterizing uncertainty in the SAC, both in general terms
and for specific applications, will be a difficult but essential task for the
Integration Project. Equally important will be the adaptation of appropriate
statistical tools that allow reasonable conclusions to be drawn even in
light of such uncertainties. The committee believes that S&T can play a
central role in reducing uncertainty—for example, through the collection
_ of
data on current contaminant conditions at the site and the development or
adaptation of procedures to validate SAC predictions. This S&T work
must be conducted concurrently with SAC development, so that results
from current versions of SAC can be interpreted properly and S&T results
can be incorporated into future revisions.
