Integrating Modeling and Monitoring for the Radioactive Waste Management Complex Page: 6 of 7
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Table 1. Vadose zone action levels for year 2001 based
on the CA simulation results.
Nuclide Surface AB BC CD
Sediments Interbed Interbed Interbed
(pCi/L) (pCi/L) (pCi/L) (pCi/L)
C-14 5.4E+05 4.8E+04 9.4E+04 1.4E+04
Discontinuities in the interbeds result in predicted BC
interbed concentrations that are sometimes greater that the
predicted AB interbed concentrations. Regular monitoring
and additional characterization of the interbeds are
planned in order to verify the non-intuitive model
predictions that the current peak C-14 concentration in the
BC interbed is higher than the peak concentration in the
III. COMPARISON OF MONITORING DATA WITH
As described in the last two sections, ongoing
monitoring is used to partially characterize the current
water concentrations in the vadose zone. In addition, the
computer model introduced above has been developed to
predict radionuclide concentrations in the vadose zone
and serve as a basis to define the corresponding vadose
zone action levels that if exceeded, would indicate a
potential that migration may be occurring more rapidly
than predicted in the PA/CA models.
The action levels are based on peak predicted
concentrations in a numerical grid block. Numerical grid
block sizes range from 62.5x62.5x0.5 m in the surface
sediments to 250x250x2 m in the CD interbed. Therefore
the predicted peak concentration in the model is actually
an average over a relatively large area.
The monitoring coverage is limited, and therefore
insufficient to guarantee that the water samples will
represent the peak concentration. Therefore, any one
sample with concentrations greater than the action level
will require some form of action, because that may not
necessarily be an actual peak.
As shown in Table 1, the peak predicted C-14
concentrations in the AB, BC, and CD interbeds are
4.8x10+4, 9.4x10+4, and 1.4x10+' pCi/L. Over the last
three years, vadose zone water analyzed for C-14 in the
interbeds have had a maximum concentrations of 26.4
pCi/L in the AB interbed, no detected C-14 in the BC
interbed, and 20 pCi/L in the CD interbeds. The measured
C-14 concentrations in the vadose zone are about 1000
times below the action levels. The over-predictions result
primarily from conservative assumptions made for CA
dose assessment. Although there will never be a guarantee
that the peak concentrations are measured, the fact that
concentrations are well below the action levels provides
confidence that the PA/CA modeling projections are
IV. FUTURE WORK
As a result of uncertainties related to the subsurface
flow and transport predictions, the mechanisms
controlling flow and transport are being studied within the
INEEL Subsurface Science Initiative to better predict the
performance of the active LLW disposal facility and the
entire SDA. The objectives of these studies include
improved characterization of the subsurface, increased
monitoring of PA/CA critical radionuclides, development
of new methods to instrument the subsurface, better
understanding of source releases, facilitated transport,
multiphase flow, and the effects of microbiology and
geochemistry on the contaminant transport. The results of
these studies will be used to better characterize water flow
and radionuclide release and migration in the subsurface
as needed to make the PA/CA modeling more defensible.
In conjunction with ongoing Comprehensive
Environmental Response, Compensation, and Liability
Act (CERCLA) remedial investigation/feasibility study,
the subsurface flow and transport simulation model is
being updated. The update will incorporate recently
collected information that was not available when the CA
simulation model was developed. The action levels will
be updated to be consistent with the results of the new
simulations. If the new simulation results are significantly
different from the current CA results, then the conclusions
of the PA and CA will be reevaluated. Updates will be
incorporated into the PA/CA annual report as needed.
IV.A. Radionuclide Release and Migration
Several research activities are underway to better
quantify the release and migration of key radionuclides.
Experiments are being conducted in a mesoscale column
(roughly 3 m tall) as well as at the bench scale in the
laboratory. Modeling is also being conducted to
complement the experimental work. Because of their
significance in the PA and CA results and because of
uncertainties regarding their behavior in the subsurface at
the RWMC, factors influencing migration of C-14 and
uranium are a key concerns in this research.
The mesoscale column is filled with soils
representative of the fill used at the SDA. Gas phase and
liquid phase migration will be monitored. One of the
major needs is to provide a basis to differentiate between
the fraction of C-14 that migrates upward as a gas and the
fraction that migrates downward in the liquid phase. This
may explain why the predicted C-14 concentrations in the
vadose zone are currently much greater than the measured
concentrations. That is, much less C-14 is migrating
downward than is predicted by the PA/CA models,
because a substantial fraction of the C-14 is migrating
upward into the atmosphere in the vapor phase.
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Seitz, Roger Ray; Mccarthy, James Michael & Keck, Karen Nina. Integrating Modeling and Monitoring for the Radioactive Waste Management Complex, article, August 1, 2002; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc890058/m1/6/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.