Integrating Modeling and Monitoring for the Radioactive Waste Management Complex Page: 5 of 7
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To meet the objectives discussed above, four vadose
zone regions are sampled in order to provide evidence
that transport in the vadose zone does not exceed the PA
and CA predictions. The regions are the surface
sediments, AB interbed, BC interbed, and CD interbed.
Figure 3 is a schematic showing the general location of
the interbeds in the vicinity of the RWMC.
35-140 ft *c Interbed
Figure 3. Conceptual stratigraphy under the RWMC.
II. DEVELOPMENT OF ACTION LEVELS
Development of action levels is required by DOE
Order 435.1 in order to provide a means to integrate the
monitoring program with modeling conducted to predict
long-term performance of a LLW disposal facility. Action
levels provide a basis with which to compare future
monitoring results with PA and CA results.
Impacts from the subsurface migration of
radionuclides dissolved in groundwater were estimated
for both the PA and CA using computer models that
described release of radionuclides from the SDA pits and
vaults and migration in the vadose zone to the aquifer.
Concentrations were predicted in the aquifer at a well
located 100 m downgradient from the SDA active pits.
Intermediate concentrations at different locations along
the transport pathway were also obtained from the
modeling results. Decay and sorption were included
throughout the model, thus reductions in concentrations as
a result of these processes are addressed in the models.
The vadose zone action levels for the LLW disposal
facility at the SDA in the RWMC are established
thresholds that indicate the possibility that the:
* rate at which radionuclides are moving out of the
disposal area is greater than expected,
* rate at which radionuclides are being transported in the
environment is greater than expected, and
* conclusions of the PA and CA may not be valid.
Fiscal Year 2001 is the first year in which the action
levels were applied at the INEEL. As the modeling is
updated and performance of the disposal facility is better
understood, the action levels may be revised.
Vadose zone and aquifer action levels were
determined based on DOE requirements and CA predicted
concentrations. Concentrations projected in the CA, rather
than the PA, were used to develop the action levels
because the CA results are more restrictive.
C-14 is the primary contributor to the predicted
aquifer all-pathways dose for the CA during the 1,000 yr
compliance period. As a mobile contaminant, C-14 is
predicted to reach the aquifer from the SDA (including
the LLW disposal area), in the very near future. As
shown in Figure 4, peak aquifer concentrations 100 m
downgradient of the SDA are predicted to be
approximately 100 pCi/L in the present and predicted to
increase to about 1,000 pCi/L over the next 100 years
(period of institutional control). The remaining curves in
the figure show the predicted water concentrations in the
various regions of the vadose zone that correspond to the
predicted aquifer concentrations 100 m downgradient of
The vadose zone C-14 action levels, for the year
2001, are listed in Table 1. If measured concentrations are
greater than the action levels, it is an indication that
assumptions used for the modeling in the PA and CA may
not be conservative. This would prompt an immediate
assessment of the need for corrective actions.
I r I
10oo - - --
I - -Surace Sediments
I -0--AB Interbed
1000 t I0--CD Interbed
I --Aqufer - lO0m downgradient
I I -ML
1,0 - -
2000 20so 2100
Figure 4. Predicted composite analysis C-14
concentrations in the subsurface through the 100 years
period of institutional control.
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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/m1/5/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.