Effects of closure cap and liner on contaminant release rates from grouted wastes Page: 4 of 9
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Effects of Closure Cap and Liner on
Contaminant Release Rates from Grouted Wastes
Andrew D. Yu, John R. Fowler and Dale T. Bignell
Westinghouse Savannah River Company, P. O. Box 616, Aiken, SC 29802
This paper describes a groundwater modeling study of
waste disposal concepts using grouted waste forms. The
focus of the study is on the effects of clay caps and concrete
vaults on contaminant migration. We modeled three waste
disposal scenarios: 1) Grouted waste was solidified in an
earthen trench and covered with soil. There was no vault
and no cap. 2) Grouted waste was solidified in an earthen
trench. The entire waste disposal facility was then closed
under a clay cap. 3) Grouted waste was solidified in a con-
crete vault and protected by the same closure as in 2.
Because of the huge contrast in hydraulic conductivities and
highly non-linear multi-phase flow characteristics, these
waste disposal concepts presented a difficult problem for
numerical simulation. Advanced fluid flow and contami-
nant transport codes were used to solve the problem.
Among the codes tested, ECLIPSE [Intera 1993, Zhou
1994] out-performed other codes in speed, accuracy
(smaller material balance errors) and capability in handling
We used nitrate as a tracer for the simulation. Nitrate does
not absorb in the solid phase and does not decay. As a re-
sult, predicted release rate based on nitrate is conservative.
We also assumed that the facility is intact for 10,000 years.
In other words, properties of the materials used for this
study do not change with time. We predicted the fraction of
initial amount of nitrate released to the water table as a
function of time. Predicted peak flux for the no vault and
no closure case was 5.8x10 per year at 12 years. If a clay
cap was installed, predicted peak flux was 8.5x105 per year
at 110 years. If the grout was disposed in a concrete vault
and covered by a clay cap, predicted peak flux became
4.4x 10' per year at 8,000 years. Both concrete liner and
clay cap can reduce the rate of contaminant release to the
water table and delay the peak time.
The focus of this study is on groundwater protection. We
assessed the performance of three unsaturated zone waste
disposal concepts. Since unsaturated zone performance
relied almost entirely on the properties of the waste form,
the liner and the closure, these predictions are generic. For
a site-specific performance assessment (PA) and design of
waste disposal facility, we need to factor in 1) inventory,
2) alternative liners such as polymer and geotextile mem-
branes, 3) degradation histories of waste form, liner and
closure, 4) hydrogeological conditions, and 5) Cost.
A field-scale grouted waste disposal program is being con-
ducted at the Savannah River Site (SRS), a U.S. Department
of Energy (DOE) facility in Aiken, South Carolina. At
SRS, a low-level radioactive waste solution containing so-
dium nitrate, other dissolved solids, and trace radionuclides
is mixed with slag, flyash, and cement to form a grout-like
material called "Saltstone" [Langton 1988]. The grout is
poured into concrete vaults constructed at the Saltstone Dis-
posal Facility (SDF). The facility is designed for the release
of contaminants in a slow, controlled manner over thou-
sands of years. The impact of SDF on the environment was
studied in a radiological PA [Westinghouse 1992]. The PA
addresses the performance requirements and objectives
mandated by DOE Order 5820.2A [U.S. DOE 1988]. One
of the performance objectives is to show that the impacted
groundwater will be in compliance with the Safe Drinking
Water Act (SDWA) [U.S. DOE 1990].
Current Saltstone disposal concept includes a concrete
vault, a clay/gravel drainage layer above the vault roof, and
a clay/gravel closure cap over the entire facility. The bot-
tom of the vault is at least 20 feet above the water table.
Contaminants initially in the grouted waste form are re-
leased to the surrounding soil. For nitrate, the controlling
release mechanisms are convection and diffusion. Dis-
solved contaminants in the unsaturated zone are carried to
the water table by infiltrating water. Flow and transport in
the aquifer system beneath the water table are simulated by
a saturated zone model. The peak groundwater concentra-
tion at a compliance point is compared to the EPA drinking
water maximum contaminant level (MCL) [U.S. EPA
1977]. The compliance point is anywhere in the aquifer that
is more than 100 meters from the facility boundary. If the
predicted peak concentration is less than its MCL for every
contaminant, the disposal concept will be in compliance.
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Yu, A.D.; Fowler, J.R. & Bignell, D.T. Effects of closure cap and liner on contaminant release rates from grouted wastes, article, August 1, 1996; Aiken, South Carolina. (digital.library.unt.edu/ark:/67531/metadc665311/m1/4/: accessed September 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.