View Document Here - Hanford Site

2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
^20
.r ..1
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
("^13
44
Appendix C - Risk Modeling of an Engineered DoFARlr2001-11
Surface Barrier for the 221-U Facility Rev. ol raft Q
Redlinc/Strikcout
Cover-rclated inputs to RESRAD are predicated upon use of a Modified RCRA Subtitle C-type
cover for the 221-U Facility or ERDF final cover, as mandated by the ERDF Record of Decision
(EPA 1995). The Modified RCRA Subtitle C-typc cover includes a 0.6-m (2-ft)-thick
compacted soil/bentonite admix with a geosynthetic membrane, a drainage layer, a vegetation
layer, and a total thickness of 5 m(16.4 ft). The effect of the geosynthetic membrane to retard
moisture penetration was ignored in the RESRAD evaluation. Two important cover modeling
parameters are admix soil density and runoff coefficient. Runoff coefficient is related to
drainage layer transmissivity, admix transmissivity, and annual inflow (precipitation). Values
from the ERDF design analysis (Casbon 1995) for admixed soil density and the transmissivity
(1 cmis) for 0.3 m(1 ft) of clean drainage gravel were used to model the cover. Drainage layer
transmissivity is converted to a runoff coefficient value for different inflows as shown in
Fi gure C- thc .-qhrrunoff coefficient increases as the transmissivity increases and as the
precipitation increases.
i u C- 1 shows coefficientc for 20.3 cm/yr and 127 cm/vr ( 8 in./vr and 50 in./yr1. The
26 cm/vr and 53.3 cm/yr ('
linearrelationship. Because
transmissivitv of 10'° cm/vr was assumed. Runoff coefficients approach 100% as liner
transmissivity decreases to the transmissivity value of a clay liner of 10'8 cm/yr. Therefore, a
single runoff coefficient of 0.98 was used for all precipitation levels.
C.1.3 Results of Modeling a Disposal Facility with Clay Admix Liner
The RESRAD runs for determination of preliminary remediation goals did not model the effects
of the cover and predicted that only radionuclides with a distribution coefficient of less than one
(carbon-14, technetium-99, and tritium (H-3]) had the potential to reach groundwater within
( 1,000 years. Uranium has a distribution coefficient.eRikreeereater than zem and was predicted
not to reach groundwater within 1,000 years.
With the clay admix cover included in the evaluation, RESRAD predicts that no radionuclides
from the 221-U Facility or ERDF will reach groundwater within 1,000 years. ^As shown in
Table C-2. this would hold true for current, double, and triple precipitation rates for the
Hanford Site.
C.2 REFERENCES
40 CFR 141, "National Primary Drinking Water Regulations;' Code of Federal Regulations,
as amended.
Final Feasibility Study for the Canyon Disposition Initiative (221-U Facility)
June 100
C-3