PERMFILE138867

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 56 Evaluation of B-Seam Impacts on Refuse Pile Leachate
• Media Type: D
• Archive: No

ASSUAJPTIONS AND BASIS OF AtJALYSIS
~:,
Impacts of leaclrate ~:ollected from the refuse pile, routed tlrrouelr the
sedimentation ponds, and discharged to the North Forlc of the Cunuison were
analyzed on the basis of the following assumptions which provide a high deEree
of conservatism.
1) Impacts were calculated assuming that undiluted leaclrate disclra[ges
from the sedimentation ponds to the North Fork during the one in ten
~,U year return period, seven day duration law Elow. The U.S. Geological
^, Survey Open File Report 84-137 (1984) states ttre ten year, seven day
M \~ ~1 low flow as 33.9 cubic Eeet per second (cfs) For the Somerset garrE,ing
station, which i:; the closest station to the refuse pile. Uisclrargc
during this assumed low flow period Ls not anticipated as the
sedimentation ponds have a storage capacity designed to retain the
runoff from a one in ten year, 24 hour duration precipitation event.
Actual discharge:; to the North Fork from the sedimentation ponds Iry
uECC since 1986 have occurred when flow in the Nortlr Fork was al' or
above 100 cfs. Teleplwne contact with 1larold Pitch of the U.S.
Geological Surve}' on 1larclr 28, 1989 indicated that the 10 year, 7 day
low flow estimate is unchanged from that used previously to estimate
F seam refuse p:;le leaclrate impacts. Therefore, flow in the lJortlr
Fork is expected to be a least 3 tunes greater during the t. irnc
leachate is actually discharged than lras been assumed in t.lris
analysis. ,
2) The flow of leaclrate was calculated by assuming that atr anrxral
infiltration of 5.7 inches flows through the maximum area of the pile
of 20 acres, generating 10.3 acre-feet per year (6.4 gpin) of leachate
The full area of the pile will not be developed Eor approxinratel}' S
years and the initial infiltration will be corrsurned in filling the
unsaturated pore space in the pile. Based on a NcWhorter-Nelson
analysis of infiltration performed for the F seam permit studies,
approximately 15 years will be required before the quantity of
leachate discharging from the pile is equal to the quantity of
infiltration through the surface of the pile. It was further assumed
for the F sears analysis tl>.ac the natural springs along the
southwestern ma[gin of the pile will generate leaclrate. The
estimated seasonal maximum discharge of the springs is 20 gprn, giving
a total assumed leaclrate discharge volume of 26 gpm. WECC personnel
have not observed. discharge of water from the rock drains beneath the
refuse pile nor Iras any saturated ground water zone been detected. in
the 3 monitor wells installed downgradient of the refuse pile (Data
from Annual Ilydrology Reports For 4later Years 1986 and 1987, pp. 12
and 18, respectively). Despite these observations, the B seam
leachate evaluation is based on the assumption that leaclrate
discharge is 26 gpm to allow direct comparison with the previous F
seam estimates and allow Eor this possibility as a future COndl ClOn.
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