PERMFILE53867

• Permit No: M1977004
• IBM Index Class Name: Permit File
• Doc Date: 8/31/1984
• Doc Name: Response to Adequacy
• From: Homestake
• To: DMG
• Media Type: D
• Archive: No

-8-
Item: Accuracy of the values and units in Table 5-2.
Res~ons~e: Table 5-2 incorrectly omits a factor 10-6 in both columns 2 and
3, the concentrations in rock material, and in leachate, respectively. The
following corrections should be made: (1) in Table 5-2, change 103 to 10-3
in column 2; multiply each number in column 3 by 10-6 (except for Ra values
which remain the same; (2) in Table 5-3, multiply mg/ml concentrations in
columns 2, 3, and 4 by 10-6 (leave Ra the same). The tables with corrected
headings are attached.
It is stressed that the projected concentrations of elements in the
drainages are extremely low and would likely be undetectable.
Item: Two items regarding the projections of leachate: first, the
representativeness of the rainfall return event in comparison to snowmelt
event in regard to producing leachate on the low-grade stockpiles was queried;
secondly, the volume of infiltrating run-off produced and the timing of
the peak run-off was discussed.
Response: The 1.7 inch rainfall event used was the maximum daily rainfall
reported in a two year span of meteorological data accumulation in the 1970's
at the project. It is assumed to be a worst-case condition. The relationship
between this event and snowmelt event is discussed later in reference to
another question. The rainfall event was used to represent conditions during
the season when maximum infiltration would be expected or the hypothetical
case under consideration. All water falling on the graded low-grade stockpile
was discussed to enter the pile.
In the absence of other information, the uniform rainfall distribution
over the 24-hour period was assumed, as was the coincidence of peak run-off and
leachate discharges. The observation regarding the noneven distribution of
rainfall is correct. The assumption regarding the average flow, while being
approximation to actual conditions, when taken with the other highly conservative
assumptions in the analysis such as unrestricted flow of the total volume
into the low-grade ore; no evapotranspiration; no dispersion or retardation;
and a constant lea hate production rate, (i.e. on the order of 10-10 mg/ml
for metals and 10-~ pCi/1 for Ra226) provided estimated concentrations in
Indian Creek that were negligibly small. Calculations using higher amounts
of inflow into the low-grade ore would have to take into account correspondingly
higher rates of run-off and flow in Indian Creek. These factors would maintain
the low concentrations.
Item: The potential effect of leachate contamination as a result of the Tow
grade ore stockpile clay cap rupturing due to settling or construction
procedures.
Res onse: As noted previously in response to one of Dr. Jim Pendleton's
geotechnical concerns, the clay cover did not enter into the calculations
of leachate concentrations. The calculations were made assuming no lower-
permeability clay cover. A rupture in the clay cover would therefore not
result in an increase in infiltration and the leachate concentrations would
remain as projected. The clay cover would, of course, reduce infiltration
into the low-grade ore material and lessen the flow-through from that used
in the hypothetical case.