GENERAL47948

• Permit No: C1981013
• IBM Index Class Name: General Documents
• Doc Date: 12/15/1982
• Doc Name: Proposed Decision and Findings of Compliance
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

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At the Allen Mine site, several historic and existing coal refuse piles are
located on the alluvial floodplain of the Middle Fork of the Purgatoire
River. Seepage from the refuse piles enters the stream-alluvial aquifer
system where it can affect water quality. An analysis of leachate from the
Allen Mine waste pile (Exhibit 6, Table 13) shows this water to be a
sodium-sulfate, bicarbonate type water with a total dissolved solids
concentration of 2599 mg/1, and a pH of 8.0. Trace metal concentrations were
low. Under low flow conditions, seepage from 16.6 acres of waste pile (8.6
acres current pile, 8.0 acres historic pile to be reused) would increase total
dissolved. solids in the river by approximately 16 mg/1. In combination with
discharge from the Allen underground workings, this results in a total
increase of 48 mg/1 at low flow. Under average or high flow conditions, the
increase would be less.
After the cessation of mining operations, mine water from the Allen Mine would
no longer be discharged, and the abandoned workings would fill with water, a
process taking up to 80 years. The piezometric surface would partially
recover. Because the underground workings lie 400 to 800 feet below the
surface, the piezometric surface of the coal aquifer would not be expected to
rise to a level where it would intersect the ground surface and thus cause
discharge. A permanent depression in the piezometric surface would exist in
the vicinity of the flooded mine workings. Given the assumption that the
Allen seam is hydrologically similar to the Maxwell seam, the depression may
extend to a distance of 2 to 3 miles from the edge of the mine workings.
When pumping .of water from the workings is discontinued, the water that
accumulates is expected to be of lower quality than that currently pumped due
to increased residence time (Exhibit 6, Table 5). The quality of the stagnant
mine water should be similar to the quality measured below the Allen waste
piles (Exhibit 6, Table 13}. Total dissolved solids are expected to equal or
exceed 2600 mg/l. This mine water would spread as a plume of degraded quality
ground water down gradient from the mine. Based on the structural contours of
the coal seam, water passing through the flooded workings would move in a
northeasterly direction. It may spread to approximately 2 to 3 miles from the
edge of the mine workings. It is anticipated that soluble materials would be
readiiy flushed from the mine cavity, but flow would be very slow away from
the mine due to the low permeabilities of the coal, causing the flushing
action to persist over an extended period of time. Since the coal seam
aquifer contains poor quality water, 806 to 1100 mg/1, and is not used as a
water supply in the general area, mining within this aquifer will not
materially damage the quality of ground water used off site.
Post-mining effects of the Allen Mine on the water quality in the
stream-alluvial aquifer system would be expected to be less than during
operations, because mine water would no longer be discharged from the
underground workings. Therefore, seepage from the refuse piles would be the
only source of pollutant and would increase the dissolved load during low
flow in the Middle Fork of the Purgatoire River by approximately 16 mg/1
(Exhibit 6, Table 7 of the permit application). This increase would be much
less under average or high flow conditions.