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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
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| Figure 14—The lower (west) adit of the Charter Oak Mine discharges 0.4 to 10 gallons of water per minute. The extent of the workings and their relationship to other workings could not be determined |
The discharge from the lower (west) adit at the Charter Oak Mine (figure 14) was monitored with a weir (small dam used when measuring water flow) and recorder for about 1 year. The discharge ranged from a minimum of about 0.4 gallons per minute to a maximum of about 10 gallons per minute during the period of record (figure 15). The greatest discharge occurred in early May during snowmelt and storm events. A small portion of the discharge was due to runoff outside the portal, but most appeared to come from within. The discharge generally declined through the rest of the year. The rapid response to snowmelt, and possibly storm events, suggests a close proximity to the groundwater recharge area and fits well with the shallow mine workings indicated on the sketch maps (figures 12 and 13).
The main (east) adit was not monitored, but flow measurements were made during several visits, mostly during late summer and early fall. Flow from this adit is always about 10 gallons per minute or less. The discharge stream disappears into the scree within a few tens of feet from the adit. When the adit was reconstructed in 1999, gravel was used to “bury” the discharge stream before it exits the portal (figure 16). It does not resurface.
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| Figure 15—The lower (west) adit of the Charter Oak Mine discharges water throughout the year. The flow ranges from about 0.4 to 10 gallons per minute. |
The workings of the Charter Oak Mine are well above the floodplain of the Little Blackfoot River. The only surface water that is likely to be in contact with the workings is a small unnamed tributary of the Little Blackfoot River that flows through the disturbed area. The stream originates several hundred feet uphill. It flows through and around several waste-rock dumps and small pits. This stream’s flow is a direct function of snowmelt and storms. Its base flow in the late fall was estimated to be about 20 gallons per minute at a point midway through the disturbed area (figure 17). The stream is contained within a straight reconstructed channel with some ponding only in the disturbed area. Seeps emerge on the north side of the creek in the recently reclaimed area. These seeps did not have iron oxide staining, vegetation appeared normal, and based on water quality data, the seeps did not contribute metals to the creek.
A series of catch basins or trenches have been bulldozed across the face of the scree slope into which the workings were driven. The trenches serve to stop rock slides from the unvegetated hill above and limit the formation of erosional channels down the steep face. One minor result of the basins formed by these grassy benches may be the increase of snow retention. Snow is also caught in the road cuts on the top and sides of the ridge east of the mines. The portal areas of the adits (especially the collapsed area at the unnamed discharging adit) also provide an area in which snow may accumulate. This snow accumulation contributes to the water available to be discharged.
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| Figure 16—The main adit of the Charter Oak Mine is just uphill from the mill. A small discharge flows from the portal (entrance) and infiltrates the coarse material near the portal. |
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