The North Star Mine Site, located just southwest of Silverton, Colorado, has historically been associated with the Animas Mining District which is part of the San Juan National Forest. The Site consists of an old mill site on the east side of the mine workings. Historically, access to the mine workings was by two portals, level 6 and level 7.
Portal 6, just above level 7, is collapsed with no visual drainage. Mine drainage exits in the North Star Mine at portal level 7. Mine level 7 portal was closed sometime in the 1980s, by placing a backfill wedge into the portal which was underlain by pipes in order to maintain drainage into the constructed channel. A closure gate was placed on the portal entrance. From portal 7, a drainage channel was constructed to convey mine drainage around the west side of the large waste rock pile into Mineral Creek. The drainage channel has not been maintained and has filled with sludge, resulting in overtopping and drainage onto the waste rock pile.
The large waste rock pile, estimated at 40,000 cubic yards, fans north to northeast with a footprint of approximately 3 acres and is actively being eroded and cut away beyond its angle of repose at its terminus with Mineral Creek. The waste pile runs north to south along the south bank of Mineral Creek for approximately 300 feet. Along this reach, the waste pile has been eroded by the river resulting in a near vertical embankment of 30 to 40 feet in height. The waste rock pile has significant levels of lead (up to 9,600 mg/kg) and arsenic (up to 480 mg/kg), which is a risk to recreators. Leachable zinc of up to 2,030 ug/l is likely contributing to degradation of the stream above table value standards downstream for aquatic wildlife. A significant collapse of the pile (5,000 to 15, 000 cubic yards) into Mineral Creek could increase contributions of zinc from the Site significantly. Leachable cadmium (up to 11.7 ug/l) and aluminum (up to 240 ug/l) also would contribute to degradation of the stream.
In 2019, an active beaver colony was observed building a dam within 50 feel below the level 7 draining portal. The beaver colony was assumed to be living somewhere inside the portal gate. The beaver dam was impending flow to the drainage channel and impounding water at a depth of up to 4 feet with an estimated 250,000 gallons of impounded mine drainage water. Mine water discharged over the crest of the 60 to 70 foot-wide dam then cuts into the large waste rock pile in several places. The discharge across the waste pile resulted in increased metals and sediment loading into Mineral Creek and subsequently into the Animas River.
Elevated metals, including aluminum, cadmium, iron, manganese and zinc are present in the acid mine water discharge from the portal. Elevated concentrations of arsenic, lead and manganese are present in the large waste rock pile.
The overall objective of this removal action is to prevent water from impounding in the North Star Mine, which will preserve current structural conditions and prevent further pressure from building within the North Star Mine. In addition to the removal of a large waste rock pile, the proposed actions include:
- Removal of beaver dam and debris within - completed in 2020
- Remove soil berm inside the portal structure
- Repair the portal structure
- Install a new gate within portal structure
- Clear, repair and reroute, as necessary, the adit drainage channel from the mine to Mineral Creek
- Remove the waste rock pile sufficiently back from Mineral Creek and stabilize
- To the extent practicable, consolidate, grade, cover and vegetate the waste rock pile
- Restore areas damaged by heavy equipment to prevent erosion
- Grade the area to enhance positive drainage away from mine workings and waste pile
- Construct site run-on controls
- Assess and implement strategies that will deter beaver populations from the adit area