Environmental management of a legacy mine site in Colorado, USA

Asarco Silverton wetland fen complex composed of intermediate and moderate-rich fen communities Crop

Asarco Silverton wetland fen complex composed of intermediate and moderate-rich fen communities Crop



David J. Heinze

PE, Principal
T: +1 303 382 5474

Randy Mandel

Technical Lead, Biodiversity and Restoration Ecology
T: +1 303 382 5461

The Asarco Silverton mine site is situated just northeast of the town of Silverton. The site includes 86 patented mining claims, lodes and associated water rights along the Animas River, around the Aspen Mine, along Arrastra Creek, in Little Giant Basin, around Silver Lake and on Kendall Mountain. 

Most of these properties are located at high altitudes in the San Juan Mountains, with some exceeding 10,000 feet, and in areas not accessible by vehicles. The principal river in the Silverton area is the Animas, which has been impacted by historic mining activity covering about 70 sections, or 45,000 acres of its drainage upstream of Silverton. Asarco property holdings total nearly 1,000 acres upstream of Silverton, or about 2% of the river drainage mining area upstream of Silverton. 

In 2009 Ramboll assisted during bankruptcy proceedings for the Asarco parent company. The Asarco Multi-State Environmental Custodial Trust received title to the Silverton site in December 2009. Ramboll was retained by the Trust to manage the environmental aspects of the site.  

Biological assessment for threatened and endangered species wetland delineation

Ramboll is currently leading the biological assessment for the mine reclamation and wetland delineation for one of the mine claims that is part of the Bonita Peak Mining District (BPMD) Superfund Site. The site includes a 3.6-acre wetland complex with intermediate sloping fens, scrub-shrub and emergent wetland communities. The wetland delineation was conducted in accordance with the US Army Corps of Engineers (USACE) 1987 Wetlands Delineation Manual and the Western Mountains, Valleys and Coast Region Regional Supplement. 

The dense vegetative understory was composed of diverse and rare hydrophytic bryophytes, graminoids and herbs with an overstory of Englemann spruce. Based on the nature of the sloping fen, the wetland boundary delineation was complex with meandering open water channels that would disappear belowground and, again, become exposed further downslope.

Because of the complexity of the wetland boundary and poor satellite imagery, the project team paired the use of sub-meter GPS units with drone orthoimagey to accurately understand and delineate the multifaceted wetland communities and their associated boundaries. 

In addition, the site required a full biological assessment to understand if threatened and endangered species would be impacted by superfund remediations efforts. State, federal, and Bureau of Land Management threatened and endangered species were included as part of the survey and assessment effort. Findings were documented in a biological assessment for the client and the US Fish and Wildlife Service as part of an Endangered Species Act Section 7 consultation. Since typical regulatory laws do not apply to superfund sites, Ramboll is working with the lead state and federal agencies to properly plan and implement a remediation effort that takes into account the rare wetland communities and wildlife habitats observed on site.  

Environmental management of a mine site

Responsibilities have included: site management and administration, monitoring of activities by the Colorado Department of Public Health, the USEPA and various stakeholder and community groups, annual site reconnaissance, oversight of mine safety closures, site characterization activities and reporting including incorporating the information into a Remedial Investigation (RI) report and representation at public meetings for the Trust.

Acid mine drainage 

Ramboll performed characterization of two waste rock piles on the mine claim that is part of a Superfund Site and is directing bench-scale treatability studies to evaluate in-situ fixation or biological stabilization of the waste rock piles. 

The characterization determined that the waste rock piles have the potential to generate acid mine drainage, creating conditions for the transport of metals from the piles into the environment. Ramboll is overseeing the bench-scale treatability studies being performed by the Colorado School of Mines and Linkan Engineering for the two different waste rock piles. The information will be used to design site reclamation to be protective of human health and the environment.  

Remote site reconnaissance

Ramboll’s Galago service undertook an unmanned aerial vehicle survey of the waste rock piles, collecting hundreds of individual aerial images. Galago processed these images to create a single high-resolution image that was used to assist in wetland delineation. The images were processed using photogrammetric methods to create a digital terrain model used to generate site topography and calculate volume estimates of the waste rock piles. 

In addition, Ramboll collected high-resolution oblique aerial images of the site. Oblique images provide views similar to images taken from a helicopter or plane and provide a more visually appealing perspective than ortho imagery (top down). These oblique images will be used to assess line-of-sight characteristics of features across the site to assist in implementation and planning of future remedial work and presentation of information to the agencies and residents of Silverton and San Juan County.

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