Formatted Title

Tailings Ponds Mine Site Restoration at High Elevations in the Colorado Rockies

Background/Objectives

The Mineral Park Settling Ponds located in the high sierras of Colorado at an elevation of approximately 11,550 feet above sea level encompassed 1.7 acres of impacted area featuring five man-made depressions or basins (settling ponds) that were devoid of vegetation, though well-vegetated berms formed the boundaries of each pond. Historical sampling of these ponds show that heavy metals were present in concentrations greatly exceeding maximum allowable concentration levels. Analytical samples collected from the largest pond revealed excessive levels of Ag, Al, Cu, and Pb [(silver, aluminum, copper, and lead)]. Based on human health risk assessment, the ponds were identified to be impacted with actionable levels of lead requiring removal action. Lead ranged from 2,230 mg/kg to 5,850 mg/kg, compared to 322 mg/kg in the background sample location.

Approach/Activities

An engineering and cost analysis completed by Trout Unlimited (TU) under the direction and oversight of the USFS and in accordance with Environmental Protection Agency (EPA)/540/F-94/009 “Guidance on Conducting Non-Time-Critical Removal Actions Under CERCLA” (EPA, 1993) determined that in situ stabilization and capping was the most cost effective and viable solution compared to phytostabilization, off-site repository, or no action (natural attenuation).

TU performed a field-scale pilot test in eight 24 ft² test plots in early October 2020 utilizing 200 lbs of sequestration reagents. Reagents were applied at a dosage ranging from 1.0% to 4.5% by weight of soil. Toxicity characteristic leaching procedure (TCLP) testing was used to evaluate performance of the Pb stabilization. Stabilization pilot trials confirmed that MTS^® addition to soil reduced leachable lead concentrations to below CDPHE SL of 5 μg/L resulting in 100% reduction of leachable lead at a reagent dosage of 2.0 wt. %.

Full-scale remediation encompassed treatment depths of approximately 2 feet within tailing pond sediments with a total volume of approximately 2,700 cubic yards.  Sequestration reagents were applied at a dosage rate of approximately 2 wt% utilizing a bulldozer and long reach excavator for mixing while adding water to the mixing to minimize dusting while also activating the chemistry resulting in the mineralization of lead and reduction in leachability. 100% reduction of lead was achieved in order to continue with the reclamation scope including landscaping and erosion control features, installation of trees, and shrubs. Visual monitoring of reclamation activities were performed for 2 years. 

Results/Lessons Learned

Vegetation took root quickly within the first year post rehabilitation and expect to have updated imagery and documentation of slope stability, drainage control, and foliage growth towards achieving site reclamation goals and long-term sustainable protection of perenial stream runoff.