CERCLA Time-Critical Removal Action

The Ute Ulay Mine and Mill Site is an inactive gold, silver, lead and zinc mining and milling operation located near Lake City, Colorado. The Site includes a flotation mill with mixed tailings and waste rock piles, several open portals and a shaft. The mill tailing and waste rock piles are situated adjacent to Henson Creek. The tailing and waste rock piles are located on both privately-owned and Bureau of Land Management (BLM) lands at the Site.

The Ute Ulay Mine and Mill is located 4 miles west of the Lake City, Hinsdale County, Colorado.
 

The Site is located in an area that is subject to heavy snow with a pronounced spring snowmelt. In addition, the Site is within an avalanche hazard area. In 2011, an avalanche occurred east of the Ute Ulay Mine and Mill Site. The avalanche filled the channel, blocked the river and backed Henson Creek up to approximately 30 feet deep, submerging a section of the waste dump adjacent to the large mill tailing impoundment. A release of tailings materials occurred when Henson Creek broke through the avalanche debris. Routine run-off events cause releases to the environment of tailings and waste rock containing hazardous substances.

Sensitive ecosystem impacts may occur in the event a large mass of waste released into Henson Creek, which is a tributary to the Lake Fork of the Gunnison.  The Lake Fork River is considered a Cold Water Aquatic Life Class 1 by the state of Colorado.  Sampling and analyses conducted by CDPHE in 2000 and 2011 indicate the presence of several known contaminants of concern, especially arsenic and lead. In addition, cadmium, copper, manganese, silver, zinc and mercury have been detected in concentrations exceeding the MacDonald PEC for freshwater aquatic ecosystems guideline related to sediment contaminants. All of the materials contaminated with hazardous substances have been left unsecured in tailing and/or waste rock piles.

Based on investigation and geotechnical analysis, the EPA, DRMS and CDPHE determined that the most significant hazard at the Site is the potential for a failure of the mine waste rock dump slope and mill tailing impoundments into Henson Creek. The stability analysis demonstrates that slope failure is likely under extreme loading events such as an earthquake or rapid drawdown of contained water, and that slopes are only marginally stable in their existing conditions. The factors of safety were calculated for several cross sections on the mine waste dump and were found to be below 1.4 and as low as 0.86 for the primary waste rock slope. Generally, slope stability should be above a factor of safety of 1.5. 

This action involves the following key elements: (1) re-grading of tailings and waste rock piles to achieve an acceptable slope stability and reduce the potential for catastrophic failure into Henson Creek; (2) re-grading to promote positive Site drainage, minimize water runon, and prevent erosion into Henson Creek;  (3) as determined appropriate, amending the mill tailings with a cement mixture to stabilize wet tailings; and (4) securing the top, accessible areas of wastes with earthen cover material, as necessary to reduce human exposure. BLM-managed land and private land on-Site will provide the area necessary to allow re-grading of waste on private land to achieve stable slope conditions. Design drawings are being developed and will show the proposed grading plan to achieve acceptable slope stability.

2.1.2 Response Actions to Date

1.      The response contractor mobilized to the Site on May 21 and equipment delivery began on May 22.  Site preparations continued with Large Format photography prior to any disturbance of the site under the Removal Action.  This satisfies the required mitigation for potential damage to historically significant features  on the site.  Additional run-on and run-off controls were put into place around the work area. 

2.      Tailings removal in the lower impoundment started on 5/27 and continued into 5/28 noon. Approximately 2000 cy (banked) tailings were removed and staged for blending with waste rock. Waste was removed to the depth of native ground. No structures or other features were unearthed.

3.      Archeological monitoring by a both BLM and a certified Archeologist with Alpine Archeological Consulting hired to monitor the site disturbance inspected the site following the initial tailings removal and found that no features were disturbed.  Furthermore, the Archeological Monitor confirmed that the prior research of Tailings Impoundment showed that the only feature of concern was the flume at the south perimeter. It was determined that additional excavation will proceed until nearing a the flume structure. Notice will be provided to the Monitor to return to the site to document this feature when operations approach that structure.

4.      Waste rock blending with tailings occurred on 5/28 and 5/29.  Once an acceptable degree of mixing occurred and the waste blend was relatively homogeneous, the waste was staged for blending with cement.   Additional blending is required for portions of the tailings based on observations of moisture and pumping in the stockpile.   

5.   Waste amending was started on 5/31 with a 3% cement addition to the waste rock/tailings blend. Placement in the excavated portion of the impoundment started on May 31 and June 1.  A total of 87 truck loads were placed.

6.      Placement is occurring the off-road trucks delivering blended waste to spread with the dozer and then waste is compacted (wheel rolled) with the loaded haul trucks each 12 to 18 inch lift.  Based on observations in the field compaction is excellent.

• Waste rock slope cut and blending with tailings and cement amending in lower tailings. 
• Borrow material will be removed from the sloped above the tailings impoundment before waste is placed over the slope. 
• Tailings removal will continue as planned in the lower impoundment and replaced after blending of waste rock and cement (as needed).