The Site is a residential property along Fourmile Creek in the Front Range of the Rocky Mountains.  The property contains, at best estimate, 30,000-40,000 cubic yards of re-processed mill tailings that were deposited in the floodplain sometime prior to 1950.

The Site is located along Fourmile Creek immediately downstream of Salina, Colorado. It is approximately seven miles west of Boulder, Colorado, along Fourmile Canyon Drive. The latitude is 40.050869, and the longitude is -105.370810. The Site is at an elevation of roughly 6,500 feet.

The mill tailings contain elevated levels of arsenic and other heavy metals which are harmful to human health and aquatic ecosystems. Several downstream residential properties obtain water directly from the creek or from shallow fluvial wells. There is also a community water diversion downstream of the Site.

Continued erosion of the mill tailings, resulting in the release of hazardous materials into the creek, threatens residential water supplies and adversely impacts Fourmile Creek's aquatic ecosystem. Due to a severe wildfire that occurred in the area in 2010, Fourmile Creek is now especially prone to flash flooding.  This flash flooding could result in a mass failure of mill tailings into the creek. 

The tailings deposit was initially estimated to be 30,000 - 40,000 cubic yards in size and contains elevated levels of heavy metals, most notably arsenic. The average concentration of arsenic is approximately 200 mg/kg with some locations exceeding 2,000 mg/kg (EPA's screening level for arsenic is 50 mg/kg).  This contaminated material readily and regularly migrates from the deposit into Fourmile Creek.

The following criteria exist for the initiation of a removal action in accordance with 40 CFR 300.415(b)(2) of the National Contingency Plan:

• Actual or potential exposure to nearby human populations, animals or the food chain from hazardous substances or pollutants or contaminants;
• Actual or potential contamination of drinking water supplies or sensitive ecosystems;
• High levels of hazardous substances or pollutants or contaminants in soils largely at or near the surface that may migrate;
• Weather conditions that may cause hazardous substances or pollutants or contaminants to migrate or be released;
• The lack of availability of other appropriate federal or state response mechanisms to respond to the release.

Work must be performed to stabilize the mill tailings before the upcoming spring snowmelt to prevent additional erosion of the tailings into Fourmile Creek.

1. Stabilizing the tailings in place.
2. Removing and disposing of a portion of the tailings and reconfiguring the remaining material so that it could be stabilized on Site
3. Completely removing and disposing of the the tailings to the extent practical.

- Crew finished excavation of final lift of rocky/tailings layer and completed final T&D of all tailings.
- Crew worked with Boulder County Engineers for the construction of the new stream channel including habitat pools, boulder clusters, and riffle drop structures.
- Crew armored slope against the road with rock.
- Crew finished backfilling excavated areas with clean fill. 
- Western ramp providing Site access was removed, while eastern ramp was only partially removed to provide access to the newly installed fire hydrant, per the property owner and Four Mile Fire Dept.
- Re-vegetation was completed including composting, hydroseeding, and tree planting. 


EPA's Superfund Technical Assessment & Response Team (START) contractor provided Site documentation services.  START also provided sampling and monitoring support.

EPA met with the newly formed Four Mile Canyon Watershed Coalition group to present on Site progress.

2.2.1.1 Planned Response Activities