MWH was responsible for the design and construction oversight for a removal action at an historic underground uranium mine in Cibola County, New Mexico. The mine is located on U.S. Forest Service property in an area with limited use for recreation and cattle grazing. The design and construction was performed in accordance with a Unilateral Administrative Order and Statement of Work issued by the United States Forest Service, for a non-time critical removal action under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).
The mine area included approximately 80 acres of mining-disturbed land, a waste rock pile of 60,000 cubic yards covering approximately 10 acres, an abandoned mine shaft, mining debris, and impacted soils and arroyos.
The project team performed a comprehensive site characterization prior to the removal action, in order to characterize the site and obtain data for the design. Radiological site characterization identified areas of the site above the designated soil cleanup limit, and delineated the extent of the removal action area. Vegetation analogs were developed for post-construction revegetation.
The specific removal action objectives for the site were set by the U.S. Forest Service and included reducing risk of gamma radiation exposure to human and animal receptors, and minimizing the release of waste material to surface water, air and environmental receptors. The selected remedy for the site included consolidating mine waste materials into an on-site repository and capping the repository with an evapotranspirative (ET) cover. Further performance standards were set by the U.S. Forest Service specific to the ET cover, including
- Minimizing infiltration into the underlying mine waste
- Minimizing erosion of the cover system and develop a vegetated cover system such that the ET cover is fully functional and requires minimal active maintenance for at least 200 years
Reclaiming an historic uranium mine in an arid environment subject to extreme temperature fluctuations and flash flooding presents unique challenges for design and construction. The repository and construction at the surrounding site were designed for long-term stability (greater than 200 years) to withstand erosive forces of wind and water. Diversion channels were designed to control surface water and provide protection of the repository from run-on. The repository the removal action were designed to blend in aesthetically and functionally with the surrounding landscape, and to sustain native vegetation. The site was designed and constructed to require minimal maintenance after vegetation is established.
MWH prepared the removal action design for regulatory review and approval, and subsequent construction. The site demanded design and construction flexibility because the total volume of mine waste present at the site was unknown, and could not be determined until the majority of contaminated materials were excavated and placed in the repository. Full-time construction oversight in the field streamlined communication between MWH and the construction contractor, resulting in minimal schedule delays resulting from changing field conditions and revised construction plans.
An aggressive schedule was set for design and construction. The entire project, from submittal of the Work Plan, through acceptance of the Construction Completion Report, lasted 21 months.
- Site characterization
- Grading and drainage design for removal action
- Development of construction bid packages & contractor selection
- Engineering support during construction
- Construction oversight
- Owner’s representative services
- Innovative technologies used during design and construction included designing a reclaimed mine site that blends in functionally and aesthetically with surrounding natural topography, while still providing erosion control and minimal long-term active maintenance.
- Aggressive schedule for design and construction and adherence to the set schedule
- The design was optimized in the field during construction to incorporate construction methods and accommodate unknown conditions encountered during construction, which resulted in minimal impacts to the construction schedule.
- Routinely met with regulatory agencies during construction to ensure design and construction methods met regulatory standards.
Feedback from the Client and Regulating Agency:
- “Excellent effort on the report. MWH did a great job of preparing a thorough document while providing a concise summary of the key details in the upfront text.” (client)
- “… the draft Construction Completion Report is excellent and quite thorough in documenting the events of the removal action.” (client)
- “Thanks for all your efforts every step along the way to make this project a success. The planning and communications have been outstanding.” (client)
- “Thanks for a most excellent tour of the project site. Everyone was very impressed with the work, and the slopes and riprap were picture perfect.” (USFS)
- “This is certainly the best looking project I have seen as OSC and I have been very impressed with the work that MWH and Envirocon has done on this project.” (USFS)
- Cibola County, New Mexico, United States
- 35.0947557, -107.8583868