The Manning Mill Site is an abandoned gold milling facility in the southern Oquirrh Mountains of Utah County, about 40 miles south of Salt Lake City near the town of Fairfield.
It is estimated that about 720,000 cubic yards of tailings were left at the site when mining operations were done. This material was generated by the Manning Mill that processed gold ore from the Mercur gold mine from the year 1890 until 1937. The tailings were disposed of in tailings ponds. The old ponds later breached and allowed the tailings to wash downstream. Because the tailings and redeposited tailings were highly susceptible to wind and water erosion, they were easily washed downstream or carried off by the wind.
The Manning Canyon Site covers about 1,470 acres (about 2.3 square miles in size), and contained six well-defined tailings deposits. If spread out, these deposits would have covered 66 acres.
What was the concern at Manning Canyon Site?
Mill tailings at Manning Canyon were determined to contain high levels of arsenic, lead and mercury. All of which posed a threat to human health and the environment. Arsenic is the contaminant that raised the largest concern. It ranged from 2,000 to 12,000 parts per million. Arsenic is toxic to plants, animals and may cause cancer in humans.
Arsenic found in the tailings at Manning Canyon is 76 times higher than allowable risk levels. The tailings material would be harmful if breathed, absorbed though the skin or accidentally swallowed.
If the tailings were left, they would pose a risk to recreational visitors - especially campers and off-highway vehicle users - and could wash downstream toward Fairfield during heavy rains or flooding.
How did BLM find out about the problem at Manning Canyon?
During the 1990s, an abandoned mine land inventory was conducted by the Bureau of Land Management and the State of Utah Abandoned Mine Reclamation Program. As a result the BLM's Utah State Office recommended additional study of the Manning Canyon tailings in order to take necessary response actions to clean up contaminated mine and mill tailings on BLM administered lands.
What has been done to study the contamination at Manning Canyon?
Between 1996 and 1998, BLM and EPA conducted preliminary assessments of the tailings impoundments, downstream deposits and the local springs which led to the conclusion that a non-time critical removal of the tailings was necessary.
A Site Investigation was conducted in 1999. The tailing piles were inventoried, sampled, and analyzed for heavy metals, such as arsenic, cadmium, and mercury.
To determine what engineering alternatives existed for a cleanup and how much they would cost, an Engineering Evaluation/Cost Analysis was written (2000). The report recommended that the tailings piles be moved into one repository capped by an impermeable material and maintained by the BLM.
During the initial phases of cleanup additional contaminated tailings were discovered. BLM conducted a geologic, soil survey, and geological survey and Removal Site Investigation in the Spring of 2004. This report was followed by an Engineering Evaluation/Cost Analysis in the Winter of 2005. The report recommended to place the new tailings in an extension of the already built repository.
What actions were taken to clean up the area?
The tailings were cleaned up in four phases.
- Phase I of the work began in 2002 with the initial construction of surface diversion channels and consolidation of tailings into one repository.
- Phase II involved construction of the repository and cap.
- Phase III consisted of removal of the lower tailings area and placement into the repository which was then capped with and impermeable fabric.
- Phase IV consisted of removal of additional tailings that were discovered during the main cleanup phase. The tailings were placed in an extension of the already built repository
All of the tailings identified on BLM property in previous investigations have now been placed in the repository. The BLM is now and will continue to monitor the repository to maintain the cap and reclamation as well as determine if ground water under and around the repository is being affected. Total cost of all phases was approximately 10 million dollars. The repository will remain closed to motor vehicles and mining indefinitely to protect the reclaimed cap from being damaged.