While Canadian mining company RISE Gold Corp was promoting the gold mine potential and alluring prospects of gaining permits to reopen the Idaho-Maryland Mine (IM Mine) in Grass Valley, CA, federal and state regulatory agencies were focusing on the polluted tailings that cover most of the 56.4 acre site and taking steps leading towards a Superfund Designation.
There has been little public disclosure of the contaminated legacy IM Mine tailings, but correspondence from the EPA dated Sept 26, 2019 indicates that IM Mine’s potential designation as a Superfund site was conditionally deferred because RISE entered into a cleanup contract with the California Department of Toxic Substance Control (DTSC).[1] On August 13, 2019, RISE Gold CEO Ben Mossman signed agreements to cleanup the site, also known as the Centennial site.[2] According to the DTSC records, RISE has been dealing with this issue from at least as early as March 13, 2019, when the first Scoping Meetings with the DTSC took place.[3]
Tests conducted in 1993 by Vector Engineering showed elevated levels of arsenic, chromium, copper, mercury, lead, and nickel over two large areas of the legacy tailings, one area originating from the Mercury gold extraction processing that took place prior to 1926, and a second area being from the Cyanide extraction processing that took place from 1936 until the mine closed around 1956.[4] More recent reports from the DTSC determined that lead, arsenic, nickel, and mercury are present at hazardous levels.[5] Contaminated tailings cover roughly 2/3 of the 56.4 acres with depths ranging from 2 to 20 feet.
As an additional complication, RISE Gold’s recent bid to re-open the mine includes plans to use the site for disposal of 1.6 million tons of mine waste rock and tailings as “engineered fill” over the course of 5 years, covering 44 acres and creating a built-up area 30 to 70 feet above current grade. However, a recently posted geotechnical report indicates that the legacy tailings are not structurally adequate for use underneath the engineered fill, so they have to be completely excavated before the dumping can take place.[6] The full extent of the contamination within the tailings is not yet clear, but even if some of the tailings are clean they will all have to be extracted and then remixed with other aggregates before they would be stable enough to be built upon as planned.[7]
Rise Gold has not yet revealed what procedures will be used to get the legacy tailings off the bedrock and safely dealt with before the new waste rock and tailings from mine operations can be deposited. Nor is it clear whether the contaminated tailings will need to be sequestered separately on site, whether they can be integrated into the engineered fill, or whether they need to be trucked to a waste disposal facility.
According to the agreement, the entire remediation process will be overseen by the DTSC. However, Nevada County and other government agencies will be overseeing all operations with respect to the IM Mine re-opening permits, which is independent of the DTSC permitting and is a separate project with a separate time frame.
[1] US EPA Transmittal of Preliminary Assessment Report,Sept 26, 2019 [2] Cleanup Agreement, Signed by RISE Aug 13, 2019 [3] Ibid., Exhibit E [4] “Contaminant Assessment of the Bouma-Erickson-Toms Property”, Vector Engineering, Nov 1993. [5] Centennial Geotechnical Report, NV5 [6] Centennial Site History, DTSC [7] Idaho-Maryland Mine Project Description, Nov 2019, pg 16