The Conjecture Mine is a historic silver and lead mine located on Gold Creek, about 6 miles south of Lakeview, Bonner County, Idaho. Gold Creek is a perennial stream that flows into Lake Pend Oreille. Mining wastes at the Conjecture Mine site are located partially on private and partially on National Forest System (NFS) lands. This action is a continuation of a comprehensive EPA/US Forest Service (USFS) approach to address mine waste contamination found in the Gold Creek watershed.
Mine operations occurred sometime after 1894 and continued periodically until 1963. Mining was conducted underground on a patented claim and adjacent NFS land, and mining wastes in the form of waste rock and mill tailings were disposed on both private and public lands. Much of the mine waste found on private land was produced intermittently from 1894 through 1951. The Conjecture Mine site is situated near the headwaters of Gold Creek and occupies more than one-third mile of Gold Creek. The upper portion of the site contains four large waste rock dumps, scattered deposits of disturbed and eroded materials, and stream sediments.
The site is mixed ownership. USFS, exercising its CERCLA authority as to the public portion of the site, entered into an Interagency Agreement with EPA to authorize work and to reimburse EPA for costs associated with cleanup of mine waste contaminated materials found on public land. EPA is addressing mine waste contaminants found on private property. Potential human health and environmental impacts from historic mine wastes present at the Conjecture Mine site include public health and safety risks, increased metal concentrations in surface water, and increased sediment load to surface water.
During the past 30 days, the following activities occurred:
- mobilized personnel and response equipment to site;
- developed site and cleanup infrastructure, including clearing and preparing office and equipment staging, operations, fuel, and storage areas, and improving existing access roads;
- cleared and grubbed approximately 1½ acres of forest land to expand the existing mine wastes repository;
- conducted site walk with USFS, USFWS, START, USCG, and EQM to ground-truth conceptual cleanup design;
- completed baseline mine waste contaminant determination of project and community roadways and established surface water quality monitoring locations;
- stockpiled woody debris removed from borrow area, camp, and repository expansion, to be used as stream bank structures and repository bio-material;
- completed removal of repository temporary cover in preparation of placement of mine waste contaminated materials;
- started screening for top soil and rock materials;
- located and installed BMPs along Gold Creek, including sediment collection basins;
- conducted two community meetings to review project history, potential site risks, cleanup activities, and construction schedule; and
- excavated approximately 7,000 cubic yards of mine waste contaminated materials from the North Waste Rock Dump (USFS) and transported the materials to the mine waste repository for condsolidation beneath a protective barrier.
Conduct removal activities as described in the project Work Plan and Conceptual Design dated 17 May 2007.
The following activities are planned for the next 30 days:
- grade the North Waste Rock Dump to provide slope stability and to manage surface water run-on and run-off;
- begin reconstruction of Gold Creek bed and banks through the North Waste Rock Dump;
- excavate an estimated 19,000 cubic yards of mine waste materials from the Southwest Waste Rock Dump;
- continue screening for top soil and rock materials, including removing bed load materials from Chloride Gulch; and
- continue air, roadways, and surface water monitoring activities and maintenance of BMPs.
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