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:
- completed excavation of an additional 500 cubic yards of mine waste contaminated materials, for a total of approximately 7,500 cubic yards, from the North Waste Rock Dump and transported the materials to the mine waste repository for consolidation beneath a protective barrier;
- completed back-hauling and grading approximately 11,000 cubic yards of topsoil to the North Waste Rock Dump;
- completed reconstruction of approximately 700 linear feet of Gold Creek stream banks, including placing instream structures, through the North Waste Rock Dump;
- excavated approximately 20,000 cubic yards of mine waste contaminated materials from the Southwest Waste Rock Dump and transported the materials to the mine waste repository for consolidation beneath a protective barrier;
- excavated approximately 7,000 cubic yards of mine waste contaminated materials from Chloride Gulch and transported the materials to the mine waste repository for consolidation beneath a protective barrier;
- continued air, roadways, and surface water monitoring activities and maintenance of roadways; and
- continued screening for top soil and rock materials.
All project activities are scheduled for completion within the next 30 days, including: excavation of all mine waste contaminated materials with consolidation at the mine waste repository; installation of the repository protective barrier consisting of soil and an HDPE liner; stabilization of all disturbed areas; and obliteration of temporary access roads.
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