The Bonanza Mine is an abandoned historical mercury mine and mill that operated intermittently from the mid-1860s through 1960. The main mercury-containing mineral is cinnabar, although metacinnabar and native mercury were also reported in the mine workings. Total recorded mercury production was 39,540 flasks (or 3,005,040 pounds).
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Except for one former building used as a residence, other mine and mill buildings are no longer present, leaving only the mill concrete foundations, calcine, and waste rock. The mine had 12 adits and more than three miles of subterranean tunnels and shafts.
Substantial environmental information exists about the Site. Numerous environmental investigations beginning in the late 1990s have been completed at or near the Site, and these investigations show that soil and sediment are contaminated by mercury, arsenic, and other metals, and that the source of these metals is from historic mercury mining, processing, and disposal operations.
During 2014, EPA will perform a removal action to mitigate the potential human health and ecological threats posed by exposure to mercury and arsenic. The removal action will consist of: closure of two adits; excavation of approximately 11,000 cubic yards of mine-waste contaminated materials from homesites, driveways, roadways, and an intermittant tributary; consolidation of excavated mine-waste materials, along with other ancillary debris against the calcine and waste rock piles; and construction of a protective barrier consisting of an impermeable geomembrane liner and a minimum of 24 inches of clean rock and/or soil placed over the contaminated material.