The Boston Placer Mine is located on BLM-administered land and comprises the hydraulic mine site known as the Boston (Bunker Hill) Mine. This site is on the east side of Greenhorn Creek - Bear River drainage and is part of the ancestral Yuba River gold-bearing gravel deposit. The inlet and outlet of the short drain tunnel are connected to an elongated series of bedrock sluice cuts that are now part of an existing wetland/reservoir area. The drain tunnel currently discharges water and sediment onto adjoining U.S. Forest Service-administered lands bordering and within Greenhorn Creek. Water and sediment sample results indicate high mercury contamination levels both in the 400-foot-long sluice tunnel and in the wetlands/reservoir area, which were created by a blockage at the inlet of the tunnel. This site presents an excellent location for a pilot remediation mercury cleanup project.
Previous/Existing Assessments
The site has undergone preliminary water and sediment sampling by the USGS Water Resources Division during FY 1999-2000. Samples were collected from the mine water discharge outlet, the principal sluicebox/drain tunnel and from a seasonal wetland formed by the blocked inlet to the tunnel which is part of the main Boston Mine hydraulic pit area. Final water and sediment sample as well as biota mercury analyses are available.
Reclamation/Reommendations
1. Sluice Tunnel Characterization: Collect data on the geological, hydrological and biological resources inside the sluice tunnel and seasonal pond. This work will begin in FY 2002 and end in 2003. Special emphasis will be directed to presence, past and potential usage of the tunnel by bats. The geological and hydrological information will be used in determining the best and most cost efficient method for controlling or eliminating the production of methyl mercury. The use of GPS equipment will provide a better determination of the tunnel and mine locations with respect to current land ownership plats.
2. Seasonal Pond Characterization: Collect additional data on the use of the pond and hydraulic pit by biological resources and assess the possibilities of removing the tunnel inlet blockage which would effectively eliminate the seasonal pond, which is one of the areas the mercury is methylizing. This work is scheduled for FY 2003. Removal of the pond may require mitigation either on or off site.
3. Public Access and Use: Access to the site is unrestricted. The site is primarily used by recreational gold miners and the curious. The potential for dermal contact with elemental mercury is high. Many of the hydraulic gold mining tunnels in the watershed have collapsed or could collapse at any time. These factors are a high risk to the general public. Mine gates, using bat friendly designs, would be erected at this site to deter public access into the tunnel.
4. Land Status: A mining claimant search as well as a potential responsible party search will be done in FY 2002 to determine past and current owners and the potential for a cooperative restoration partnership.
5. Restoration Methods Being Considered:
a. Tunnel/Pond Sediment Removal
Removal of sediment located on the tunnel floor and sidewalls and in the seasonal pond area would be accomplished using a high-pressure water system beginning in the most upstream areas. The mercury contaminated sediment would be collected in a sump and vacuumed into tanks for additional treatment. Any surface flows normally flowing through the tunnel would be diverted. After all the sediment is removed, an impervious cap would be applied to prevent further deposition of mercury into the tunnel floor fractures/crevasses. Water flow through the tunnel would be restored. Water analysis would continue to assure tunnel outflow is not contaminated. This process was used in a larger tunnel and proved to be a costly process.
b. Pond and Tunnel Bioremediation
There has been some thought that using specific chemical or bacterial processes to remove or debilitate the methylization process could be a workable solution. However, additional study will be needed to support this theory. Estimated cost unknown.
c. Bypass Tunnel Water Flow
In most hydraulic placer gold mine tunnels, there is water outflow either from seepage or redirected water courses or both. The mercury rich sediments when roiled by recreational miners causes the mercury to be more available to the methylization processes. Precluding public access into the tunnels with gates and collecting the water and transporting the water through piping to the outflow end of the tunnel could control the higher outflow mercury concentrations thus stabilizing downstream mercury transport.
Environmental Risks Narrative
The Boston Hydraulic Mine is located on unpatented BLM administered land but adjoins large patented hydraulic and drift placer gold mines. The 40+ acre mine site contains a 200+ foot long sluice box tunnel containing high concentrations of elemental and methyl mercury in the tunnel sediments. The blocked inlet of the sluice tunnel created a small seasonal pond and wetland area which also contains high levels of methyl mercury in the water and biota. The sluice tunnel and several deep, vertical shafts presents a physical safety concern to the public. Skin contact with the mine water discharge could pose a human health concern to recreational miners.
Risks: Water Quality, Physical Safety
AMLIS #: CA020180003
Location: N39 13 48, W12054 41
Field Office: Folsom
Congressional District: 01
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