ACID MINE DRAINAGE AND ASSESSMENT OF RECENT REMEDIATION EFFORTS AT THE BLUE LEDGE MINE, SISKIYOU COUNTY, CALIFORNIA

The Blue Ledge Mine in Siskiyou County, California exploited a polymetallic massive sulfide deposit from 1906 to 1919, producing over 60,000 tons of waste rock, most of which remains on a 20^o to 40^o slope at the site. The massive sulfides are composed primarily of brecciated pyrite and chalcopyrite with lesser amounts of arsenopyrite, bornite, galena, pyrrhotite and sphalerite. Standing pools of water within abandoned adits of the Blue Ledge Mine have pH values ranging from 1.2 to 3.5, total dissolved solids that vary seasonally from 300 to over 2000 ppm, and temperatures between 10^o and 20^oC. Geochemical data collected bi-weekly from effluent in the adits and the surface waters around the mine have established a seasonal trend in the metal concentrations, pH, and total dissolved solids. During the dry summer months, there is an increase in acidity, with the lowest pH measured at 1.2, and an increase in the concentrations of As, Cd, Cu, Fe, Pb, and Zn in standing pools of water within the adits. At the same time, there is no water movement in the run-off channels from the mine except during rare summer rain events. Longer duration precipitation events beginning in October flush concentrated acid mine drainage (AMD) from the adits, along with remobilizing precipitated sulfates from the waste rock piles, into run-off channels. These run-off channels empty into nearby Joe Creek, lowering the pH and inducing toxic metal loading in the stream. The high gradient of Joe Creek rapidly oxygenates the water resulting in the precipitation of numerous iron oxides and hydroxides in the streambed.

In September 2006, the Environmental Protection Agency and U. S. Forest Service performed remediation efforts at the site. The waste rock piles were terraced and material moved to create a settling pond and direct AMD into a run-off channel. Rip-rap in the run-off channel consists of marble boulders which became coated with iron precipitates in less than three weeks. Unfortunately, monitoring of water chemistry of AMD from the Blue Ledge Mine indicates minimal impact of the remediation to date. This may be due to the overwhelming release of AMD from the disturbance of the waste rock piles during remediation and/or the large size of the rip-rap. Future monitoring of the geochemistry of AMD from this site will be used to develop future remediation designs.