GROWTH EXPERIMENTS OF ACIDITHIOBACILLUS FERROXIDANS TO RESOLVE OXIDATION RATES OF SULFIDE MINERALS AT THE ABANDONED BLUE LEDGE MINE, SISKIYOU COUNTY, CALIFORNIA

The Blue Ledge Mine in Siskiyou County, California exploited a polymetallic massive sulfide deposit for copper from 1906 to 1919, producing over 60,000 tons of crushed waste rock, most of which remains on a 20^o to 40^o slope at the site. Today, the mine site is abandoned with exposed waste rock and open shafts that readily produce acid mine drainage that has adversely affected Joe Creek downstream of the abandoned mine site. The purpose of this study is to investigate the rate of oxidation of sulfide minerals by microbial activity in laboratory experiments. Waste rock containing pyrite (FeS₂) and sphalerite (ZnS), the two dominant sulfide minerals at Blue Ledge Mine, were collected from the site, then crushed and sieved. The medium sand-sized fraction of each sample was placed in a sterile growth chamber, mixed with an ammonium sulfate medium containing pure pyrite, inoculated with Acidithiobacillus Ferroxidans, and incubated at 30°C. Control samples were also prepared following the same procedure, but were not inoculated.

Samples were removed systematically from incubation at two weeks, one month, and six months and analyzed for total dissolved solids and metal concentrations. Visual microbial growth and appearance of the solutions were noted. Solutions from the growth chambers were processed using EPA Method 3005A and concentrations of calcium, cadmium, copper, zinc, arsenic, iron, and lead were determined using an Inductively Coupled Plasma Optical Emission Spectrometer in the Analytical Chemistry Laboratory at Southern Oregon University.

Metal concentrations (except calcium) experienced a significant increase (36 to 64 percent) within the first two weeks of incubation and concentrations continued to increase steadily, but at reduced rates. Acidithiobacillus Ferroxidans is a common microbial strain present both inside the mine shafts and in waste rock piles at the Blue Ledge abandoned mine site. Quantifying their rate of oxidation of the exposed sulfide minerals on the waste rock piles further explicates the complexities of mitigating acid mine drainage from similar abandoned mine sites.