BIOGEOCHEMICAL PROCESSES IN THE NATURAL ATTENUATION OF ACID-CONTAMINATED GROUNDWATER AT DAVIS PYRITE MINE IN ROWE, MASSACHUSETTS

Davis Pyrite Mine in Rowe, MA, collapsed in 1910 and was abandoned shortly thereafter. Since that time, acidic runoff from the flooded mine shafts and saturated tailings piles has attained a state of dynamic equilibrium such that pH and dissolved solids have remained relatively constant. This self-regulation, together with evidence of sulfate reduction in groundwater, indicates that the mine effluent may be remediated naturally by biogeochemical processes. Such processes may offer a means to an environmentally friendly restoration of the site, avoiding a costly and labor-intensive engineered infrastructure. A multi-disciplinary project studying microbiological, hydrogeological and geochemical factors leading to a natural decrease of acidity, sulfate, iron and other dissolved trace metals began in 2003. Initial observations suggest acidic drainage through waste rock piles is modified by 1) microbial communities that reduce sulfate or Fe(III) levels, thereby raising pH and sequestering trace metals as sulfides, 2) uncontaminated groundwater flowing through bedrock fractures that dilutes the acidity and forces precipitation of many contaminants, or 3) weathering of silicate minerals in the soil or bedrock that neutralize acidic water.

New data from multi-level wells monitored since April 2003 show a relatively narrow contaminated groundwater plume confined by bedrock on either side of the drainage channel. There are consistent differences in chemical composition between groundwater flowing through the bedrock and shallower groundwater regardless of seasonal fluctuations. The greatest contamination is in shallower groundwater in tailings piles, which contains >900mg/L sulfate, >100mg/L iron and pH < 3. Less contaminated groundwater averaging <40mg/L sulfate, <5mg/L iron and pH > 4.7 is consistently present in wells on the periphery of exposed tailings piles. Sulfate reduction by biochemical processes is suggested by seasonally consistent lower ORP, acidity and dissolved metals in groundwater flowing through bedrock. Moreover, anaerobic, sulfate-reducing bacteria have recently been identified at Davis Mine. The long-term goal of the Davis Mine Project is to identify factors contributing to the natural remediation of acidic drainage in the hopes they can be encouraged at other mining sites.