STIMULATING THE NATURAL REMEDIATION OF ACID-MINE DRAINAGE (AMD) AT DAVIS MINE, ROWE MA

Acid runoff from the rapid oxidation of exposed sulfide minerals is a problematic phenomenon in many parts of the world, most commonly seen as acid mine-drainage (AMD) from open-pit mines for coal, gold, and other sulfidic ores. Reversing or mitigating the generation of the acidity and the subsequent transport of metals in the runoff has been a principal goal in recent decades. Most of the extant remedies involve substantial engineering efforts such as elaborate pump-and-treat systems, constructed wetlands, or permeable reactive barriers (PRB). Although these can be effective, their utility is limited to large mining operations, they are expensive and labor-intensive to operate, and the resultant alteration to the landscape may be objectionable. Such solutions are also not appropriate or desirable in locations where the phenomenon is caused by natural seepages from sulfide-bearing lithologies (acid rock-drainage, or ARD).

Davis Mine in western Massachusetts is an abandoned pyrite mine that has been discharging acidic effluent for 100 years from a small (0.3 ha) area into an adjacent brook that has been rendered toxic to normal aquatic biota. Studies of the microbial populations within the mine area demonstrate that iron-and sulfate-reducing bacteria are present in stream sediments and shallow soils. Laboratory experiments have stimulated these bacteria to precipitate metal sulfides in controlled laboratory experiments by adding select organic compounds (glycerol or Postgate’s solution) to closed microcosms. The pH rises and the redox potential decreases as a result. These outcomes have been successfully duplicated in a groundwater field experiment whereby several boreholes surrounding a monitoring well were filled with a mixture of cow manure, shrimp compost and pelleted dolomite. After approximately eight months of equilibration, groundwater chemistry exhibited a distinct rise in pH (from 3.5 to 4.9), a decrease in redox potential (from 500 mV to 300 mV), and a concomitant drop in concentration of Fe and some trace elements. Further testing will be needed to confirm the stimulation of Fe or SO₄-reducing bacteria as the ultimate cause of the changes, but this technique has promise for a simple and effective way of enhancing the natural attenuation of AMD.