PRECIPITATION OF TRACE METALS FROM STREAMWATER DRAINING AN ABANDONED SULFIDE MINE IN ROWE, MASSACHUSETTS

Davis Mine, once a productive pyrite mine, is presently an acid mine-drainage site located within northwestern Massachusetts. Water flowing from the abandoned mine shaft is highly acidic (pH ~3) and carries large loads of trace metals downstream. These metals precipitate out as part of iron-rich rock coatings along the bed of Davis Mine Brook. To determine bioavailability of the metals, a modified sequential extraction technique removed four fractions from the rock coatings, corresponding to exchangeable, acid-extractable, oxyhydroxide-bound, and organic-bound metals. These fractions were analyzed for concentrations of zinc, aluminum, manganese, copper, and lead using ICP-OES.

Aluminum is the most abundant trace element in the coating (average 2,080 mg/kg) followed by zinc (1,489 mg/kg), manganese (1,428 mg/kg), lead (803 mg/kg), and copper (257 mg/kg). The various metals have principal associations in different extractions. Zinc and manganese are bound primarily in the exchangeable fraction. Lead is associated mostly with the acid-extractable fraction. Aluminum, manganese, and lead are found in the oxyhydroxide fraction, and the organic-bound fraction contains a large proportion of aluminum and zinc. For all extractions, the highest concentrations occur at sampling site A4, which is upstream of the point where the mine effluent enters Davis Mine Brook. The concentrations of aluminum and copper increase with distance for about 500 m from the confluence with the mine effluent. However, zinc and manganese for all extractions had concentration maxima at site Well 2, about 300 m downstream from the confluence. Lead concentrations do not vary systematically downstream.

Zinc, manganese, and copper have high bioavailability, since they are held primarily in the fractions that can be remobilized under slight changes in pH. Aluminum and lead are less available, but can be released into the stream water by changes in redox potential. The highest concentrations of metals are found in areas where the direct impact of the mine drainage is low, perhaps because of the low total mass of precipitate at these locations.