IN SITU AND LABORATORY STUDIES OF NATURAL ATTENUATION OF ACID MINE DRAINAGE

Keywords: Acid mine drainage, in situ microcosms, natural attenuation

Once the largest pyrite mine in Massachusetts, Davis Mine is located 4 miles north of the town of Charlemont. The primary sulfide minerals in the ore are pyrite, sphalerite, chalcopyrite; with trace amounts of pyrrhotite, galena and covellite. Bacterially mediated pyrite oxidation generates acidic drainage (pH 2.4 – 4.0) with high concentrations of SO₄^2-, Fe_total, Zn²⁺, Cu²⁺, and Pb²⁺. The goal of this multidisciplinary investigation of Davis Mine is to evaluate the biogeochemical processes of natural attenuation. This paper reports the results of our studies using microcosms representing subsurface conditions.

In situ microcosms (ISMs) were installed into the aquifer materials at the field site. Laboratory batch microcosms (LBMs) were constructed using site groundwater and sediments. Bromide was added to the three ISMs as a tracer. Glycerol, nitrogen, and phosphorous were added to ISM 1 and formaldehyde was added to ISM 2, which served as a killed control. ISM 3 served as an unaugmented control. LBMs were prepared in triplicate with killed controls and incubated at 16 ^oC. Water samples from the microcosms were monitored for pH, Fe (II), Fe (III), sulfate, ORP, Br^- and DOC for 50 days. A decrease in Br^- was detected in all ISMs, due to dilution with groundwater. ISM 1 showed that addition of glycerol and nutrients benefits biological activity (average pH 6.38, ΔORP = -345 mV, ΔDOC = -12 mg/L). In the killed control (ISM 2), the pH remained acidic (average pH 4.32) and the ORP remained near 122 mV. Although ISM 3 was not augmented with glycerol and nutrients, the pH increased 0.79 units and ORP decreased 326 mV. DOC followed similar trends to Br^-. The LBMs had similar behavior, with highest increases in pH and decreases in ORP in the glycerol and nutrient ammended microcosms.