PHYSICAL AND CHEMICAL LIMNOLOGY OF AN ACIDIC PIT LAKE AT THE ELIZABETH COPPER MINE SUPERFUND SITE, VERMONT

The Elizabeth mine offers unique opportunities to investigate the interplay among sulfide oxidation, photoreduction, and other geochemical processes in the geochemical evolution of an acidic pit lake because of physical characteristics of the lake. The mine exploited a steeply dipping massive sulfide orebody. Ores comprised pyrrhotite and chalcopyrite and were hosted by siliciclastic sedimentary rocks and amphibolites. An open pit is filled by a long (380 m), narrow (< 25 m), and shallow (< 8 m) lake, which is divided into two parts by a rockslide. Half of the northern part is sheltered by overhanging rock. The southern end serves as a decant point, and discharges for most of the year. Ca (25 - 60 mg/L), Mg (4.4 - 10.5 mg/L), K (2.9 - 7.7 mg/L), SiO₂ (3.5 - 8.4 mg/L), and sulfate (92 - 240 mg/L) are the major dissolved constituents of surface waters; Fe (0.3 - 0.7 mg/L), Al (1.0 - 1.9 mg/L), Cu (0.5 - 0.7 mg/L), and Zn (0.2 - 0.3 mg/L) are important minor dissolved constituents. Depth profiles from June 2003 reveal a homogeneous water column with minor differences between the north and south in pH, specific conductance, and dissolved oxygen; pH shows slight increases with depth in both parts. The pH, specific conductance, and dissolved oxygen in the north averaged 3.5 ± 0.2, 0.50 ± 0.01 mS/cm, and 8.5 ± 0.4 mg/L, respectively, whereas those in the south averaged 3.9 ± 0.1, 0.50 ± 0.01 mS/cm, and 8.0 ± 0.2 mg/L, respectively. The temperature of the sheltered northern part ranged from 15.9 at the surface to 8.8 °C at the bottom (4.3 m), whereas that in the southern part ranged from 23.0 at the surface to 14.9 °C at the bottom (6.4 m). The oxidation-reduction potential of the water column was fairly homogeneous with depth, but was significantly different between the sheltered north (550 ± 4 mv) and the exposed south (506 ± 3 mv), probably reflecting enhanced photoreduction of iron in the south.