GEOCHEMISTRY OF A FLOODED MINE PIT WITH ANOXIC BOTTOM WATER IN AN ESTUARY, CALLAHAN MINE SUPERFUND SITE, MAINE

The Callahan Zn-Cu-Pb mine, near Harborside, Maine, produced ore from an open pit (1968 to 1972) in the restricted Goose Pond estuary that was dammed and drained during mining, but has since been flooded in 1972. Fresh surface water interacts with mine-waste piles on land and drains into the estuary. The water column in the pit (total depth ~ 90 m) was sampled three times spanning an annual cycle (April 2007, August 2007, June 2008) at up to ten depths per sampling. The water column was dominated by a permanent halocline with tidally influenced less saline water (24 to 31 ppt) down to a depth of approximately 15 m above saline bottom water (~ 35 ppt). Temperature was constant (~ 1.6 °C) below 20 m depth. A permanent redox boundary was encountered that separated oxic shallower water from anoxic bottom water at depths varying by sampling event between 47 and 63 m. The pH dropped from 7.3 to 7.8 at the surface to 6.7 below the redox boundary. Sulfate showed limited variation with depth (2300 to 2750 mg/L) even below the redox boundary. Below the redox boundary, dissolved sulfide was encountered (2.6 mg/L) and reached a concentration of 9 mg/L near the bottom. Iron above the redox boundary was dominated by particulate ferric iron (0.033 to 0.063 mg/L). Near the boundary dissolved ferrous iron, produced by reductive dissolution of ferric iron particulates, reached a maximum (0.18 to 0.59 mg/L) then decreased with depth. Dissolved Zn and Cu were the most important trace metals, reaching maximum concentrations near the halocline (0.160 and 0.005 mg/L, respectively) and minimum concentrations below the redox boundary (0.0002 and <0.0001 mg/L, respectively). Dissolved Cd (max. 0.0009 mg/L) showed similar variations to those of Zn and Cu. Lead was dominated by particulate species (< 0.001 mg/L) and showed a steady decrease with depth. The stagnant water beneath the permanent halocline acts as a physical trap for particulates, although some bypass the pit in the shallow, tidally influenced zone. The halocline also permits the establishment of anoxic bottom water with significant dissolved sulfide due to bacterial sulfate reduction. The resulting sulfide scavenges and sequesters trace metals from the water column. Thus, the flooded pit acts as a stable physical and chemical sink for metals in the estuary.