THE GEOCHEMICAL CYCLING OF CU, FE, AND ZN IN A MEROMICTIC, MARINE PIT LAKE, CALLAHAN MINE SUPERFUND SITE, MAINE, USA

The Callahan Zn-Cu-Pb Mine in Brooksville, Maine produced ore enriched in pyrite, chalcopyrite, and sphalerite from an open pit in a dammed tidal estuary (Goose Cove) during 1968-1972. The pit was about 180-300 m wide and 97 m deep. The dam was breached in 1972 and the pit filled with seawater. To understand the seasonal hydrodynamics and geochemistry of the pit lake, temperature sensors were deployed at multiple depths and continuous temperature records were obtained from April 2007 to June 2008. The water column was sampled in April and August 2007 and June 2008. Water samples were analyzed for acid-soluble and dissolved constituents, including stable metal isotopes of Cu, Fe, and Zn. Profiles of temperature, dissolved oxygen, and dissolved sulfide indicated that the pit lake is permanently stratified, and that the redox boundary became shallower (from 63 to 47 m) during the study period. Concentrations of dissolved oxygen between 10 and 60 m decreased by 150 µM between the April and August 2007 samplings, whereas a mixing event ventilated depths between 10 and 35 m between the August 2007 and June 2008 samplings. Dissolved sulfide reached a maximum of 315 µM in the bottom water, and values of pH decreased from 7.3-7.8 in the surface water to 6.7 in the bottom water. Dissolved concentrations of Cu were < 50 pM in the surface water, increased to 75-130 pM at 20 m, and then decreased to < 2 pM below 60 m. Preliminary isotopic data from June 2008 indicated that dissolved δ⁶⁵Cu decreased from 1.6 ‰ at 10-40 m to -0.7 ‰ at 80 m. Dissolved concentrations of Fe were low in the surface water and reached a maximum of 10 µM at 50 m in June 2008, and then decreased to about 3 µM as sulfide increased. Values of dissolved δ⁵⁶Fe were about -1.6 ‰ at depths < 40 m and increased to -0.7 ‰ at depths below 60 m. Like Cu, dissolved Zn was lower (< 1.2 µM) in the surface water, increased to a maximum of 6.5 µM at 20 m in June 2008, and then decreased to < 0.03 µM below 60 m. Isotopic data from June 2008 indicated that dissolved δ⁶⁶Zn decreased from 0.4 ‰ at 40 m to ~ 0 ‰ below 65 m. Seasonal changes in metal concentrations and isotopic signatures with depth likely are due to a combination of mixing, scavenging by particles, and dissolution and precipitation of mineral phases across the redox boundary.