ISOTOPE TRACERS AND METAL TRANSFER IN A MARINE ESTUARY ZONE: THE CALLAHAN MINE US-EPA SUPERFUND SITE, MAINE

Pb, Sr, and Nd isotope variations and mineralogical and trace element data chronicle metal redistribution among host rocks, ore, waste piles, estuary sediments and aquatic biota at the Callahan Zn-Cu-Pb mine. The open-pit mine produced ~800,000 tons of ore from massive sulfide hosted by Cambrian volcanic rocks. About 5 million tons of sulfide-bearing waste and unprocessed ore remain on-site providing a unique opportunity to establish impacts of mining in an estuary environment. The waste piles contain galena, pyrite enriched in Cu, Pb, and As, sphalerite enriched in Cu, Pb, and Cd, and jarosite, anglesite, and goethite. Sediment cores from open estuary ponds at the site contain <1-3% pyrite (framboidal), goethite, rare sphalerite, and have variable contents of Zn (88-1590 ppm), Cu (14-268 ppm), Pb (4-1070 ppm), As (4-55 ppm), and Cd (up to 83 ppm). Bivalve shells from estuary sediment cores have variable contents of Zn (6-443 ppm), Cu (5-74 ppm), and Pb (1-76 ppm). Clam tissue has 0.71 to 43 ppm Pb (US-EPA, 2008); some values exceed the US-FDA safety tolerance level (1.7 ppm Pb, ATSDR). Pb, Sr and Nd isotope compositions vary widely, linking host rocks, ore, sediment and clams shells. For example, Pb isotopic signatures of sediments (²⁰⁸Pb/²⁰⁷Pb=2.4223-2.4696; ²⁰⁶Pb/²⁰⁷Pb=1.1544-1.1965) span a range from host rocks (²⁰⁸Pb/²⁰⁷Pb=2.4817-2.5466; ²⁰⁶Pb/²⁰⁷Pb=1.2254-1.2537), massive sulfide, and waste (²⁰⁸Pb/²⁰⁷Pb=2.4224-2.4350; ²⁰⁶Pb/²⁰⁷Pb=1.1566-1.1646). This isotopic range also overlaps signatures of bivalve shells (²⁰⁸Pb/²⁰⁷Pb ~ 2.43-2.45, ²⁰⁶Pb/²⁰⁷Pb ~ 1.16-1.18) and water from the former open pit and adjacent to the site. The bivalves likely incorporated Pb and other metals by ingestion of fine particulate matter in addition to dissolved metals. The chemical and isotopic variations in estuary sediments reflect core depth, bulk mineralogy, and proximity to waste piles. The Pb isotope and trace element data show a link between mining and ore processing activities and the fate of metals that are redistributed in estuary sediments and aquatic biota. Within the site and immediately adjacent (0-2 km), sediment and shells show elevated metals, however, less than 4 km away from the mine site, the impact of mining is essentially indistinguishable from geologic background.