SULFIDE MINERAL OXIDATION AND METAL FLUX IN THE INTERTIDAL ZONE, ELLAMAR MINE, PRINCE WILLIAM SOUND, ALASKA

The disposal of sulfide-bearing mine waste in marine and transitional-marine environments has long been practiced in some areas, but fluxes of acid and potentially toxic elements are difficult to evaluate because of rapid dispersion in marine waters. The Ellamar mine in eastern Prince William Sound, Alaska, produced 270 thousand tons of copper ore from 1900 to 1920 and ore and waste material are currently distributed as a rubble pavement in the intertidal zone near the historic Ellamar pier. Although sulfide-rich fragments containing abundant chalcopyrite, pyrite, pyrrhotite, and sphalerite are partly to completely oxidized, there is little evidence of acid or metal flux in the water column of Ellamar Bay.

In order to evaluate in situ reactions in the intertidal zone during a falling tide, eight shallow (0.3m) pits were excavated perpendicular to the shoreline over a distance of 31.2m . Six of pits in the inclined beach surface were below water-table and filled with ambient groundwater. Pit water pH ranged from 3.3 to 5.5, generally decreasing toward the shoreline where pit waters have normal marine conductivities. Al (16 mg/L) and Fe (48 mg/L) are strongly enriched in the saline, low pH waters near the shoreline, and values drop off sharply within ~12 m upslope of the low-tide line. Jarosite is the primary precipitate in the shallow subsurface at 11.7 to 20.3m above low-tide line. Ba (140 ug/L), Pb (0.8 mg/L), Tl (9.2 ug/L), and Zn (2.2 mg/L) also are strongly enriched in the saline, acidic waters near the low-tide line. Cu (2.9 mg/L) is most strongly enriched in intermediate zones.

These results show significant metal mobilization and acid production in marine intertidal zones and metal influx into Ellamar Bay may introduce significant quantities of potentially toxic elements into an important aquatic ecosystem.