ENVIRONMENTAL GEOCHEMISTRY OF TRACE ELEMENTS ASSOCIATED WITH ABANDONED GOLD MINES, UNGA ISLAND, ALASKA

An investigation of the environmental geochemistry of water and sediment at the abandoned Apollo and Sitka gold mine sites on Unga Island, Alaska was undertaken in order to understand the impact of the mine workings and tailings on the environment. Both of the abandoned mines are located in the same watershed which contains two anadromous streams that empty into Delarof Harbor.

Water, streambed sediment, and precipitates were collected from seven streams at and near the mine sites and soil samples were obtained within four stream drainages. All samples were analyzed for major and trace element concentrations by ICP-MS. The water samples were also analyzed for anions by IC and H and O isotopes by stable isotope mass spectrometry. In situ measurements of pH, SC and T were made at each sampling site. Alkalinity was measured on site by titration.

At the Apollo mine site pH increases from 6.26 at the main adit to 6.53 downstream before it enters the main stream channel. Alkalinity is low (27-29 mg/L) and sulfate ranges from 146.1 to 146.9 mg/L. Dissolved elemental concentrations of Al, Cu, Zn and Pb decrease with distance from the mine with the exception of a small increase in Al and Zn 300 feet downstream of the main adit. Aluminum, Fe and Zn concentrations in streambed precipitates decrease with distance from the adit and with increasing pH. The precipitates also have an increase in concentrations of Cu, Mg, Cd and Mn as a function of distance downstream away from the adit.

At the Sitka mine site pH ranges from 6.25 upstream of the mine site to 5.21 just below the main waste rock pile. Alkalinity is 30 mg/L up stream from the waste rock to <10 mg/L at the waste rock site. Sulfate ranges from 2.8 to 36.0 mg/L, with the higher concentrations being closer to the mine site. Dissolved Al, Cu, Zn, Mg and Pb concentrations increase with decreasing pH. Streambed sediments from upstream of the waste rock pile have lower concentrations of Al, Fe, Pb, and As than those below the waste rock pile.