Hydrochemical, and Pb- and Sr- isotopic constraints on the precipitation and dilution of metals in acidified mine water: an example from the abandoned Dongjin Au-Ag-Cu mine, Korea

Abstract

The geochemical properties (acidity, elemental composition, and isotopic composition of Sr and Pb) of surface water in the watershed of the abandoned Dongjin Au-Ag-Cu mine, central Korea, vary as the water drifts downstream and reacts with refuse ore piles. These properties also change due to adsorption and co-precipitation of metals on Fe-Al precipitates, and dilution by unpolluted waters. The patterns of variation of dissolved ions were divided into two groups (1 and 2). Group-1 ion (Al, Cu, Cd, and Pb) levels are increased only in acidified water after reaction with refuse ore piles, and their concentrations are regulated largely by precipitates of amorphous Fe-Al hydroxides, which strongly adsorb or precipitate these metals in mildly acidic to neutral conditions. The selective adsorption of Pb on Fe-Al precipitates is supported by Pb isotope data, suggesting that Pb from ore is dominant only in acidic water (pH 6.0) even with high contents of other ions from dissolved sulfide minerals because of strong adsorption of Pb from ore. Group-2 ions are composed of the major cations (alkali and alkaline-earth), anions, and trace elements including Zn and Mn. Their concentrations and 87Sr/86Sr ratios tend to decrease with distance from the mine adit, indicating that group-2 ions are less dependent on pH and are strongly controlled by mixing with unpolluted, dilute water from the surrounding areas. A combination of Pb and Sr isotopes is useful for tracing the geochemical evolution of surface water in acid mine drainage.