Dilution and removal of dissolved metals from acid mine drainage along Imgok Creek, Korea

The dilution factors (Di) and removal fractions (Ri) of pollutants from acid mine drainage (AMD) were quantitatively estimated using two different methods, the conservative component and mass balance method, along Imgok Creek in Korea. The conservative component method assumes that SO4 is a perfectly conservative component and calculates Di and Ri from the concentration ratios of SO4. The mass balance method solves the simultaneous equations relating the concentrations of dissolved components to their precipitation stoichiometries to obtain Di and Ri. The results from both methods are little different, indicating that SO4 concentration is a good indicator of dilution for Imgok creek. The calculated Diʹs of pollutants quickly decrease from the site of AMD input to the site a few km downstream, but then remain more or less constant over the reaches farther downstream. This is because Di loses its sensitivity in the reaches where difference in SO4 concentration between the main stream and combining tributaries significantly diminishes. The calculated Riʹs show that approximately 90, 95, and 75% of the original Fe input were removed from the streamwater in October 1996, April 1997, and October 1997, respectively. Aluminum was almost completely removed in April 1997, but only 50% of the original Al was removed in October 1997. The removal of Fe was due to the precipitation of schwertmannite or ferrihydrite and Al due to amorphous Al4(OH)10SO4. The maximum removal fraction of dissolved SO4 was only 5%. The other metals from AMD were significantly removed from the stream water only in April 1997. These metals were removed not by precipitation but by adsorption on and/or coprecipitation with Fe/Al-compounds. The relatively abundant freshwater supply in April 1997 might raise stream pH higher than the adsorption edge and consequently, contribute to rapid metal attenuation by forcing not only more precipitation but also more adsorption of the dissolved metals.