A quantitative estimation of the factors affecting pH changes using simple geochemical data from acid mine drainage

The behavior of heavy metals in acid mine drainage (AMD) is mainly controlled by pH values. Therefore, a quantitative estimation of factors affecting pH values in AMD is very important in predicting the behavior of those metals. Many different factors cause pH changes in streams affected by AMD and we quantitatively estimated those factors by making simple equations from geochemical data collected from the Dalsung mine. In a stream from that mine, the pH values decrease as the stream flows downstream from the AMD source, which is different from normal streams affected by AMD. The stream shows low pH ranges (4.04-5.96), high electrical conductivity (1,407-1,664 mu S/cm), and sulfate concentration (680-854 ppm). Most ion concentrations decrease or do not show noticeable changes mainly due to dilution. The change of the iron content is most significant, even though the concentration of iron is relatively low compared with other ions. The iron concentration (13.4 ppm) becomes almost 0 ppm due to precipitation. Schwertmannite is the dominant precipitated phase downstream and whitish basaluminite is observed in the upstream. From our pH estimation, precipitation is the most important process lowering pH values from 5.96 to 4.04. The dilution factor was calculated by the concentration changes in sulfate ions. Dilution increases pH values, but compared with the precipitation factor, the contribution of the dilution factor to pH is relatively small. Alkalinity is the main factor that buffers hydrogen, which is released by precipitation. The redox changes, which were calculated from the pH and Eh values, also affect pH at each sampling site. The trend of estimated pH changes is almost identical to the observed ones, but the values are slightly different. Some errors are expected mainly due to the uncertainty in the observed Eh values and the chemistry of the added water for dilution.