Influent Water Quality Affects Performance of Passive Treatment Systems for Acid Mine Drainage

Passive treatment of acid mine drainage (AMD) relies on biological, geochemical, and gravitational processes to neutralize acidity. Published design guidelines use sizing 'rules of thumb' based on AMD loadings at design flows. Using average performance data for 82 treatment systems, we used regression modeling to investigate the influence of influent net acidity and water loading on alkalinity generation by five treatment system types. Alkalinity generation increases with influent net acidity loading for all system types. Influent net acidity loading can be deconstructed into concentration and water loading components. In bivariate models, water loading was a predictor of alkalinity generation for all five system types but net acidity was significant only for vertical flow systems (VFs). In multivariate models using both components as performance predictors, both influenced alkalinity generation. These relationships were strongest for anaerobic wetlands (AWs), VFs, and open limestone channels; anoxic limestone drains and limestone leach beds demonstrated these influences less consistently. These results reflect the geochemical mechanisms governing the performance of limestone-based passive treatment system: solubility of limestone decreases as dissolved reaction products and pH increase.