Abstract: Acid drainage from exposed pyritic material such as coal mine tailings is reduced through the controlled application of anionic detergents which inhibit the bacterium Thiobacillus ferrooxidans. The controlled application of anionic detergent comprises an initial application of anionic detergent in sufficient quantities to satisfy the adsorptive capacity of clay or clay-sized particles overlying the pyritic material coupled with the application of an anionic detergent inhibitor of Thiobacillus ferrooxidans from a slow release matrix containing the inhibitor. The two applications are to intercept infiltrating water up-gradient of the pyritic material or strata. The amount of matrix employed is calculated in accordance with a described formula. The method can be used to reduce acidity in drainage from pyritic mine tailings, coal mines, coal refuse piles and construction sites at which acid drainage is a problem.