Four test conditions and a set of controls were used to examine the best conditions for microbial cap formation. The following conditions were executed in triplicate (1) inoculation of three columns with Ultramicrobacteria (UMB’s) and addition of liquid nutrient (BHI + sucrose); (2) inoculation with UMB’s, liquid and solid nutrients (Stillage concentrate), (3) addition of the same as the previous set plus calcium and (4) only liquid nutrient was added. The control maintained a constant pH around 3, an Eh higher than 415 mV, an oxygen level always higher than 5.1 mg/L. However, the Fe content in general decreased over time which was unexpected. The series with UMB’s, liquid and solid nutrients maintained a lower Eh than the other series for the longest period of time followed by the series of columns with liquid nutrient only. Al1 conditions tested were generally better than the controls in increasing the pH and also in reducing the Eh of the effluent but nothing can be concluded with respect to the dissolved oxygen and iron contents due to continuous variations in the results. There was a significant decrease in the redox potential of the permeates, however none of the conditions tested completely reduced the permeability of the tailings over a long period of time. The scavenging of the oxygen and the increase in pH by the heterotrophs help reduce AMD generation by preventing growth of Thiobacillus ferrooxidans. The use of the UMB, Klebsiella oxytoca a heterotrophic strain, is not essential since the results of uninoculated columns with liquid nutrient only were equivalent to the columns with liquid nutrients and UMB’s. It is likely that the nutrients increased the metabolic activity of the indigenous heterotrophs already present in the tailings.

Key words: Acid Mine Drainage, Ultramicrobacteria, Microbial Plugging, Microbial Capping