Los Bronces has two ROM dump leach operations called Leach I and Leach II. Leach I was commissioned in 1998 and currently produces 12,700 ton per year. The copper mineralization in these dumps is largely sulfides, namely 60% chalcocite/covellite, 30% chalcopyrite and the balance is oxides. The Leach II plant, commissioned in early 2002, was built to treat around 700 million ton of marginal ore having 0.45% total copper. The copper mineralogy of this material is estimated to be 70% chalcopyrite, 20% chalcocite/covellite and the balance oxides. Current copper production is 33,700 ton per year. Early in 2004 it was decided to conduct a complete column test work program, aimed at reducing power, acid and lime consumption at the Leach II plant. The results of this work, partially reported at IBS 2005, have been successfully applied to the commercial operation since the end of 2005. As a consequence, the projected reduction in operating costs for 2006 of US$ 5.5 million was accomplished. The main results of the column test work program, the modifications implemented together with the results obtained at the operation, including a molecular characterization of the microbial population, are discussed in this paper.