The National Tunnel is a part of the Central City / Idaho Springs Superfund site. Because passive treatment is an important possibility for removal of contaminants from the water, the USEPA and the Colorado Division of Public Health and Environment (CDPHE) have been sponsoring a bench-scale study of different organic substrates for sulfate-reducing bioreactors (SRBRs). The substrates being tested include ethanol, woodchips and hay, woodchips and corn stover, and crab-shell chitin. After 6-18 months of operation, all of the reactors are showing significant amounts of sulfate reduction. In all of the reactors, Cu and Zn are removed to below their respective ambient water quality criteria of 0.010 and 0.100 mg/L. As is commonly observed in SRBRs, Mn removal is significantly less, with the exception of the chitin reactors. The reason for chitin's superior Mn removal may be the dissolution of calcite from the crab shell. In the chitin reactors, Ca has increased from 210 to 870 mg/L and alkalinity has increased from zero to up to 5,000 mg CaCO 3 / L. Furthermore, the pH of the effluent leaving the chitin systems averages 6.9. In most SRBRs, Mn is precipitated as MnCO 3 and significant removal does not occur until the pH is raised to between 7 and 8. This is the case in the other types of SRBRs being tested at the National Tunnel, as their Mn removal efficiencies have only approached 50 % at pH values ranging from 6.5 to 7.5. However, in the chitin reactors, 86% of the influent Mn is being removed from 21.5 mg/L to an average of 3 mg/L. The high removal is very similar to the removal of Mn in pulsed limestone beds that are maximized for the dissolution of calcite. These chitin reactors have been operating for six months while the other substrate reactors have been operating for over one year.