One potential drawback of compost-based passive bioreactors, which is a promising biotechnology for acid mine drainage (AMD) treatment, is the transport of dissolved organic matter (DOM)-metal complexes in surface waters. To address this problem, the objective of this study was to assess the maximum capacity of organic substrates to release soluble DOM-metal complexes in treated water. The reactivities of DOM in maple wood chips and sawdust, composted poultry manure, and leaf compost were quantified toward Cd2+, Ni2+, Fe2+, and Cu2+ using fluorescence quenching. The DOM showed the highest reactivity toward Fe, but a limited number of available sites for sorption, whereas DOM-Cd complexes exhibited the lowest fluorescence quenching. Overall, the DOM from a mixture of wastes formed higher concentrations of DOM-metal complexes relative to sole substrates. Among DOM-metal complexes, the concentrations of DOM-Ni complexes were the highest. After reaching steady-state, low concentrations of DOM-metal complexes were released in treated water, which is in agreement with theoretical predictions based on geochemical modeling. Therefore, in addition to physicochemical characterization, fluorescence quenching technique is recommended for the substrate selection of bioreactors.