An experimental system was constructed with a limestone-buffered organic substrate (LBOS) to study the effects of passive treatment of low pH (<3), ferric irondominated acid rock drainage (ARD). Low pH, ferric iron-dominated ARD was passed through the LBOS for two years and the mechanism of acidity neutralization and alkalinity generation studied. Influent ARD was completely neutralized at a limestone dissolution front within the LBOS. Consequently, the total alkalinity generated in the LBOS was more than double the total alkalinity previously realized in any other passive treatment system. Three reaction zones surrounding the limestone dissolution front were identified: 1) an overlying oxide zone, characterized by iron oxyhydroxides and no limestone; 2) a transitional zone at the dissolution front, characterized by aluminum hydroxysulfate and partially dissolved limestone; and 3) an underlying sulfide zone, characterized by ubiquitous sulfide minerals and pristine limestone. The majority of the influent iron was removed in the oxide zone through hydrolysis and precipitation of ferric iron. The acidity generated through the precipitation of ferric oxyhydroxides was balanced by the dissolution of aluminum hydroxysulfate. Aluminum dissolved from the oxide zone was reprecipitated in the transitional zone in the presence of limestone. The acidity generated during the precipitation of aluminum hydroxysulfate was balanced by the dissolution of limestone. Ferrous iron generated in the oxide and transitional zones was removed in the sulfide zone, mainly as framboidal pyrite. The sulfide zone formed ahead of the limestone dissolution front at pH >6.5. As the limestone was completely removed from the transitional zone, the dissolution front advanced deeper into the LBOS. As a consequence, the leading edge of transitional zone overprinted the sulfide zone, while the oxide zone overprinted the trailing edge of the transitional zone. Therefore, with complete migration of the limestone dissolution front through the substrate, the LBOS will evolve towards oxide zone material. Several trace elements (As, Cr, Cu, U, Cd, Co, Ni, Zn) were also sequestered in LBOS. As the substrate evolved toward oxide zone material, uranium, chromium, copper, cadmium, cobalt, nickel, and zinc were remobilized and migrated with the limestone dissolution front. Arsenic was not remobilized once sequestered.