Acid rock drainage (ARD) is commonly generated when meteoric waters interact with sulfide minerals disseminated in exposed mine tailings and coal piles. Passive treatment systems have been used often to remediate sites impacted by ARD, although large tracts of land are normally required for this remedial strategy to be effective when treating low pH, highly mineralized ARD. The vertical flow constructed treatment wetland, consisting of an organic layer overlying a limestone drain, is a common design that has been employed to reduce the land area required for constructed-wetlands.

Vertical flow wetlands (VFW) are a limestone-based passive treatment option for the remediation of acid rock drainage (ARD) from coal and ore mines. They function to neutralize proton acidity and generate alkalinity. Vertical flow systems are not traditionally designed to remove mineral acidity in situ; this process typically occurs in a downstream aerobic wetland. In coal-related ARD, mineral acidity comprises >85% of the total acidity. Therefore, by design, VFW neutralize only a small portion of total acidity in ARD internally.

In this study, acidity neutralization and alkalinity generation reactions were examined using an experimental mesocosm system modeled after VFW, but containing limestone-buffered organic substrate (LBOS). Highly acidic (1304 mg·L-1 CaCO3 equivalents), low pH (2.4), ferric iron-dominated (95 – 100 % ferric iron; 92 – 237 mg·L-1 total iron) ARD was passed through the system for two years. Effluent pH was consistently near-neutral (6.4) and alkaline (619 mg·L-1 CaCO3 equivalents) with >97% of the influent acidity removed (15 mg·L-1 iron). Total alkalinity generated in the system (1832 mg·L-1 CaCO3 equivalents) was more than four times greater than traditional VFW systems, because mineral acidity, as ferric iron and aluminum, was neutralized at relatively low pH (<4.5) simultaneously with proton acidity. Although ferric iron oxyhydroxide and aluminum hydroxysulfate precipitated in the presence of limestone, armoring and passivation of the limestone was not observed, nor was there any reduction in permeability. Therefore, LBOS-amended treatment systems may be able to effectively treat low pH (<3.0), ferric iron-dominated ARD over long periods of time.