Hydrobiogeochemical interactions in 'anoxic' limestone drains for neutralization of acidic mine drainage

Processes affecting neutralization of acidic coal mine drainage were evaluated within `anoxic' limestone drains (ALDs). Influents had pH≤3.5 and dissolved oxygen <2 mg/l. Even though effluents were near neutral (pH>6 and alkalinity>acidity), two of the four ALDs were failing due to clogging. Mineral-saturation indices indicated the potential for dissolution of calcite and gypsum, and precipitation of Al³⁺ and Fe³⁺ compounds. Cleavage mounts of calcite and gypsum that were suspended within the ALDs and later examined microscopically showed dissolution features despite coatings by numerous bacteria, biofilms, and Fe–Al–Si precipitates. In the drain exhibiting the greatest flow reduction, Al-hydroxysulfates had accumulated on limestone surfaces and calcite etch points, thus causing the decline in transmissivity and dissolution. Therefore, where Al loadings are high and flow rates are low, a pre-treatment step is indicated to promote Al removal before diverting acidic mine water into alkalinity-producing materials.