Abstract: Constructed wetland treatment system effectiveness has been limited by the alkalinity-producing, or acidity-neutralizing, capabilities of systems. Anoxic limestone drains (ALD's) have allowed for the treatment of approximately 300 mg/L net acidic mine drainage, but current design guidance precludes using successive ALD's to generate alkalinity in excess of 300 mg/L because of concerns with dissolved oxygen. “Compost” wetlands designed to promote bacterially mediated sulfate reduction are suggested as a means of generating alkalinity required in excess of that produced by ALD's. Compost wetlands create two basic needs of sulfate reducing bacteria; anoxic conditions resulting from the inherent oxygen demand of the organic substrate, and quasi-circumneutral pH values resulting from the dissolution of the carbonate fraction of the compost. However, sulfate reduction treatment area needs are generally in excess of area availability and/or cost effectiveness. Second generation alkalinity-producing systems demonstrate that a combination of existing treatment mechanisms has the potential to overcome current design concerns and effectively treat acidic waters ad infinitum. Successive alkalinity-producing systems (SAPS) combine ALD technology with sulfate reduction mechanisms. SAPS promote vertical flow through rich organic wetland substrates into limestone beds beneath the organic compost, discharging the pore waters. SAPS allow for conservative wetland treatment sizing calculations to be made as a rate function based on pH and alkalinity values and associated contaminant loadings. SAPS potentially decrease treatment area requirements and have the further potential to generate alkalinity in excess of acidity regardless od acidity concentrations.

Abstract: Because of its location in the sensitive Coromandel Peninsula, strict water management and environmental requirements had to be met on the Golden Cross Mine Project. This led to the development of new technologies for cyanide recovery and the adoption of advanced water management and water treatment systems. This paper discusses the water management and treatment system adopted for contaminated water at Golden Cross. While permit discharge levels must be and are met for mine discharge waters, the ultimate success of the water management system is demonstrated by the results downstream; biological surveys show no changes to the resident aquatic life in the river.

Abstract: The water treatment performances of two anoxic limestone drains (ALDs) were evaluated. Anoxic limestone drains are buried beds of Limestone that are intended to add bicarbonate alkalinity to flow-through acid mine drainage. Both ALDs received mine water contaminated with Fe2+ (216-279 mg L(-1)) and Mn (41-51 mg L(- 1)). Flow through the Howe Bridge ALD increased alkalinity by an average 128 mg L(-1) (CaCO3 equivalent) and Ca by 52 mg L(- 1), while concentrations of Fe, K, Mg, Mn, Na, and SO42- were unchanged. The Morrison ALD increased alkalinity by an average 248 mg L(-1) and Ca by 111 mg L(-1). Concentrations of K, Mg, Mn, and SO42- all decreased by an average 17%, an effect attributed to dilution with uncontaminated water. Iron, which decreased by 30%, was partially retained within the Morrison ALD. Calcite dissolution was enhanced at both sites by high P- CO2. Untreated mine waters at the Howe Bridge and Morrison sites had average calculated P-CO2 values of 6.39 kPa (10(- 1.20) atm) and 9.24 kPa (10(-1.04) atm), respectively. At both sites, concentrations of bicarbonate alkalinity stabilized at undersaturated values (SICalcite = 10(-1.2) at Howe Bridge and 10(-0.8) at Morrison) after flowing through approximately half of the limestone beds. Flow through the second half of each ALD had little additional effect on mine water chemistry. At the current rates of calcite solubilization, 17.9 kg d(-1) CaCO3 at Howe Bridge and 2.7 kg d(-1) CaCO3 at Morrison, the ALDs have theoretical effective lifetimes in excess of 20 yr. By significantly increasing alkalinity concentrations in the mine waters; both ALDs increased metal removal in downstream constructed wetlands.