Hydrogeochemical Investigation of Metal Evolution in Circum-Neutral Mine Discharge

We investigated the removal of metals from a circum-neutral mine discharge over a 0.5-km distance in a series of sequentially interconnected ponds from the exit of an 8-km tunnel that drains a gold mine. The geochemical and mineralogical processes controlling the evolution of metals in the drainage ponds were the focus of this research. We measured the pH and concentrations of dissolved inorganic carbon (DIC) and alkalinity in the aqueous phase and the concentrations of Ca, Mg, Na, Fe, K, Mn, Sr, and Zn in the aqueous phase and in sediments on the beds of the ponds. We calculated the saturated indices of metal oxides, metal carbonates, and the partition coefficients (D) for Mg, Sr, Mn, and Zn. The pH of the mine discharge increased while the concentrations of DIC and alkalinity decreased over the 0.5-km distance. The concentrations of Ca, Mg, Na, Fe, K, Mn, Sr, and Zn generally decreased while their concentrations in the sediment phase increased. Mineralogical and acid-base accounting and modeling results showed that all the metals in equilibrium with the aqueous phase were oversaturated, and the dominant mineral in the precipitated sediments was calcite. Forward geochemical modeling in combination with the Dₘg, DSᵣ, DMₙ, and DZₙ showed a low ionic strength solution indicating that these metals were removed from the mine discharge by adsorption and coprecipitation of calcites. Forward geochemical modeling results indicated that the groundwater exiting the tunnel had reached equilibrium with respect to ferrihydrite and goethite, suggesting that the precipitation of Fe-oxides were responsible for removing these metals from solution. Calcite precipitation removed about 16% of Ca²⁺, while 15% of Sr and 5% of Mg were scavenged from the mine water by coprecipitation with calcite and/or adsorption into calcite lattices. About 52% of total Fe from the mine water was scavenged through amorphous Fe(OH)₃ and goethite precipitation. About 23% of the Zn in the mine water was removed by sorption on Fe-oxides surfaces or coprecipitation with calcite, and 30% of the Mn in the mine water was removed by rhodochrosite precipitation and/or coprecipitation with calcite. Settling ponds, therefore, provide a framework where minerals precipitate, are adsorbed, or coprecipitated, such that mine water at acceptable contaminant level is released into the natural environment.