Geochemical Evolution of a Surface Mine Lake with Alkaline Ash Addition: Field Observations vs. Laboratory Predictions

Abstract

In the Eastern Middle Anthracite field of Pennsylvania, a formerly acidic (pH = 3.6) surface mine lake (initially approximately 45,000m3 in volume) is being reclaimed using fluidized bed combustor (FBC) ash. The pH of the water in the pit dramatically increased when the alkaline ash was added. The pH of the water is now well buffered, and has not dropped below a value of 11.0 since March 2000. Analysis of data from samples collected over the past six years indicate that the lake’s alkalinity is controlled by carbonate, silicate, and hydroxide reactions. The relative importance of these factors varies with ash input, and can be determined in a predictable fashion. Laboratory tests determined that the mass of CaO was more significant than the particle surface area on the pH of the solution. Using only alkaline material, the transition between caustic and carbonate alkalinity was apparent, though this did not account for interaction with silicate minerals, which should be considered when using alkaline ash for reclamation. Field data indicate that with time, the pH will again decrease but will be buffered by calcite present on both the upper walls of the mine pool and within pores of the FBC ash. Less than 1% of the ash is currently used to increase the pH and alkalinity, so a large reserve exists for long term buffering capacity.