Development and testing of a model for simulating weathering and trace elements release from fixated scrubber sludge utilized in abandoned coal mine reclamation site

In this paper we present the first approximation to a fully coupled model for simulating chemical reactions between engineered coal combustion byproducts (specifically fixed scrubber sludge-FSS) and ambient groundwater. If FSS is placed at a mine site as a hydrologic barrier over pyritic refuse, it would interact with acidic mine water at the bottom and soil water at the top of the FSS. A coupled reactive transport model was developed for simulating reactions at the exposed edges of the FSS over long time frames. In our modeling, emphasis was placed on the release and fate of toxic trace elements such as arsenic, boron, barium, and zinc, which are commonly present in FSS. The parameters associated with dissolution of trace element hosts were calibrated and optimized using data from column leachate experiments on core samples extracted from a reclaimed abandoned mine land site in Indiana. In an effort to evaluate the leaching potential of these trace elements, reactive transport simulations were conducted and compared with historical chemistry data. The model successfully reproduced many characteristics observed in the field, such as initial rises in the concentrations of arsenic, boron, and zinc, following by continuous decline toward steady state by the end of simulation. Also, simulation results indicated that the weathering rate was slightly higher at the top of the FSS where it is in contact with ambient soil water than at the bottom of the FSS. While the preliminary modeling results are promising, comparisons between model calculations and historical data point to the fact that there must be other sources or mechanisms that accommodate rapid releasing of trace elements. Further model development will require explicit inclusion of mechanisms for sorption/desorption of the potentially toxic elements.