Reactive Transport Modeling of the migration of a Pyrite Slag Leachate through an Aquifer in the Vicinity of a Canal with a Time-Dependent Stage

Abstract

A plume of pyrite cinder leachate originates in an aquifer in the vicinity of a canal. Besides being slightly acidic, the plume carries dissolved concentrations of As, Zn and Ni. The canal has a variable stage, either leading the aquifer to drain into the canal or canal water infiltration into the aquifer. In order to assess to what extent sorption to iron hydroxides combined with seasonal reductive dissolution could be responsible for contaminant patterns observed in the field, a 2D reactive transport model using MODFLOW and PHT3D was setup. The model is a cross-section to the canal, and a time-dependent river boundary is used to simulate the varying stage of the canal. The model incorporates diffuse double layer sorption on iron hydroxides and cation exchange. It maximizes the potential reductive dissolution of iron hydroxides that could ensue from the oscillatory infiltration drainage patterns. Comparison of field data and model results shows that the processes incorporated in the model account for only part of the results, but are insufficient to reproduce all aspects of field patterns. Possible causes for the incomplete reproduction of field data are identified. Through the way in which they affect the flow field, conductivity and temporal discretization of the canal stage boundary condition are seen to affect redox and acidity sensitive reactions. Effects of the flow field on dilution are also apparent. Ultimately, the conductivity and temporal discretization cause changes of several orders of magnitude in contaminant concentrations arriving at the canal. Mechanisms accounting for this behavior are identified and explained.