Variably saturated reactive transport of arsenic in heap-leach facilities

A variably saturated reactive transport software package, UNSATCHEM, has been modified for the purposes of simulating As oxyanion transport in gold mine heap-leach facilities. Gold-ore deposits in the Carlin Trend, located in northeastern Nevada and western Utah, are characterized in part by the occurrence of As-bearing minerals. The large-scale mining methodology used on the Carlin Trend includes the use of large-scale heap-leach facilities in which an extractive solution is used to dissolve the gold from the ore. This solution also is capable of dissolving other minerals, including those that contain As. While these facilities are generally environmentally benign compared with sulfide waste rock dumps associated with other mining provinces around the world, controlling the flux of As from the heap-leach facility after mining has ceased is of interest to the mining and regulatory communities. We present three examples that use the As transport capable version of UNSATCHEM to simulate the effect of three commonly applied heap-leach facility closure methodologies. The results demonstrate the importance of including geochemistry in transport simulations and illustrate the highly non-linear behavior that is produced from a coupled flow and reactive geochemistry numerical solver.