The development of an unsaturated flow and reactive transport model for arsenic in Carlin-Trend heap leach facilities

An effort to develop a numerical unsaturated flow and reactive transport model in gold heap-leach facilities is presented. To develop the physics and chemistry of arsenic transport in highly heterogeneous and reactive gold ore, the adsorption characteristics of As(III) and As(V) are determined for two samples of Carlin-Trend heap material. The pH- and redox chemistry-dependence of adsorption for these oxyanions precludes the use of standard isotherm approaches to model the experimentally derived data. A new Langmuir isotherm formulation that includes pH-dependence is presented for both As(V) and As(III) data. This model is used in a modified version of UNSATCHEM to create a new numerical flow and reactive transport model for arsenic in variably saturated media. In addition to the incorporation of the pH-dependent isotherm formulation into the code, oxygen transport, pyrite oxidation, arsenian pyrite oxidation, and iron hydroxide precipitation are also new modifications required to simulate the reactive transport of arsenic. A series of numerical experiments is presented which illustrate the dominate geochemical and physical processes which govern arsenic oxyanion transport in heap leach facilities.