Geo-fluid analysis for the natural purification mechanism of Arsenic and Iron in Yotsugi mill tailings pond

The presence of arsenic in mine/rock drainage is an important issue in environmental research. In Ningyo-toge mine, mine/rock drainage is conducted to Yotsugi Mill Tailing Pond in which several toxic ions such as arsenic and iron can be naturally oxidized and removed as precipitates. In this study, the dynamic behavior of arsenic and iron in the Yotsugi Mill Tailings Pond at Ningyo-toge mine was investigated. For the geo-fluid analysis, a geosphere fluid modeling software (GETFLOWS) was utilized. GETFLOWS can model the dynamic behavior of contaminants in terms of advection-dispersion in porous media, chemical reactions and adsorption, for both point source and non-point source pollutants. We first reproduced the overall area flow through the advection-dispersion analysis of a non-reactive silicate ion by assuming the groundwater inlet. Position and amount of entering groundwater were set by taking into account the available data of groundwater levels and the mass balance for water. Following this, the total concentrations of arsenic and iron were reproduced by including in the advection-dispersion model, the first order kinetics for the oxidation of As(III) and of Fe(II) by the dissolved oxygen, whose concentration was set according to data previously obtained by sampling and analysis. Based on the previously observed natural oxidation of Fe(II) to Fe(III) and subsequent precipitation of Fe(III) as ferrihydrite, the adsorption of As(V) or As(III) on ferrihydrite was also included in the simulation by considering a surface complexation model. Finally, the influence of natural phosphate ion on the adsorption of As(V) onto ferryhydrite was also included in the model by considering a competitive adsorption mechanism. The profile concentrations of As(III) and Fe(II) determined by simulations were found to be in good agreement with the values measured on the field. Result also highlighted the key role of phosphate in the natural removal of the arsenic from the pond: where high concentrations of phosphates were present in the pond, the natural removal of arsenic would be dramatically diminished.