Modeling and Evaluating the Performance of River Sediment on Immobilizing Arsenic from Hydrothermally Altered Rock in Laboratory Column Experiments with Hydrus-1D

Large volumes of excavated rock are produced as a result of road and railway tunnel construction in Hokkaido, Japan. Due to the geological condition of this region, these rocks have often undergone hydrothermal alterations, causing them to contain elevated amounts of hazardous elements including arsenic (As). Therefore, these excavated rocks are potentially hazardous waste, and proper disposal methods are required. In this article, performance of unsaturated river sediment on immobilizing As from hydrothermally altered rock is evaluated using laboratory column experiments and Hydrus-1D. The results reveal that the river sediment significantly reduces As migration. Arsenic retarded by river sediment was observed in three patterns. The first was an adsorption onto minerals originally contained in the river sediment. The next pattern was a combination of reduction of As generation by oxidation of As bearing-minerals, irreversible adsorption, and adsorption onto newly precipitated Fe oxy-hydroxide/oxide. The last pattern led to a further depletion of As leached from the rock layer due to a shift in the majority of the As generation mechanism from dissolution to oxidation in combination with a low concentration of oxygen in the rock layer. These patterns were satisfactorily evaluated by a Hydrus-1D model with reversible and irreversible adsorptions. The information from this work is effective in designing and establishing a reasonable technique for the disposal of hydrothermally altered rocks.