Effects of Additional Layer(s) on the Mobility of Arsenic from Hydrothermally Altered Rock in Laboratory Column Experiments

Hydrothermally altered rocks are frequently encountered when tunnels are constructed in Hokkaido, Japan. High concentraions of hazardous elements, such as arsenic (As), are often released from these rocks into the surrounding environments. Therefore, the rocks are considered potentially hazardous waste. This article describes the effects of water content and oxygen (O-2) concentration in relation to additional layer(s), i.e., surface covering and bottom adsorption layers, on As leaching by using laboratory columns with water content and O-2 concentration sensors. The results show that the use of additional layer(s) has a significant effect on lowering As migration. This was due not only to the adsorption capacity of As by the adsorption layer but also to the water content and O-2 concentration inside the rock layer. The accumulation of pore water was increased in the rock layer in cases with additional layer(s), which resulted in lower O-2 concentration in the rock layer. Consequently, the leaching of As by the oxidation of As-bearing minerals in the rock layer was reduced. Moreover, a longer water-resident time in the rock layer may induce precipitation of Fe oxy-hydroxide/oxide. These results suggest that the geochemical conditions of the rock layer affect As leaching and migration