Frequency distribution of naturally occurring arsenic leached from excavated rocks of Hokkaido Shinkansen tunnels between Shin-Hakodate-Hokuto and Oshamambe

Excavated rock produced during tunnel construction projects in volcanic areas and sedimentary rock of marine origin pose the potential hazard of introducing naturally occurring arsenic (As) into the surrounding soils and groundwater. Thus, appropriate management and/or countermeasures against As mobility is required by comprehending the leaching characteristics of the As in excavated and sedimentary rock. This study focused on the Neogene Miocene sedimentary rock of marine origin that is widely distributed between Shin-Hakodate-Hokuto and Oshamambe Stations along the Hokkaido Shinkansen in Hokkaido, Japan. The probability distribution of the leaching concentration of As from the rock excavated from the tunnels between the above stations was evaluated via batch leaching tests using crushed rock as a countermeasure against the rock containing naturally occurring toxic elements. A statistical evaluation of this probability distribution was conducted using a vast amount of test results. The results showed that the As leaching concentration followed a logarithmic normal distribution depending on the geological formation. The concentration was influenced by various factors, including the As content, key chemical composition in the rock, such as calcium (Ca), iron (Fe), and sulfur (S), as well as the pH and electrical conductivity (EC) in the leachate. In addition, the thickness of the overburden was found to impact the As leaching concentration. These results suggest that reasonable countermeasures against the risks of As leaching into the surrounding environment can be devised by considering the probability distribution of the As leaching concentration.