Hydrothermally altered igneous rock and mudstone formed under the sea are often rich in arsenic (As). Therefore, As leaching from these rocks excavated during construction work may affect the surrounding soils and groundwater. High-volume, low-concentration contaminants contained in the waste rock as muck have been a major concern in Japan since the Soil ContaminationCountermeasures Law was enforced in 2003.The aim of this study is to develop ways to reduce As concentration leached from muck by controlling the geochemical conditions. Batch leaching experiments were carried out to determine the leaching behavior of As as a function of mixing period, solution pH, oxygen concentration, or mixing ratio of different rocks. The results showed that the As concentration in the leachate approached almost constant after 1 to 7 days mixing and increased with the As content in the rock, and that the maximum As leaching was observed at around pH 9. The concentration of oxygen gas and the mixing ratio of different rocks also affected the As leaching by changing the leachate pH.This indicates that the solution pH as well as the As content in the rock greatly affects the As leaching from rocks.Keywords: Leaching, Oxidation, and Environmental Hokkaido, in the north of Japan, has many active volcanoes, hot springs, and abandoned mines.This implies that hydrothermally altered rock is widely distributed throughout Hokkaido. The arsenic (As) content of altered rock is known to be higher than that of unaltered rock (Tanaka, 1988), and it may induce higher As concentrations in seepage through altered rock layers. In particular, the leachate through impounded altered rock excavated in relation to tunnel or dam construction, called muck, often displays noticeably elevated As levels. This is because the geochemical conditions of the rock have been changed from anoxic to oxic and because the specific surface area of excavated rock is dramatically higher than that of the intact rock, causing an enhancement in the mobility of As (Smedley and Kinniburgh, 2002;Yoshimura and Akai, 2003).The extensive As pollution of groundwater as in the cases of Bangladesh (Acharyya et al., 2000; Ahmed et al., 2003;Akai et al., 2004;Anawar et al., 2003;Nickson et al.,1998;Nickson et al., 2000; Stollenwerk, 2003), West Bengal (Acharyya et al., 2000;Das et al., 1996;Dowling et al, 2002;Nickson et al., 2000), and other locations (Chen et al., 1994;Kim et al., 2000; Kelley et al., 2005;Nimick, 1998;Peters et al., 1999;Robertson, 1989;Schreiber et al., 2000;Smedley and Kinniburgh, 2002;Yoshimura and Akai, 2003;Welch et al., 1988;Welch and Stollenwerk, 2003;Williams et al., 2005) is a well known concern worldwide. There are also reports of As pollution of groundwater, sediments, soils, and/or rocks in Japan (Shimada, 1996;Masuda et al., 1999). The origin of the As has been reported to be As-containing minerals in sediments (Akai et al., 2004;Dowling et al, 2002;Kim et al., 2000;Peters et al., 1999;Shimada, 1996). There has been exhaustive research into the beha...
