2017 Volume 58 Issue 4 Pages 264-278
Under construction of the Hokusatsu tunnel（4.85km in length）, which transects through the Miocene Shibi-san granitic pluton（granodiorite） and the Cretaceous Shimanto Group, distributed in the northwestern part of Kagoshima Prefecture, large amount of water inflow to the tunnel occurred . The water contains As up to 0.3mg/L.
Microscopic observation and electron probe microanalyses have revealed that the granodiorite contains As minerals as arsenopyrite（46.6-48.7wt%As）and löllingite（71.6-74.0wt% As） . These minerals are found in fresh rocks of hornblende-bearing biotite granodiorite as primary minerals. Arsenopyrite occurs as euhedral to anhedral forms of 50-800 μm in length, while löllingite takes irregular shapes of 30-50 μm across. Comparing ionic compositions of waters for the streams above the tunnel and the inflow to the tunnel, it is considered that As release has occurred to the groundwater in the surface layer of the weathered granodiorite by oxidative dissolution of arsenopyrite, which is widely dispersed in the peripheral part of the Shibi-san granitic pluton. Then, the As-bearing groundwater has moved downward through the fracture zone, and happened to inflow to the tunnel.
It is important for the disposal of excavated rocks in tunnel construction that very fine grains of arsenic minerals occur in the fresh granitic rocks, especially in Miocene ilmenite-series granitic plutons distributed sporadically in the Outer Zone of Kyushu, Japan.
