Experimental and Numerical Study of the Groundwater Quality in Altered Volcanic Rock Area

Abstract

Groundwater geochemistry associated with the Hakkouda Tunnel in the Sankaku-dake Mountains, Aomori prefecture, northern Japan, was surveyed in an experimental and numerical study to understand the mechanism forming the groundwater quality in altered volcanic rock mass. We already made clear that the groundwater quality in this area corresponds with the mineral assemblages in the rock masses. Altered volcanic rock samples were tested by batch-leaching test to estimate rock-water reaction model in equilibrium conditions, in which 100 g crushed rock samples, under 10 mm in diameter, were shaken with 500 g pure water. The results of this test show that the chemical quality of the leaching water from an altered rock sample is similar to that of groundwater distributed in same rock mass. The numerical simulations using PHREEQC verified the results of the leaching test for altered volcanic rock samples. The calculation results support that the water reacts with rock (minerals) in equilibrium. From these results, we can estimate that the quality of fissure groundwater in hard rock mass depends on the equilibrium reaction with co-existing minerals in aquifer and that the geochemical homogenization of groundwater by diffusion and/or dispersion do not influence the groundwater quality .