Geochemical characteristics and origin of groundwater draining into a large-scale tunnel, northern part of Iwate Prefecture

摘要

The purpose of this study is to investigate sources and style of migration of spring water discharging from fracture zones in a large-scale tunnel occurring at 150 to 200 meters below the surface of the ground. The water samples collected were spring water 200 meters below the surface and subsurface water 100 meters below the surface in the tunnel, well water, river water and precipitations. The properties of the water samples are discussed on the basis of concentrations of the major chemical components, including Na+, K+, -Ca2+, Mg2+, Cl-, NO3 , SO42-and HCO3- and the oxygen and hydrogen isotopic ratios of those water. The oxygen and hydrogen isotopic ratios of the spring water in the tunnel have almost constant values of ..18O= -11.0 ‰ and ..D= ..........................-70 ‰, respectively, and the isotopic ratios are lower than those of subsurface water, well water, river water and precipitations. On the other hand, the oxygen and hydrogen isotopic ratios of subsurface water show weak seasonal variation. The residence time of the spring water in the tunnel is estimated to be about over 2years. Based on the results of concentration ratio for major chemical components of water in the area, river water and well water having a short residence time and spring water having a long residence time. The subsurface water is thought to have been formed by water/rock interaction between water of meteoric origin with Quaternary and Tertiary volcanics in a short time, while the spring water is thought to be water formed by reaction of subsurface water with Paleozoic ..Mesozoic to Quaternary strata over a long period.