地熱変質地帯における地すべり地の地下水の化学および同位体組成の変動-別府市乙原地すべり-

Abstract

The landslide at the Otobaru area, Beppu City, occurred twice in 1943 and 1969. A partof this area, even now, is affected considerably by thermal activities.Variations in chemical and isotopic compositions of waters from the Otobaru area and itsvicinity were investigated from 1977 to 1983 and 1986 to 1987.The results are as follows: (1) electric conductivity data suggest that the two kinds oflow-concentration water and high-concentration water exist in the landslide area, (2) theexistence of two groundwater aquifer in the landslide area and its vicinity is inferred fromtritium data, (3) variations in chemical composition of waters from the horizontal borehole areaccompanied by the rise and decline of water table, (4) the waters from the landslide area andits vicinity are in equilibrium with montmorillonite, (5) the most waters under 10 1 atm. Of Pco2are saturated or supersaturated with calcite, and (6) there is no detectable contribution ofgeothermal water to the waters from the landslide and its vicinity.And our hypothesis on the mechanism for the formation of calcium sulfate type water isalso presented.