ABSTRACT

The major chemical and stable isotopic compositions of hydrogen, oxygen, and sulfur (δD, δ¹⁸O, δ³⁴S) were analyzed from water samples of wells and natural springs in the northern foot area of Mt. Fuji and the adjacent Misaka and Tanzawa Mountains to investigate the origin of deep well waters and water-rock interactions that affect the water quality. The water in the samples was considered to have originated through the mixing of meteoric water with very small amounts of altered seawater that had been trapped in the pore spaces in the basement rock, the so-called green tuff formations. The major chemical components of the original waters are considered to be affected by the dissolution of anhydrite and/or gypsum, the weathering of plagioclase, precipitation of calcite, and the cation exchange reaction of smectite. In the distributed volcanic products of Mt. Fuji, the weathering of olivine appears to be an important factor in water quality. The δ³⁴S values of the SO₄^2– ions of well water were higher in the Misaka and Tanzawa Mountains and lower at the foot of Mt. Fuji, indicating the presence of anhydrite/gypsum of different origins.