ABSTRACT

The aim of this study was to elucidate groundwater recharge areas and flow systems in the Matsumoto City area and its surroundings in the Matsumoto Basin, central Japan, using ⁸⁷Sr/⁸⁶Sr ratios, dissolved inorganic ions, trace elements, and stable isotopes (δ¹⁸O and δ²H). Distinct differences in ⁸⁷Sr/⁸⁶Sr ratios were observed among sediments and rocks, spring water, and river water between the Hida Mountains in the west and the Chikuma Mountains in the east of the Matsumoto Basin. The ⁸⁷Sr/⁸⁶Sr ratios were consistently higher in the Hida Mountains than those in the Chikuma Mountains. Comparison of ⁸⁷Sr/⁸⁶Sr ratios in rocks and river water with those in groundwater and spring water indicated that groundwater in the Matsumoto City area is primarily recharged by the Chikuma Mountains, whereas groundwater in the Azumino alluvial fan area is likely recharged by the Hida Mountains. Spatial distribution of groundwater ⁸⁷Sr/⁸⁶Sr ratios in the Matsumoto City area showed slightly higher values in the northern part, which gradually decreased toward the southern part, possibly reflecting differences among alluvial fan deposits. The ⁸⁷Sr/⁸⁶Sr ratios were slightly higher in the Metoba River and slightly lower in the Susuki River and nearby spring water, suggesting that groundwater in the southern part of the Matsumoto City area may be influenced by recharge from the Susuki River or nearby springs. By integrating multiple tracers, the groundwater flow system in the Matsumoto City area was classified into six groups, and the groundwater recharge areas, water-quality formation processes, and recharge elevations for each group were determined.