Hydrobiogeochemical evolution along the regional groundwater flow systems in volcanic aquifers in Kumamoto, Japan
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ORIGINAL ARTICLE
Hydrobiogeochemical evolution along the regional groundwater flow systems in volcanic aquifers in Kumamoto, Japan Takahiro Hosono1,2 · Shahadat Hossain3 · Jun Shimada1 Received: 18 September 2019 / Accepted: 27 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Kumamoto is known as the largest groundwater city in Japan. Geochemical modeling (saturation indices and mineral stability diagram) was applied in this area for better understanding a hydrogeochemical evolution in volcanic aquifers in regional scale with additional constraint from stable isotopic dataset. Geochemical evolutionary model was interpreted along the water flow dynamics. In total 136 water samples were collected from wells, springs, and rivers in and around the major groundwater flow lines for geochemical analyses. Our results indicated that plagioclase is the major weathering reactant in aquifers with secondary important weathering minerals of pyroxenes. These reactions facilitate current hydrochemical signatures and produce secondary minerals of kaolinite or halloysite and smectite in later stage. Observed hydrogeochemical processes can be distinguished into two distinct criteria along flow regime. The first processes are material loads from the surface (mixing of contaminants and river waters) and initial stage of silicate weathering, resulting in the formation of CaHCO3, Ca-NO3–HCO3, and Ca–Mg–Na-HCO3 type waters. These processes prevail in aquifers at the recharge to lateral flow zones with relatively shorter residence time of 55 years. Microbiological reduction reactions dominate over these areas and salinization occurs at the coast that changes aquifer waters to be Na-Cl type. Proposed models and approach shown in this study may be useful and applicable in interpreting systems in other volcanic aquifer systems at similar climate conditions and for sustainable water resources management. Keywords Groundwater · Volcanic aquifer · Saturation indices · Silicate weathering · Cation exchange · Flow dynamics
Introduction
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12665-020-09155-4) contains supplementary material, which is available to authorized users. * Takahiro Hosono hosono@kumamoto‑u.ac.jp 1
Faculty of Advanced Science and Technology, Kumamoto University, 2‑39‑1 Kurokami, Kumamoto 860‑8555, Japan
2
International Research Organization for Advanced Science and Technology, Kumamoto University, 2‑39‑1 Kurokami, Kumamoto 860‑8555, Japan
3
Atomic Energy Centre, Chittagong, Bangladesh Atomic Energy Commission, 1018/A, Bayezid Bostami Road, East Nasirabad, Chittagong 4209, Bangladesh
Kumamoto is the largest groundwater user area in Japan (Fig. 1). About one million people in and around the area completely depend on groundwater for their all purposes (e.g., Taniguchi et al. 2019). The area comprises Kumamoto City and its surrounding districts and towns, extended to the western flank of Aso caldera mountains in central Kyushu (
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