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Abstract:
Mountain Block Recharge (MBR) has been previously investigated focusing on the semi-arid regions. However, there has been few research on MBR in the warm humid regions with the steep topography like Japan. We applied a multi tracer approach to evaluate the role of MBR on the groundwater in a basin, Azumino, Nagano Prefecture, Japan. Azumino is located in a central area of Japan and has an area of 2,232 km2 with an elevation ranging from 495 m to 3,192 m, underlain by permeable alluvial deposit with annual average precipitation of 1,110 mm and annual average temperature of 11.8 °C. We sampled groundwater at 33 sites, river water at 16 sites and spring water at 6 sites from July to September 2020. We analyzed the inorganic solute concentration, oxygen-18 and deuterium isotopic ratios, SF6 and CFCs concentrations on all water samples. We applied End Member Mixing Analysis (EMMA) to estimate the contribution ratio of MBR to the groundwater in Azumino basin using SF6 and oxygen-18 as tracers, and the contribution ratio of MBR shows 38% in average ranging from 0% to 62%. The results show that the MBR is an important factor on the groundwater in a warm humid basin with a steep topography, and SF6 is an effective tracer to separate MBR from the groundwater recharged by the river water discharged from the headwater.
