Contribution of precipitation to groundwater flow systems in three major alluvial fans in Toyama Prefecture, Japan: stable-isotope characterization and application to the use of groundwater for urban heat exchangers

The isotopic compositions (D and ¹⁸ O) of 177 precipitation samples collected at seven observation stations in Toyama Prefecture and one station in Gifu Prefecture in the northern part of central Japan were determined. The source and characteristics of the isotopes were clarified and their contribution to the groundwater flow systems of three major alluvial fans in the same area were investigated. The δD and δ ¹⁸ O values ranged from −113.3 to −26.7‰ and − 16.4 to −4.2‰, respectively. Precipitation samples collected from May to September (summer) and November to March (winter) plotted along two meteoric water lines, with d-excess = 10 and 30, respectively. Conversely, precipitation samples collected in April and October, and some samples in November to March, plotted between the two meteoric water lines. The contribution of precipitation to the groundwater systems was modelled based on the assumption that groundwater is a mixture of major river water and precipitation. According to the observed δ ¹⁸ O values for the precipitation, river water, and groundwater samples, the contribution of local precipitation to groundwater reservoirs ranged from 5 to 100%. Groundwater flows near the rivers did not always originate from 100% river runoff; however, the contribution of river runoff to groundwater decreased with increasing distance from the rivers, and groundwater flows far from the river were generated mainly by precipitation. The possibility of using groundwater for a ground-source heat pump system, for air conditioning in houses and to melt the snow on roads, is also discussed.