Hydrogeochemical controls and recharge-flow system of groundwater in the lowland of the Fukui-Ohno Basin, central Japan, based on carbon isotopic signatures

Abstract

• River water mixing and CO 2 dissolution in groundwater was modeled with δ 13 C tracers. • Decomposition of both C3 and lacustrine organic matter was related to CO 2 dissolution. • Modeling identified similar amounts of vertical and horizontal groundwater recharge. Carbon stable isotopic compositions of dissolved inorganic carbon (δ 13 C DIC ) and chemical compositions of groundwater were used to investigate the processes of supply of DIC to groundwater in the lowland area of the Fukui-Ohno Basin and to examine constraints on the groundwater recharge-flow system. The groundwater in the basin is dominantly of Ca-HCO 3 type with relatively high concentrations of Ca 2+ and HCO 3 − and lower concentrations of Mg 2+ ; this groundwater chemistry can be attributed to weathering within the aquifer of mainly anorthite and some pyroxene at partial pressures of CO 2 that are typical of soils. The δ 13 C values of the CO 2 gas initially equilibrated with DIC in the groundwater were calculated to be −28.6‰ to −22.8‰, indicating that the groundwater in the lowland basin has two main sources of carbon: (1) C3 organic matter (δ 13 C ≈ −29‰), distributed mainly in mountains surrounding much of the basin and in aquicludes within the basin; and (2) organic matter in shallow lacustrine deposits within the lowland basin (δ 13 C ≈ −20‰). The results of modeling accounting for these two sources demonstrated that roughly equal proportions (50 ± 20%) of DIC in the groundwater are sourced from lacustrine deposits and C3 organic matter. The distributions of these types of organic matters in and around the lowland basin suggest that there are two major and roughly equal components of groundwater recharge: a vertical component of downward percolation of precipitation in the lowland area and a mainly horizontal component of groundwater flow originating in the mountains around the study area and flowing through aquifers into the lowland basin.