Geochemical and isotopic imprints of groundwater evolution in mountainous areas of Maniwa City, Okayama Prefecture, Japan

Groundwaters mainly along the Asahi River in Maniwa City, a mountainous area in Okayama Prefecture, have been supplied as tap water to the residence. To examine the source and water quality of the groundwaters, chemical and isotopic (δD and δ¹⁸O) analyses were conducted on 58 shallow groundwater and 30 river water samples collected from 51 water intake facilities twice during winter (February–April) and summer (August–September) in 2017. Three hot springs were also sampled. The water temperatures of the groundwater in the northern area was almost constant throughout the year, whereas seasonal temperature variation was observed in the other area. Groundwater samples were plotted along a line with slope 3 in a δD vs. δ¹⁸O diagram. Both δD and δ¹⁸O of the groundwater showed tendencies to be low in the north and high in the south due to altitude effect. The groundwater sampled here had low electric conductivity and samples in the north and south were rich in SiO₂ and Ca²⁺–HCO₃ ⁻, respectively. This strongly reflects the influence of regional geology, containing diatomite in the north and limestone in the south. These results indicate that the groundwater in Maniwa City flows near the surface and that their residence times are several months except for groundwater in the northern mountains. From the major and trace components, the groundwater in Maniwa City are judged to be safe as drinking and agricultural water.