Seasonal Variation in Groundwater Quality Revealed by the Multi-tracer near the Coastal Area of Sendai, Japan

Abstract

　To elucidate the characteristics of groundwater near the coastal area in Sendai and the variation in its quality, water samples were collected every month since December 2018. Water quality assessment of the open-channel flow (hereafter referred to as the channel) indicated the presence of Na-Cl with extremely high electrical conductivity. Shallow groundwater contained (Na+Ca)-HCO₃, while deep groundwater contained Na-(Cl+HCO₃) or Na-HCO₃. The water quality of the channel and shallow groundwater exhibited seasonal variations owing to precipitation. The quality of deep groundwater was almost constant with no seasonal variation; however, S-3, which is a deep well located near the seashore, differed slightly from other deep groundwaters in that the Na⁺ and Cl^- concentrations in this well were higher than those at other sites, perhaps owing to the effect of sea salt. The δ¹⁸O and δ²H values were relatively low in S-3; therefore, its recharge elevation was likely higher than that of other sites owing to the effect of altitude. Additionally, the residence time of deep groundwater estimated using CFCs was approximately 70 years.