In this research, we aimed to examine the effect of unsteady recharge process by river water during an extreme rainfall event on a groundwater environment based on 8-year monitoring results of groundwater level, major water quality, and oxygen and hydrogen stable isotopic ratios (δ¹⁸O and δD) in Okayama alluvial plain, coastal Seto Inland Sea.

Hydraulic heads in the deeper groundwater with depths of 10–20 m were below the sea level. It suggests groundwater depression by artificial pumping. The main recharge source of groundwater under steady state was estimated to be river water in the upper plain, while it was rainwater and seawater in the lower plain. However, the deeper groundwater indicated extremely low δ¹⁸O for 2 years in the central plain, and the highest EC was observed in the lower plain after the maximum flood. These results suggest that an unsteady recharge process by river water in the central plain and seawater intrusion from the tidal river in the lower plain occurred due to the largest hydraulic gradient between river water and the deeper groundwater. Based on the Darcy flux estimated by the hydraulic gradient for a year after the flood, the transport time of flooding river water to the deeper groundwater in the central plain was estimated to be 169–405 days. This value was similar to the time that has elapsed before the minimum δ¹⁸O was observed in the deeper groundwater from the flood (300 days). This result supports the unsteady groundwater recharge process by the extreme flood.