ESTIMATION OF GROUNDWATER RECHARGE AND SALINIZATION IN A COASTAL ALLUVIAL PLAIN AND OSAKA MEGACITY, JAPAN, USING δ18O, δD, AND Cl−
Keywords:Coastal groundwater, Stable isotope, Chloride, Seawater intrusion
Seawater intrusion and salinization are the most common problems of water pollution in coastal groundwater. To clarify the process of groundwater salinization in a coastal alluvial plain, we estimated the groundwater recharge and salinization process in Osaka coastal groundwater using δ18O, δD, and Cl−. Water samples were collected at 14 boreholes of 9 plots with depth of −5 to −60 m amsl in March 2015. The δ18O values and Cl− concentrations of the groundwater varied spatially from −6.4 ‰ to −4.7 ‰ and 17 mg/L to 5193 mg/L, respectively. Based on Cl− concentrations, the maximum mixing ratio of seawater with the concentration of 1800 mg/L into groundwater was estimated to be 29% about 3 km inland from the shoreline at the depth of about −40 m. The relationship between δ18O and Cl− of groundwater and seawater indicated three types of end members: seawater with high Cl− and δ18O, a groundwater source with lower Cl− and lower δ18O (−5.6 ‰), and a groundwater source with lower Cl− and the lowest δ18O (−7.2 ‰). According to the relationship between the altitude of groundwater recharge and δ18O established by previous research, two types of groundwater recharge and source areas were estimated to be coastal lowland and upland with altitudes <10 m amsl and surrounding hill 18 km inland from the shoreline with altitudes of 100 m amsl.