ESTIMATION OF GROUNDWATER RECHARGE AND SALINIZATION IN A COASTAL ALLUVIAL PLAIN AND OSAKA MEGACITY, JAPAN, USING δ18O, δD, AND Cl−

Abstract

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.