The spreading of reverse-osmosis desalinated seawater (DSW) in the Israeli Coastal Aquifer was studied using groundwater modeling and stable water isotopes as tracers. The DSW produced at the Hadera seawater reverse osmosis (SWRO) desalination plant is recharged into the aquifer through infiltration pond at the managed aquifer recharge (MAR) site of Menashe, Israel. The distinct difference in isotope composition between DSW ( δ 18 O = +1.41; δ 2 H = +11.34 ‰) and the natural groundwater ( δ 18 O = −4.48 to −5.43 ‰; δ 2 H = −18.41 to −22.68 ‰) makes the water isotopes a preferable tracer compared to widely-used chemical tracers, such as chloride. Moreover, this distinct difference can be used to simplify the system to a binary mixture of two end members: desalinated seawater and groundwater. This approach is especially robust when spatial data of stable water isotopes in the aquifer is scarce. A calibrated groundwater flow and transport model was used to predict the DSW plume distribution in the aquifer after 50 years of MAR with DSW. The results show that after 50 years 94 % of the recharged DSW was recovered by the production wells at the Menashe MAR site. The presented methodology is useful for predicting the distribution of reverse-osmosis desalinated seawater in various downstream groundwater systems.