Abstract: The southern Coastal Plain in Israel incorportates a transitional fringe of the desert in which three different chemical types of groundwater are found: (1) near-surface waters from springs along the Besor River course: (2) shallow- to moderate-depth waters from the slightly westward-dipping Pleistocene coastal aquifer (this aquifer, which consists of sandstone layers of the Kurkar Group, is recharged in the Coastal Plain); and (3) deep waters of the westward-dipping Upper Cretaceous Judea Group carbonates, which are recharged in the mountains in the east. A thick aquiclude of Upper Cretaceous-Tertiary rocks separates the Judea Group aquifer from the overlying coastal aquifer in the southern Coastal Plain. Isotopically light oxygen and depleted deuterium characterize the Judea Group waters, as expected from high-altitude recharge. The isotopic composition of the Coastal Plain waters is variable, but for the most part enriched in 18O and D. Within the southern Coastal Plain aquifer a southern subgroup comprises waters more depleted in heavy isotopes than those of either the northern or eastern subgroups. The Besor waters are isotopically similar to the Judea Group waters, reflecting their origin in the mountain region, and flow through the surficial river gravels and sands. It is suggested that leakage of the Besor waters into the underlying southern Coastal Plain aquifer results in mixing of the two water types. The most prominent chemical feature characterizing the groundwater of the southern Coastal Plain is Na+Cl− \textgreater 1. This Na+Cl− ratio can be maintained only by a continuous input from a non-marine source of Na. The most plausible source of this Na is the dissolution of feldspar derived from the windblown loess deposits which cover the area and/or leaching of trona minerals found in the unsaturated zone, combined with base-exchange processes.