- Intensive use of the Coastal Plain aquifer in Israel, as well as its ongoing contamination by agrochemicals and domestic and industrial waste, has resulted in the deterioration of its water quality. As a result, suggestions of abandoning this aquifer, which is a major source of potable water, have been seriously considered. The Coastal Plain aquifer is divided into a number of calcareous sandstone units separated by continental and marine lenses composed of silt and shale. Whereas the uppermost units are unconfined and are recharged directly from land suface by precipitation, wastewater, and runoff, the lower aquifers are partly confined. Although the presence of subaquifers is geologically well documented, the aquifer has been traditionally managed as a single water reservoir. The prevailing notion has been that the separation into subunits exists only up to 5 km from the shoreline, whereas, in the east, the aquifer is considered unlayered and uniform. However, hydraulic data, isotopic age determinations and variations in chemical composition, suggest that the clay lenses form an effective hydraulic partition between the different sand units, both in the western and eastern parts of the aquifer. The results of this study indicate that the prevailing conceptualization of the aquifer as one homogeneous water body is incorrect.