Hydraulic Connections Among Subaquifers of the Coastal Plain Aquifer, Israel

The intensive use of the Coastal Plain aquifer, Israel, as well as its ongoing contamination by domestic and industrial waste and agrichemicals has resulted in the deterioration of its water quality. The Coastal Plain aquifer is divided into a number of calcareous sandstone units, separated by silt and shale lenses. Whereas the shallow, uppermost units are unconfined, the lowermost aquifers are partly confined. Although the presence of hydrological subunits is geologically well‐documented, the aquifer has been traditionally managed by the Israel Water Authority as a single water system. This study focused on the deeper units of the Coastal Plain aquifer where contamination may be less significant. The hydraulic connections between the overlying shallow units and the deep units were examined along two regions parallel to the main ground‐water flow direction, from east‐southeast to west‐northwest. The working hypothesis was that if hydraulic separation by the silt and shale lenses is effective, chemical and isotopic variations can be traced, and ground‐water‐level variations in each unit are likely to be different. The observed difference in hydraulic heads, distinct patterns of temporal water‐level variations, different C‐14 and tritium activities, and diverse chemical compositions suggest that the silt and shale lenses form an effective hydraulic partition between the shallow and deep sand units, both in the western and eastern parts of the aquifer. Our limited observations indicate that although the nature of the hydraulic separation and its effects are spatially variable, the prevailing conceptualization of the aquifer as one water body lacking vertical separation is incorrect.