Chemical and Isotopic Variations with Depth: a Detailed Saturated Zone Profile of a 140m Thick Coastal Aquifer

A percussion borehole was constructed through the saturated zone of the Mediterranean coastal aquifer in Tel Aviv, Israel, penetrating its three subaquifers and the upper part of the underlying Saqiye aquitard. The research site was previously subjected to direct industrial contamination and is currently exposed to the industrial contaminants in the outskirts of the densely populated Tel Aviv metropolis. Here we report the results of a large variety of analysis conducted on the 140m saturated profile that included field measurements (e.g. dissolved oxygen, pH, temperature, conductivity), major elements (e.g. Cl-, SO42-, HCO3-, NO3-, Ca2+, K+, Na+), trace elements (e.g. Pb, Fe, Cu) and radium isotopic measurements (223Ra, 224Ra, 226Ra). A clear distinction between the units becomes evident along the vertical profile; the upper phreatic unit (A) appears to be more saline relative to the central unit (B) (TDS of 734 and 670 mg/L, respectively). The deep unit (C) is significantly more saline in its lower part (up to 860 mg/L). We observed two nitrate peaks in the central zones of subaquifers B and C. The high nitrate peaks are associated with low Na/Cl and high Ca/Cl ratios. The 224Ra/223Ra ratio also changes with depth; in the upper and the lower subaquifers the relatively low 224Ra/223Ra ratios (50) indicates a larger fraction of a uranium source whereas in the central zone of the aquifer high 224Ra/223Ra ratios reflect rather a predominant thorium source for the dissolved radium. The data obtained through this borehole allows a rare investigation of the heterogeneity of water quality and composition in a coastal aquifer. The data provides characterization of different end-members along the saturated zone and also indicates the different proportions of lateral versus vertical flows of groundwater in a porous media.