In order to identify the origin of the salinity and formation of saline plumes in the central part of the Mediterranean coastal aquifer of Israel, we determined the elemental and boron, strontium, and oxygen isotopic compositions of fresh and brackish groundwater (Cl up to 1500 mg/L). We distinguish between two key anthropogenic sources: (1) sewage effluents used for irrigation with high Na/Cl, SO4/Cl, and B/Cl ratios and low Br/Cl ratios relative to seawater ratios, low Δ11B values (0~10?) and high Δ18O values (>-4?); and (2) imported water from the Sea of Galilee that is artificially recharged to the aquifer with high Br/Cl (3¿10-3) and Δ18O values (-1?) and a low 87Sr/86Sr ratio of 0.70753. The brackish groundwater from the saline plumes have relatively low Na/Cl ratios (0.5--0.8) and high Ca/Mg, Mg/Cl, and Ca/(SO4+HCO3)(>1) ratios relative to seawater ratios; marine SO4/Cl and Br/Cl ratios; Δ11B values of 24.8--49.9?; Δ18O of -2.95? to -4.73?; and 87Sr/86Sr ratios of 0.708275--0.708532. The composition of most of the investigated groundwater from the saline plumes differs from those of the anthropogenic sources, imported water, fresh uncontaminated groundwater (87Sr/86Sr of 0.70866, Δ11B of 20--30?), and saline water from the adjacent Eocene aquitard. Only in areas of artificial recharge does local groundwater have high Br/Cl and Δ18O values that are typical to the Sea of Galilee. The linear relationships between chloride and most of the ions, including B and Sr, the relatively high Δ11B(>30?) and low Δ18O(<-4?) values, and the chemical signature of the saline plumes (e.g., marine Br/Cl and SO4/Cl ratios), suggest that (1) mixing processes control the chemical composition of the brackish water within the aquifer, and (2) the saline postulated end-member has a chemical composition that resembles modified seawater with a marine and higher Δ11B values, and a 87Sr/86Sr ratio of <0.7083. We propose that most of the salinization phenomena and the formation of saline plumes in the inner parts of the coastal aquifer are derived from upconing of underlying natural saline water bodies and enhanced by overexploitation and draw-down of the overlying fresh groundwater. ¿ 1999 American Geophysical Union