干旱区生态环境知识资源中心

Recent concerns about the rapid deterioration in the quality of water resources in Israel, in addition to the widely documented anthropogenic acidification of precipitation in many parts of the world, have put a ’’state of the art’’ review of our knowledge of the chemistry of precipitation in Israel on the order of the day. Electrolyte content in precipitation in Israel is inversely related to average annual precipitation, and thus is 3-6 times higher in southern than in central Israel. The major reason for these differences is the differential enrichment in components of continental origin, such as Ca2+ and HCO3-. The chemical composition of precipitation is determined by the balance between components of marine and continental origin. Na+ and Cl- are the major ions contributed by sea spray, and therefore most of the precipitation exhibits Na/Cl (molar) ratios close to that of seawater. Precipitation in the southern parts of the country is an exception. Other ions of partly marine origin are Mg2+, SO42-, K+, and, to a limited extent, Ca2+. Marine contribution to precipitation decreases with distance from the sea. The most important component of continental origin in precipitation in Israel is Ca2+, similar to precipitation in other semiarid areas surrounding the Mediterranean. Calcium in precipitation is often associated with dryfall (dust) originating in North African desert areas. NH4+ concentration levels are relatively low, and thus, the importance of this ion as an acidity-neutralizing agent is small compared to central/northern Europe. Precipitation in Israel is neutral to mildly alkaline, with an average pH of 6.5 +/- 0.8. Mildly alkaline rain is the rule in central and southern Israel. In the Golan Heights, Galilee, and the northern coastal region, most of the precipitation events are acidic. On Mt. Carmel, 40% of the rain events examined during three rainy seasons were strongly acid, and in more than 65% of the events, pH was less than or equal to 5.6. Acidity in precipitation in Israel is determined primarily by excess SO42-. Nitrates have only a subordinate role. Synoptic analysis as well as sulfur isotopic data suggest that the excess sulfate is a long-range import from over the Mediterranean Sea, either of anthropogenic European or of biogenic/marine dimethyl sulfide (DMS) origin. In southern Israel, an isotopic amount effect was observed, whereby rain in rainier years was isotopically lighter delta(18)O than precipitation in the less rainy years. delta(34)S values of precipitation were found to be in the range of 0-15 parts per thousand, thus indicating that precipitation sulfate consisted of a mixture of marine and nonmarine sulfate. Cl and Sr isotope ratios in precipitation were determined as well.