WATER SALINIZATION IN ARID REGIONS - OBSERVATIONS FROM THE NEGEV DESERT, ISRAEL

The processes affecting salinization of precipitation, surface water, vadose water and groundwater were studied in the Negev desert, Israel. Observations spanning 18 years included the collection of rainfall at three rain sampling stations, flood water at six flood stations, vado se water from four coreholes penetrating chalk formations, and groundw ater from 16 monitoring wells tapping the chalk aquitard, Dissolved ca rbonate dust and evaporation of the falling raindrops result in Ca(HCO 3)(2) facies and increased ion concentration of the rainwater with res pect to inland, more humid regions. The exposure of flood water to eva poration during flood events is minimal. The observed Ca(HCO3)(2) faci es and salt enrichment by a factor of three to five in the flood water with respect to precipitation results primarily from interactions of the flood water with the chalk and limestone bedrock, including ion ex change on Na- and K-bearing minerals and the dissolution of calcite, g ypsum and halite. The presence of these salts at and near land surface results from the complete evaporation of rainwater in land surface de pression storage areas following most rain events, Except for a small portion moving through the low permeability chalk matrix, most of the vadose water moves through preferential pathways and is typically not exposed to evaporation. This dual movement of water accounts for the N aCl facies of vadose water and the variable rates of isotopic depletio n and salt dilution observed in the underlying heterogeneous groundwat er in the saturated zone. Although the variable mixing with low-salini ty, isotopically depleted water percolating from the fractures account s for the depleted isotopic composition of the groundwater, its relati vely low solute content cannot modify the groundwater NaCl facies. Con sequently, only groundwater salinity in the chalk is reduced by the pr eferentially flowing water, but the Ca(HCO3)(2) facies prevailing in t he rainwater and flood water disappears, and the NaCl imprint from the vadose zone prevails. (C) 1997 Elsevier Science B.V.