CHARACTERIZATION OF A DESERT AQUITARD - HYDROLOGIC AND HYDROCHEMICAL CONSIDERATIONS

The northern Negev desert has become a prime area for siting potentially contaminating facilities, including chemical industries and the Israeli national site for isolation and treatment of hazardous waste. The area is underlain by chalk formations having low matrix permeability. However, abundant fractures create preferential pathways for contaminant migration from the land surface to ground water in the chalk aquitard and, possibly, to adjacent aquifers. Geologic investigations preceding the construction of these industries assumed that the combination of the arid climate and low matrix permeability of the aquitard was sufficient to prevent any contaminant movement into its ground water. Solute transport through fractures and dissolution channels was never considered. However, contamination has been recently observed in the aquitard. Observations regarding the seasonal fluctuation of ground-water levels, oxygen-18 and deuterium composition, and the measurable tritium (0.3 to 15.9 TU) provide the basis for the following conclusions: (1) Ground-water recharge by winter precipitation and runoff water occurs rapidly despite the regions's aridity and the low permeability of the overlying chalk matrix, (2) recharge occurs along fractures in all regions and is not restricted to hydrologically upgradient areas, and (3) recently recharged water is being mixed with the base flow of older water. Older ground water may flow along smaller, less developed fracture systems or through the porous matrix, whereas recent water probably moves along wider fractures or karst channels formed underneath ephemeral streams.