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Water percolation and tracer migration through the vadose zone underneath an ephemeral channel were studied using a vadose zone monitoring system (VMS) and application of a multitracer test. The VMS included flexible time‐domain reflectometry (FTDR) probes for continuous tracking of water content profiles, and vadose zone sampling ports (VSPs) for frequent sampling of the deep vadose pore water at multiple depths. The VMS was installed directly under an infiltration pond with several infiltration rings containing a traceable solution. Water content measurements by the FTDR probes allowed detailed visualization of the vadose wetting process; VSP samples allowed the establishment of tracer breakthrough curves at various depths. Flow velocities and fluxes were calculated from both the wetting process and the tracer breakthrough curves. The multitracer experiment revealed an unsteady flow pattern strongly affected by the layered structure of the sediments. The tracer breakthrough curves indicated domination of a mobile–immobile flow mechanism controlling contaminant migration across the vadose zone. The experimental setup demonstrated the ability of the VMS to provide real‐time monitoring of water flow and contaminant transport in the vadose zone.

This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four drydrilling holes were bored through the vadose zone. Core and auger samples, collected at 30-to 50-cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6-11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth.Close to land surface, the pore water is strongly evaporated (15180 = +5.94%0) and highly concentrated (--•29 meq CI/100 g rock), but changes gradually with depth to a more dilute concentration (-4 meq Cl/100 g rock) and isotopically depleted composition (151•O = -4.4%o), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a hydrologic barrier. •J,•a,,l•,,11 LI•JII, 11 ari Ll•Jllari I i •,.71 LI •,Cl LIII•,11 L and isolation of hazardous waste has been operating there since 1975. The aridity of the area (50-200 mm precipitation/ ,,, J LEGEND O Sands 8, AlLuvium (Ouat. Hotoc.) Ku Kurkor (Ouot. Pteist) NATIV ET AL.: WATER RECHARGE AND SOLUTE TRANSPORT 261 207-225, 1992. Zimmermann, U., D. Ehhalt, and K. O. Munnich, Soil-water movement and evapotranspiration: Changes in the isotopic composition of the water, in Isotopes in Hydrology, Proceedings of the IAEA Symposium, Vienna, pp. 567-585,

SUMMARYBadland areas are usually regarded as impermeable zones which generate high runoff and are very vulnerable to sheetwash and rainsplash. To test those considerations sprinkling experiments using two rainfall simulators were carried out on slopes of varying aspect in the northern Negev (Israel). For one unit 1.5 m2 plots were used with rainfall of natural characteristics at 36 mm/hr intensity and 43-48 minute duration, runoff being recorded and water/sediment samples taken every 5 minutes. The second unit was used on 30-50 m2 plots but rainfall energy production was below that of natural rainfall. Results show that due to the high stability and strong flocculation of clay-rich aggregates rainsplash is ineffective in surface sealing so that infiltration capacities remain high despite intense, prolonged rainfall. Aspect differences are reflected in variation of surface properties despite homogenous bed-rock, which cause marked differences in hydrological response. North-facing slopes respond more quickly, more frequently and produce more runoff than south-facing slopes. Non-uniform runoff generation is also seen within plots of one aspect reflecting subtle variations in surface properties. Comparison of rainfall intensity and duration used during the experiments with those prevailing under natural conditions shows that under present day conditions surface flow in the Zin valley badlands must be extremely infrequent and denudation rates very low.