Ofer Dahan

Water Recharge and Solute Transport Through the Vadose Zone of Fractured Chalk Under Desert Conditions

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 dry-drilling 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 (δ18O = +5.94‰) and highly concentrated (∼29 meq Cl/100 g rock), but changes gradually with depth to amore dilute concentration (∼4 meq Cl/100 g rock) and isotopically depleted composition (δ18O = −4.4‰), 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.

Water salinization in arid regions—observations from the Negev desert, Israel

Abstract 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, vadose water from four coreholes penetrating chalk formations, and groundwater from 16 monitoring wells tapping the chalk aquitard. Dissolved carbonate dust and evaporation of the falling raindrops result in Ca(HCO 3 ) 2 facies and increased ion concentration of the rainwater with respect to inland, more humid regions. The exposure of flood water to evaporation during flood events is minimal. The observed Ca(HCO 3 ) 2 facies 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 exchange on Na- and K-bearing minerals and the dissolution of calcite, gypsum and halite. The presence of these salts at and near land surface results from the complete evaporation of rainwater in land surface depression 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 NaCl facies of vadose water and the variable rates of isotopic depletion and salt dilution observed in the underlying heterogeneous groundwater in the saturated zone. Although the variable mixing with low-salinity, isotopically depleted water percolating from the fractures accounts for the depleted isotopic composition of the groundwater, its relatively low solute content cannot modify the groundwater NaCl facies. Consequently, only groundwater salinity in the chalk is reduced by the preferentially flowing water, but the Ca(HCO 3 ) 2 facies prevailing in the rainwater and flood water disappears, and the NaCl imprint from the vadose zone prevails.

Particle transport in unsaturated fractured chalk under arid conditions

A series of field and laboratory experiments were conducted to study the mechanisms of particle detachment and transport from fractures in vadose chalk. Experiments of intermittent flow events along fracture surfaces were carried out in the laboratory. In the field, water was percolated from land surface via a discrete fracture into a compartmental sampler installed inside a horizontal corehole located I m below the surface. The mass, size distribution, and composition of the particles drained from the fracture voids were examined along with flow rates and salt dissolution. Two boreholes penetrating the underlying saturated zone were sampled and analyzed for colloidal concentration and composition. Most of the particle and solute release at the drained effluents occurred during the first several hours of flow, but erratic pulses of particles were still observed after long periods of time. Most of the detached particles had a mean diameter of >2 microm, while the mobile colloidal phase in the groundwater had a mean diameter of approximately 1 microm. Mineralogical composition of the groundwater colloids and the particles detached from the upper vadose fracture were similar. Laboratory observations demonstrated the importance of the existence of a coating layer, made of weathered particles and salts, on particle detachment. The results of this study suggest that: (1) particle detachment causes flow-rate variability in the unsaturated fracture; (2) the mechanisms of particle detachment and salt dissolution within the fracture are linked: and (3) although most of the detached particles are large and likely to accumulate inside fractures, some colloidal particles also eroded from the fracture void and are likely to be transported to the groundwater.