Ronit Nativ

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.

Field observation of flow in a fracture intersecting unsaturated chalk

Flow through a natural fracture crossing unsaturated chalk in an arid region was investigated in a field experiment using a specially designed experimental setup. The setup allowed complete control of the flow domain inlet and outlet. Water flux into and out of the fracture was measured in small segments of the fracture openings, and flow trajectories were identified using seven fluorobenzoic acid tracers. A 5 day percolation experiment on a 5.3 m long fracture showed significant spatial and temporal variability of the flow regime. Flow through fracture openings did not reach a steady state either in individual segments or across the entire flow domain, although the boundary conditions were kept relatively steady for the entire duration of the experiment. Flow trajectories within the fracture plane varied over time; however, most of the flow was confined to small sections of the fracture. Over 70% of the flux was transmitted through <20% of the studied fracture openings. Observations from the tracer tests suggest that flow paths can coexist near each other without water mixing, probably because the fracture fill generates unconnected flow paths across the main fracture void.

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.

Salinization and dilution history of ground water discharging into the Sea of Galilee, the Dead Sea Transform, Israel

Abstract The mechanism governing salinization of ground water discharging into the Sea of Galilee in Israel has been the subject of debate for several decades. Because the lake provides 25% of the water consumed annually in Israel, correct identification of the salt sources is essential for the establishment of suitable water-management strategies for the lake and the ground water in the surrounding aquifers. Existing salinization models were evaluated in light of available and newly acquired data including general chemistry, and O, H, C and Cl isotopes. Based on the chemical and isotopic observations, the proposed salt source is an ancient, intensively evaporated brine (21- to 33-fold seawater) which percolated through the valley formations from a lake which had formed in the Rift Valley following seawater intrusion during the late Miocene. Low Na:Cl and high Br:Cl values support the extensive evaporation, whereas high Ca:Cl and low Mg:Cl values indicate the impact of dolomitization of the carbonate host rock on the residual solution. Based on radiocarbon and other isotope data, the dilution of the original brine occurred in two stages: the first took place ∼30 000 a ago by slightly evaporated fresh-to-brackish lake water to form the Sea of Galilee Brine. The second dilution phase is associated with the current hydrological regime as the Sea of Galilee Brine migrates upward along the Rift faults and mixes with the actively circulating fresh ground water to form the saline springs. The spatially variable chemical and isotopic features of the saline springs suggest not only differential dilution by fresh meteoric water, but also differential percolation timing of the original brine into the tectonically disconnected blocks, registering different evaporation stages in the original brine. Consequently, various operations to reduce the brine contribution to the lake may be differentially effective in the various areas.

Hydraulic calculation of groundwater flow velocity and age: examination of the basic premises

Abstract The equations applied to the calculation of groundwater flow velocity and hydraulic age are based on assumptions that are frequently invalidated or not satisfactorily met by actual field conditions. Problem areas that account for significant errors in the calculated values of groundwater flow velocity and age include: (1) poorly defined hydraulic variables; (2) lack of single recharge and discharge areas; (3) extremely small or nonexistent discharge areas and entrapment of ground water in ‘dead’ volumes in deep confined systems; (4) topographic and geologic controls affecting the flow path; (5) fractures/conduits affecting groundwater flow in supposed homogeneous porous media; (6) concealed structural and lithological barriers; (7) mixing of ground water of various ages; (8) slow hydraulic response to climatic or geologic changes. Checks based on physical, chemical, and isotopic data are advocated as a means to validate the basic assumptions used for calculating groundwater flow velocity and age. Hydraulic and isotopically determined groundwater ages provide essential and complementary hydrologic information. Both ages have to be computed with the utmost care, and the validity of the basic premises in each case study must be checked.

Impact of intermittent rainwater and wastewater flow on coated and uncoated fractures in chalk

Two coated and two uncoated slices from the fracture surface of an unsaturated chalk were exposed to short flow events (24, 8, and 9 hours) of industrial wastewater and/or synthetic rainwater, followed by long drying periods (weeks). The topography of the fracture surface was shown to be unstable due to the detachment of colloidal and large-sized particles during the first 3-7 hours of flow. Following rainwater flow, erosion was more pronounced on the coated than on the uncoated surface (mean erosion of 0.313 and 0.134 mm, respectively). Interaction with industrial wastewater generated a skin of organic matter and gypsum that collapsed following contact with rainwater, leading to a deeper erosion of the uncoated surface (1.238 mm) than of the coated one (0.549 mm). Erosion was measured using a laser-scanning system and was calculated from high-resolution topographical maps (elevation z # 6 0.01 mm) generated by Geographic Information System (GIS, ARCInfo) prior to and following the flow experiments. The mean thickness of the erosion was found to be strongly correlated with the thickness of a layer calculated from the total accumulated mass of particles and soluble salts released from the fracture surface. This relationship can be used to evaluate fracture surface erosion in large field and laboratory experiments.

Sequential biodegradation of TNT, RDX and HMX in a mixture

We describe TNTs inhibition of RDX and HMX anaerobic degradation in contaminated soil containing indigenous microbial populations. Biodegradation of RDX or HMX alone was markedly faster than their degradation in a mixture with TNT, implying biodegradation inhibition by the latter. The delay caused by the presence of TNT continued even after its disappearance and was linked to the presence of its intermediate, tetranitroazoxytoluene. PCR-DGGE analysis of cultures derived from the soil indicated a clear reduction in microbial biomass and diversity with increasing TNT concentration. At high-TNT concentrations (30 and 90 mg/L), only a single band, related to Clostridium nitrophenolicum, was observed after 3 days of incubation. We propose that the mechanism of TNT inhibition involves a cytotoxic effect on the RDX- and HMX-degrading microbial population. TNT inhibition in the top active soil can therefore initiate rapid transport of RDX and HMX to the less active subsurface and groundwater.

On the variability of fracture surfaces in unsaturated chalk

The topographical variations in a fractured chalk surface were studied by laser scanning in laboratory experiments after cycles of immersion in tap water and air drying. The surfaces were found to erode by up to 0.295 and 0.352 mm following wetting for 10 min and 14 hours, respectively. Topographical changes were related to the nonuniform release of particles from the surface under no-shear flow conditions. The total amount of particles released decreased exponentially with time during a 96-hour experiment, from 11 mg/L following a wetting period of 10 min (at the beginning of the experiment) to 1 mg/L following a wetting period of 48 hours (at the end of the experiment). These preliminary results suggest that under conditions of variable water content (highly significant in arid and semiarid regions), the aperture, roughness, and flow channels of fractures in soft rocks are transient properties. In the field site, fracture apertures are >1 order of magnitude smaller than expected from the laboratory observations. This discrepancy is related among other possible reasons, to partial wetting and drying cycles and to the development of flow channels within the filling materials rather than along the fracture surfaces.

Impact of coating and weathering on the properties of chalk fracture surfaces

The surface topography of a natural fracture intersecting a chalk formation was mapped using a high-resolution laser-scanning device, and its roughness was evaluated using five different criteria. Two coated and two uncoated slices of the natural fracture surface were encased in flow cells and exposed to short (24 and 8 hours) interactions with percolating synthetic rainwater and industrial wastewater, followed by long drying periods (weeks). These flow experiments simulated the intermittent infiltration of surface runoff and industrial effluents typical of the study area and resulted in erosion of the fracture surfaces and modification of their roughness. The temporal variations in surface roughness were evaluated using three commonly used criteria (standard deviation from mean heights, differences between maximum and minimum heights, and the root-mean-square roughness) and two newly developed measures, namely, the slopes on the surfaces and a calculated roughness-related ratio (RRR). The mean slope maps and RRR calculations were found to be good criteria for evaluating surface topography and its uniformity across the fracture surface. The coated surfaces were significantly rougher, had greater relief, more heterogeneous topography, and a larger surface area than the uncoated surfaces. The results suggest that coating and fast weathering (caused by intermittent flow events) significantly change roughness, surface area, relief, and the heterogeneity of a fracture surface. These, in turn, may affect the spatial distribution of flow paths across that fracture surface. Consequently, when exploring the properties of fracture surfaces and fracture flow, the impact of surface skin and variations of the surface properties over short timescales should be taken into account.

Tracer diffusion from a horizontal fracture into the surrounding matrix: measurement by computed tomography

The vertical diffusion of NaI solution from a horizontal fracture into and within the surrounding matrix was tracked and quantified over time using an artificially fractured chalk core (30x5 cm) and a second-generation X-ray computed tomography (CT) scanner. The different tracer-penetration distances imaged in the matrix above and below the horizontal fracture are indicative of a greater tracer mass penetrating into the lower matrix. The enhanced transport in the matrix below the fracture was related to the Rayleigh-Darcy instability induced by the density differences between the heavier tracer solution in the fracture (1.038) and the distilled water that had initially resided in the matrix. Our observations suggest that below the fracture, the tracer is propagated by an advection-diffusion process that is characterized by both higher rates and higher concentrations relative to its propagation by diffusion above the fracture. The experimental results suggest that the prediction of contaminant migration in a rock intersected by both vertical and horizontal (e.g. along bedding planes) fractures is difficult because of density effects that result in different solute-penetration rates.