Fracture and bedding plane control on groundwater flow in a chalk aquitard

Shallow groundwater in the northern Negev desert of Israel flows preferentially through a complex system of discontinuities. These discontinuities intersect what would otherwise be a massive, low-conductivity, high-porosity Eocene chalk. Vertical fractures and horizontal bedding planes were observed and mapped along approximately 1,200 m of scanline, 600 m of core and 30 two-dimensional trace planes. A bimodal distribution of size exists for the vertical fractures which occur as both single-layer fractures and multi-layer fractures. A bimodal distribution of log transmissivity was observed from slug tests conducted in packed-off, vertical intervals within the saturated zone. The different flow characteristics between the horizontal bedding planes and vertical-type fractures appear to be the cause of the bimodality. Two distinct conceptual models (discrete fracture network) were developed based on the fracture orientation, size, intensity and transmissivity statistics derived from field data. A correlation between fracture size and hydraulic aperture was established as the basis for calibrating the simulated model transmissivity to the field observations. This method of defining transmissivity statistically based on prior information is shown to be a reasonable and workable alternative to the usual conjecture approach towards defining transmissivity in a fractured-rock environment.     

Flow and transport predictions during multi-borehole tests in fractured chalk using discrete fracture network models

Multi-borehole pumping and tracer tests on the 10 to 100-m scale were conducted in a fractured chalk aquitard in the Negev Desert, Israel. Outcrop and core fracture surveys, as well as slug tests in packed-off intervals, were carried out at this site to obtain the parameters needed for construction of a stochastic discrete fracture network (DFN). Calibration of stochastic DFNs directly to the multiple borehole test data was inadequate. Instead, two equivalent deterministic DFN flow models were used: the vertical-fractures (VF) model, consisting of only vertical fractures, and the fractures’ intersections (INT) model, consisting of vertical and horizontal fractures with enhanced transmissivity at their intersections. Both models were calibrated against the multi-borehole response of one pumping test and their predictions were tested against three other independent pumping tests. The average accuracies of all transient drawdown predictions of the VF and INT models were 65 and 66%, respectively. In contrast to this equality in average drawdown predictions of both models, the INT model predicted better important breakthrough curve features (e.g., first and peak arrival times), than the VF model. This result is in line with previously assumed channeled flow, derived from analytical analysis of these pumping and tracer tests. Ronit Nativ, deceased, may her memory be blessed.     

Runoff and erosion processes and rates in the Zin valley badlands,northern Negev,Israel

Badland 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.     

Dilution tests in a low-permeability fractured aquifer: matrix diffusion effect

A point dilution test is commonly used in single-borehole tracer experiments designed to determine the Darcy velocity of a formation. This method is based on the concept that, in a borehole, a tracer's concentration declines as a consequence of the water flux. Based on theoretical simulations and field observations, this study indicates that for low-permeability, yet highly porous fractured formations, the common practice of excluding the effect of diffusive mass flux between the dissolved tracer within the borehole and the surrounding matrix may lead to significant errors in the assessment of the Darcy velocity. This conclusion was confirmed by a model adapted to simulate experimental data collected from a tracer test performed in a vertical, large-diameter (25-cm) borehole drilled along a subvertical fracture intersecting a chalk formation.     

Correlating fracture trends and hydraulic head using semivariogram cloud analysis

Fracture trends (defined as kilometer-scale linear features interpolated between field observations of fractures along their strikes) often have a dominant orientation. Finding a correlation between this orientation and hydraulic data could shed light on their hydraulic influence. A significant correlation between head residuals from first-order regional drift and the orientation of 2- to 4-km-long fracture trends was found in a study site in the Negev, Israel, using the semivariogram cloud analysis. Correlation of head residuals rather than the head itself implies that the orientation of the fracture trends controls the anisotropy and heterogeneity at this scale, mainly because the fracture trends define the orientation of blocks, which differ in their hydraulic properties. Preferential transmissive pathways are probably shorter than the full extent of the fracture trends, causing a relatively high head difference along the trends on the 2- to 4-km scale. Fracture trend density and additional data from short-range hydraulic tests helped characterize two blocks separated by a fault zone. The identification of hydraulic features on a kilometer scale is necessary for better modeling of regional ground water flow and transport. Hydraulic tests at this scale are not feasible, thereby rendering combined analyses of head and structural data, such as the one presented here, essential.     

Faecal indicators and pathogens in selected New Zealand waterfowl

Freshly excreted droppings from Canada geese (n=80), black swans (n=80), ducks (n=80) and gulls (n=80) were collected from sites around New Zealand. The droppings were enumerated for Escherichia coli, enterococci and Salmonella spp., and for the presence/absence of Cryptosporidium spp. Overall prevalence of E. coli and enterococci in samples was 95% and 94%, respectively. Cryptosporidium spp. was detected in 2% of the samples, whereas no Salmonella spp. were detected in the survey. Preliminary estimates of daily microbial outputs suggest that ducks will produce the highest loadings of E. coli and enterococci per bird, whereas Canada geese will produce the highest loadings of Campylobacter spp. per bird. This study provides the first set of indicator and pathogen counts for one of the largest sources of diffuse faecal contamination of natural waters in New Zealand.