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...distribution which is unknown. Whereas the slope of the potentiometric surface provides the general direction of the driving force maintaining ground water flow, the spatial pattern of the fracture systems controls the actual ground water flow-paths, which may locally zigzag to follow the most permeable routes. In contaminated areas, the identification of these preferential pathways is crucial for (1) determining the areas that are likely to be affected by contamination in the future, (2) selecting appropriate monitoring or remediation zones through which contaminants are likely to migrate, and (3) reliably predicting contaminant migration rates. This prediction is derived from the modeling of ground water flow and contaminant transport in the study area. The models simulating ground water flow in the fractured water-bearing units require information about the prevailing hydraulically active fractures, their orientation, spacing, hydraulic aperture, or hydraulic conductivity. As the occurrence of enhanced permeability due to fractures is largely unpredictable, the permeability of fractured formations can be determined reliably only on the basis of field measurements (van der Kamp 2001).
A variety of methods have been developed over the years to locate and hydraulically characterize subsurface fracture systems. Fracture location techniques draw on anomalous contrasts and response of the fractured zone with respect to the surrounding rock matrix. They include both surface geophysics, e.g., seismic reflection (Hsieh et al. 1993), refraction of S and P waves (Gburek et al. 1999; NRC 1996), ground-penetrating radar (Hubbard et al. 1997), or various logs run in boreholes, e.g., video, natural gamma, neutron, caliper, tube-wave, and televiewer logs (Hardin and Chang 1987; Keys 1989; Morin et al. 1997; Paillet 1994). Identification of the hydraulically active fractures may be carried out through borehole logs such as electrical conductivity (EC), temperature, and radon, relying on the changing fluid features near the contributing fractures (Cook et al. 1999; Morin et al. 1997; Paillet 1994). Hydraulic characterization of the active fractures can be carried out using heat-pulse logs (Morin et al. 2000; Young 1995) and pumping, slug, and point-dilution tests (Barker and Black 1983; Hanor 1993; Maloszewski and Zuber 1993; Rubin et al. 1996; Sen 1986; Shapiro and Hsieh 1998). A more complete characterization can be obtained by testing more than one borehole, either through pumping tests or through tracer tests (D'Alessandro et al. 1997; Gernand and Heidtman 1997; Novakowski and Lapcevic 1994).
In this paper, we present a collection of surface and borehole techniques that enabled us to characterize the hydraulic properties of fractured chalk formations. Because some of the methods provided similar types of information, they are compared, and conclusions regarding the most effective and cost-beneficial method are presented.
Methods
The study area is located in the northern Negev desert, Israel (Figure 1), where a thin Holocene loessial cover is underlain by Eocene chalk formations with low matrix permeability (~0.002 m/day). The abundant fractures crossing the chalk have been identified as responsible for the migration of industrial contaminants to the underlying ground water (Nativ and Nissim 1992; Nativ et al. 1995, 1999). Potential leakage of the contaminated ground water from the chalk into adjacent aquifers is of major concern.
[FIGURE 1 OMITTED]
Identification of the Prevailing Fracture Systems in Outcrops
Recognizing the importance of preferential ground water flow in chalk, considerable effort was made to identify the major hydraulically active fracture systems crossing the industrial complex. As part of this work, aerial photographs of the study area dating back to the early 1970s, before it was leveled off for the industrial site, were processed using a special algorithm to identify surface lineaments that might represent major fractures or fracture zones (Karnieli et al. 1996). Field verification of the lineaments was carried out by mapping the orientation of faults, fractures, and joints intersecting the chalk formations outcropping along ephemeral streams, man-made cuts, and a trench 190 m long and 4 m deep. Attention was focused on large-extension, through-going, multilayer fractures and joints (Bahat 1987a, 1987b; Bloomfield and Nygaard 2001), the depth of which in some areas reaches 100 m below land surface (bls). They may be intersected by other shallow (related to unloading) or deeper-seated fracture systems. The intersection of such systems could allow contaminant transport to great depths over long distances.
Drilling Coreholes into the Fracture Systems
When the fracture systems were field-mapped along the outcrops, it became clear that aside from fractures oriented along the bedding planes,
most fractures were vertical or subvertical. Drilling deep coreholes in the fracture planes (to monitor preferential flow and transport) and keeping the coreholes aligned with them seemed technically impossible. Consequently, inclined coreholes were drilled to ensure intersection with the vertical fracture sets below the water table. Six 8.9 cm coreholes were drilled in three clusters, each pair consisting of one open shallow corehole (~40 m deep; RH-1, RH-11, and RH-23) and one deep corehole (~100 m; RH-101, RH-111, and RH-123) cased throughout the depth of the nearby shallow corehole. Consequently, a continuous 100 m long core and 100 m long in-hole testing section were obtained for each cluster. Most of the coreholes were drilled normal to the prevailing orientation of the local fracture system (60[degrees]), at an angle of 22[degrees] from the vertical, and at a distance from the target fracture system, such that the corehole would intersect the fracture plane at a depth below the water table. By doing so, these inclined coreholes also intersected the horizontal bedding planes, along which fractures had been observed in the outcrops. In the RH-11/RH-111 corehole cluster, an additional borehole (RH-11b) was also drilled normal to the second dominant fracture system (340[degrees]) to allow for its testing. In the RH-1/RH-101 cluster, the RH-1 corehole was drilled vertically.
Core Mapping for Fracture Distribution, Inclination, Orientation, and Ground Water Flow Signs
The chalk cores were mapped in great detail for fracture distribution and spacing. Because the coreholes were inclined, the orientation of each fracture could be determined using a device that aligned the fractured core at its original inclination (22[degrees]) and orientation (155[degrees]) (Figure 2). Using the faint bedding, the compactional direction of trace fossils, and the indications of direction of stratification deduced from the trace fossil Zoophycos, the bedding planes in the inclined cores could be reconstructed and the fracture inclination (dip) and orientation (strike) could be measured using a Brunton compass.
[FIGURE 2 OMITTED]
Ground water flowing through fractures in chalk is likely to modify the fracture surfaces by enhancing weathering and dissolution of the fracture walls. The resulting larger aperture is expected to further increase the hydraulic conductivity and, in turn, the flow channeling through the fracture. Higher oxygen levels in the flowing ground water are expected to result in precipitation of trivalent iron as iron oxide on the fracture surfaces. The transport of filling materials from land surface into the fracture voids is also facilitated by the enlarged aperture and enhanced ground water flow velocity. The occurrence of weathering, dissolution, and coating on the fracture surfaces was described for each...
NOTE: All illustrations and photos
have been removed from this article.
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