Israel’s national water authorities realized that polluted groundwater south of Beer-Sheva might eventually make its way northwest into Israel’s southern coastal aquifer — a substantial fresh water reserve that stretches west of Beer Sheva. Thus an evaluation of the seriousness of the pollution threat and its amelioration became a major national project taken up by scientists of Israel’s academia.
For over 20 years, from 1995 to date, the basic problems underlying groundwater pollution were probed by teams of hydrologists, geologists, and microbiologists headed by Prof. Eilon Adar, Dr. Alex Yakirevich, Dr. Zeev Ronen, Dr. Ofer Dahan and Dr. Noam Weisbrod of the Blaustein Institutes of Desert Research, Ben-Gurion University of the Negev, Beer-Sheva and by the late Prof. Ronit Nativ of the Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot. Because there was no comprehensive portrayal of the hydrogeology and the subsurface flow pattern in the area, resolving the pollution problem was a broad, challenging task, eventually involving more than 30 national and international scientists and graduate students studying and elaborating on the spatial and temporal flow as well as the transport distribution of contaminants in the groundwater.
The researchers realized that if pollution was making its way down into the chalk aquifer from the surface, it was necessary to obtain an improved picture of the structural geology of the chalk deposit and the nature of water passage. In particular, the properties and structures of the natural discontinuities or fractures crossing the solid chalk had to be clarified, as these apparently provided high permeability channels for deep percolation and groundwater movement. Some basic factors had first to be clarified: the fractures had to be located and mapped, their apertures and structures determined; and subsequently the flow of groundwater and pollutants through them had to be quantified.
For documenting surface and subsurface fracture locations, geologists use visual inspections of the exposed rock surfaces, ultrasound and tomography techniques, ground-penetrating radar, and the lowering of digital video cameras into experimental boreholes drilled through the rock. Their studies clearly demonstrated two kinds of fractures, those contained along the bedding planes of the chalk, which are usually oriented at an incline and rotation with respect to the horizontal, and single-layer as well as multi-layer vertical fractures that cut across numerous bedding planes. The fractures run in different directions and inclinations, with some of them intersecting one another.
Moreover the cracks can have different gaps that can be partially or completely blocked by fine material entering along with rain and flood waters. Thus water and pollution movement through a natural fracture system is complex and can proceed along a multiplicity of paths with varying rapidity. In addition, rain water entering the chalk, which has likely picked up dissolved minerals and salts or even pollutants from the surface, can itself modify the exposed walls lining the fracture. [1]
The senior researchers, together with their students in the late 1990s, carried out a multiyear study of 23 wells scattered throughout the chalk formation in central northern Negev. Changes in the levels and chemical compositions of the well water in response to precipitation and floods were documented. In addition, the team also looked for signs of water flux through the solid chalk deposit using isotopic and composition studies of samples taken from cores. Both these investigations showed that there was no way that sufficient water could move down directly through the chalk matrix to account for the observed seasonal increases and decreases in water table levels. These basic studies clearly demonstrated that most of the water would have to pass through the fractures above the aquifer (known as the vadose zone), but which were formerly thought to be blocked by fine sediments or not sufficiently interconnected to conduct water down from surface. These were the pathways for pollutant entry, as well.
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