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Clearly, liquid-based pollution moves through fractures to reach the aquifer together with percolating rainwater. However, some contaminants are taken up by the particulate material — soil, clay, and other solids — that impregnate cracks and do not reach the aquifer as a fluid. Dr. Noam Weisbrod, Dr. Ofer Dahan, Prof. Ronit Nativ and Prof. Eilon Adar have studied the movement of these small particles — known to researchers as colloids. Their work has been carried out in field as well as in laboratory experiments designed to mimic the conditions found in upper cracks through which rainwater passes intermittently (the vadose zone) and in deep chalk fractures holding groundwater.
Flow through natural chalk voids is extremely difficult to predict. Their wall profiles are not uniform, with the spaces available for fluid flow increasing or decreasing as one moves along, or up and down the fracture. In addition, wall surfaces can be coated with natural or toxic industrial salts picked up by rainfall and left behind when the water evaporates. These precipitated salts can surround and glue together particles that have come down from the surface or have broken away from the chalk matrix, thereby acerbating the blockage of flow. The nature of these processes clearly required detailed study.
The investigations into colloid transport and detachment in the upper unsaturated layers of fractured chalk took advantage of field set up built earlier for water flow experiments. Here they also used entry ponds containing artificial rainwater, but they examined the exit samples for their particle content. The researchers found that particle release occurred mostly during the first several hours of flow. Since this was accompanied by an increased concentration of dissolved salts (as approximated by electric conductivity), it was postulated that the colloid release was made possible by the moving water dissolving the precipitated salts lining the crack which had cemented the weathered particles to the walls.
By pumping water from the water table, the researchers found that the mean diameter of the floating colloid particles was 1 micrometer, while most of the particles that detached from the upper fracture leading to the aquifer were much larger. Apparently the large particles do not make their way down to the water table due to the inner fracture structure but provide additional filling materials that block the narrow crack openings.
In laboratory studies on rock cores having a fracture running down their entire length, the aforementioned group found that colloid transport through the liquid-filled fracture was very rapid, often exceeding the speed of ionic tracers that were dissolved in the water. This hinted at the relative danger of free particulate matter that has taken up toxic contaminants in the upper fractured chalk before entering the aquifer region.
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