Back to Water > Water Pollution
Because polluted water and effluents were obviously moving through the fractures cutting through the Ramat Aviv chalk rock, water experts at the Zuckerberg Institute for Water Research, the Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev and researchers at the Seagram Center for Soil and Water Sciences at the Hebrew University took on the study of how this takes place.
Because the direct measurements of water transport through chalk fractures were never carried out in the field, the researchers designed a novel approach to determine the spatial distribution and rate and direction of water movement down a fracture. Choosing a visible crack exposed on a rock ledge, they set up a line of 21 water-containing compartments along the top of the fracture. In addition, they bored a horizontal hole along the crack from the vertical wall of the ledge, in which they inserted a row of 21 sampler compartments. Therefore, the rate of movement of water down from the upper compartments (known as percolation ponds) to the lower samplers could be determined. Furthermore, by placing different chemical markers (or tracers) in the upper cells, they could follow the direction of movement and determine which of the upper compartments served as sources for the water that filled the lower sampling compartments.
These initial experiments — carried out by Dr. Ofer Dahan, along with senior researchers Prof. Ronit Nativ and Prof. Eilon Adar — affirmed the complexity of fracture flow. Only about 50 percent of the upper ponds showed significant flows of water, with the highest flows coming from only four non-consecutively positioned entry ponds, clearly indicating the variability of downward water movement along the crack length. In fact, over 70% of the liquid flow came from less than 20% of the entry ponds.
In addition, the flow rates out of each of the upper ponds varied with time over the five-day experiment in entirely unrelated patterns. Some started losing water at reasonable rates but subsequently slowed down and speeded up many times over. One of them, for example, saw no movement for the first 24 hours, with the flow then building up in an irregular manner to reach high levels. The researchers believe that this last temporal variation could have resulted from trapped air or the movement of fine filling material in the fracture void during the experiment. The tracer measurements showed another nonuniformity in flow properties, namely, that water exiting the entry ponds did not descend vertically. It often took multiple diagonal pathways directed at various angles, even when originating from the same pond.
Thus movement down a single fracture was found to be extremely complex, depending on the crack’s varying widths, the foreign materials that block flows, and changes in flows determined by the effect of water on the crack properties themselves. And these early studies did not include another major complexity, the detour of fluid movement into new, intersecting fracture planes.
Previous section Next section
