Tap Water Truths

When you open a water tap, chances are good that the water is being pumped from underground reserves called aquifers. Like a giant sponge, spaces in the porous rock or sand beneath the surface hold water that soaks in from above ground. Many aquifers, especially those near heavily populated coastlines, are threatened by overpumping, which causes the nearby seawater to be sucked in as fresh water is removed. Weizmann scientists have now revealed that this meeting of salt and fresh waters might negatively affect underground water quality even before it becomes too salty to drink.

The study was performed by Prof. Brian Berkowitz, postdoctoral fellow Dr. Ishai Dror and Tal Amitay, all of the Institute’s Environmental Sciences and Energy Research Department, and Dr. Bruno Yaron of the Agricultural Research Organization - Volcani Center.

The team found that certain chemicals typical of those in industrial and agricultural waste are capable of mixing into seawater.

“Most scientists had assumed that these pollutants would behave either like oil slicks, floating on the seawater, or like sludge, sinking to the bottom,” says Berkowitz. “But we found that a mixing process similar to wave action shakes up the water and chemicals like oil and vinegar in a bottle, forming micro-emulsions - tiny drops of one liquid suspended in another. The implication was that relatively large amounts of these chemicals could be distributed throughout seawater, which theoretically could then be carried into the groundwater.”

But the story does not end there. In the experiments, chemicals added to the salt water refused to stick around with the salt, rushing into the freshwater like salmon in mating season.

In these experiments, glass tanks were divided in half, horizontally, by a sand barrier. In some tanks, freshwater containing a cocktail of five chemicals was placed on one side and clean freshwater on the other, while in other tanks the chemical mix was added to saltwater, with clean freshwater placed on the other side. When the water on the “clean” side of the barriers was analyzed after a period of time, the scientists found that the contamination levels in water that bordered on saltwater were many times higher that those from the all-freshwater tanks. One particular chemical compound did not cross the barrier at all in the freshwater trials, but did so in the salty ones. Though some straying to the other side was to be expected, clearly another process was at work.

A phenomenon known as salting out is to blame. The salt ions fill in the spaces between the water molecules, shutting out all other molecules and thereby driving the droplets of pollutants into the fresh- water, where they can co-exist more easily.

“Aside from being a cool demonstration of a scientific principle, the experiment reveals only what happens in a glass tank in the lab, not in the far more complex underground systems,” Berkowitz emphasizes. He estimates that at least a year of additional lab work is required, testing different combinations of barriers, water flows, chemicals and minerals, before they can begin to check whether real-life aquifers might be under threat or how they could be protected.

Prof. Berkowitz’s research is supported by the Sussman Family Center for the Study of Environmental Sciences; the Angel Faivovich Foundation for Ecological Studies; the Brita Fund for Scholarships Research and Education, the Feldman Foundation; the P. and A. Guggenheim-Ascarelli Foundation; Mr. and Mrs. Michael Levine, Pinckney, NJ; and the late Mrs. Jeannette Salomons, the Netherlands. He is the incumbent of the Sam Zuckerberg Professorial Chair in Hydrology.