Looking at the blue expanses wrapping the globe, one can hardly imagine that a planet covered mostly by water could experience water shortages. Yet 97 percent of Earth’s water is too salty for drinking or irrigation, and much of the rest is locked up deep underground or in ice caps. Meanwhile, a burgeoning world population leads to increasing water consumption. By 2025 at least 40 percent of Earth’s population may face serious health and economic problems if it relies solely on natural freshwater resources. In a survey conducted by the International Council for Science in more than 50 countries, environmental experts ranked freshwater scarcity as a 21st-century issue second only to global warming. Water experts believe that to meet the soaring demand, humankind must find smarter ways of using its water supply. Weizmann Institute researchers are developing scientific approaches to efficient and sophisticated water management.
Detection Goes Underground
Chemicals seeping in from toxic waste dumps, factories, gas stations, and other sources threaten underground water reservoirs. For example, one barrel of oil leaking slowly through the ground can pollute thousands of times its volume in freshwater. Considering that most of the world’s accessible freshwater is stored in underground aquifers, it is vitally important to safeguard these hidden reservoirs. Scientists usually rely on mathematical models to predict the movement of pollutants through layers of rock and soil. However, existing models are generally unreliable, largely because geological formations are extremely varied and complex.
Two professors in the Weizmann Institute’s Environmental Sciences and Energy Research Department - Brian Berkowitz, a hydrologist, and Harvey Scher, a physicist - are developing new mathematical models for predicting the movement of fluids and pollutants underground. On the basis of a theory previously used to model the passage of electrons through a disordered semiconductor, the scientists are seeking to discover how particles move through rock layers of different flow resistance. This research could help predict the consequences of leaks from nuclear waste canisters, evaluate the potential spread of contaminants from a new factory, or help engineers devise strategies for pollution containment.
Plotting the movement of pollutants through cracks or heterogeneous sediments is a tremendously uncertain business, data on the arrangement of the ground beneath the surface being generally limited and sketchy. Computer simulations help fill in the blanks, as does interdisciplinary collaboration among scientists. Using nuclear magnetic resonance imaging of fluid flow through rock fractures, and incorporating lab and field test data into their models, Berkowitz and Scher were able to predict not only how fluids move through cracks in the rock, but also how the cracks are dissolved and eroded as the fluid flows downward. The scientists have discovered that fluids flow especially quickly through such maze-like fissures and have developed models that reflect the risk of potentially catastrophic contamination.
In a separate project, Prof. Scher is studying the interface region known as the capillary fringe - a unique layer sandwiched between the rock formation that holds gases and water, and the stratum underneath that is saturated with water. He is developing models, based on theories describing the movement of a mixture of fluids through porous media, to describe the structure of the fringe and predict the transport of particles through this layer. An understanding of the capillary fringe will provide a better picture of how fluids move through the ground and how pollutants make their way into groundwater. Moreover, this research could clarify how bacteria residing underground may help degrade pollutants.
Managing coastal aquifers is a crucial issue for seaside communities the world over. Near coastlines, seawater tends to seep into drinking water sources. Deep wells intended to pump freshwater from underground may gradually suck in salt water from the sea, endangering entire aquifers. And the problem is not limited to the coastline. Pockets of saltwater locked underground, away from the sea, may similarly endanger inland aquifers.
Prof. Brian Berkowitz is developing new quantitative models that specifically describe fluid flow and the movement of chemicals in rock formations near aquifers. This research may lead to improved management of freshwater resources along the coast or near underground sources of salinity. Such management is a particularly acute problem in Israel, a country prone to severe water shortages that derives about two-thirds of its water supply from groundwater - mainly the Coastal Aquifer, which extends some 130 kilometers along the Mediterranean, and the Yarkon-Taninim Aquifer, which runs parallel to the coastal one, under the Judean mountains. A sequence of drought years, as well as chronic overpumping, have already severely endangered the water quality in these underground reservoirs, and preventing their further deterioration is a matter of top national priority.
Averting the Metal Menace
Arsenic poisoning of drinking water has become a problem in many parts of the world, at times turning into a massive public health crisis, as occurred in Bangladesh in the 1990s. Chromium contamination has threatened water supplies in the United States; one famous case was featured in Erin Brockovich, a film in which the title character uncovers the poisoning of an entire town by a power company. These are examples of metal ion pollution, which has ravaged numerous communities around the world. Metal ions are among the deadliest water pollutants, harmful even in low quantities and tough to detect. Scientists are developing methods to detect the ions and remove them from water.
Laws against water pollution are only as good as the sensors that monitor compliance. New types of sensors for the real-time measurement of toxic metal levels in rivers and other fast-moving bodies of water are being investigated by Prof. Avi Shanzer of the Organic Chemistry Department and Prof. Israel Rubinstein of the Materials and Interfaces Department. The scientists have designed a gold electrode coated with a single layer of tightly packed metal-ion-binding molecules. When specific metal ions come into contact with the electrode, they bind to it, triggering an electrical response. Successfully tested in the laboratory with iron and copper, this metal sensor could in principle be developed into a device for detecting other toxic metals in water.
In a different approach, Prof. Shanzer is developing a method for removing metals from water. His team has demonstrated the possibility of synthesizing tailor-made organic molecules that bind to particular metal ions and has already successfully synthesized selective binders for copper, cobalt, nickel, lead, mercury, and cadmium. Currently available ion-exchange purification columns have a low capacity because they become plugged up by non-relevant metal ions; but columns based on Shanzer’s method - due to the high selectivity of his binders - are expected to have a high capacity. As a result, they should effectively remove even trace amounts of metal ions. The columns could be used, for example, to purify wells and aquifers that contain traces of poisonous ions.
A project launched by the late Prof. Abraham Warshawsky of the Organic Chemistry Department could help both detect and fight metal ion pollution. In Warshawsky’s method, water flows through a tube filled with synthetic beads that contain numerous holes. Inside the holes, two kinds of synthetic molecules, called ligands, are engaged in detection: one ligand is a collector molecule, designed to bind with a polluting metal; the other is a reporter molecule, which emits intense fluorescent light when bound to the metal. By manufacturing ligands that grab onto different ions, the scientists can make the system sensitive to a variety of pollutants. This work was conducted in Warshawsky’s lab by postdoctoral fellow Dr. Ying Wang, in collaboration with Dr. Gilad Haran of the Chemical Physics Department. The amount of light emitted by different pollutants is picked up by a photosensor, which indicates the chemicals in the water that have reached harmful levels. Metal-binding ligands may also be used to clean up pollution: if they hold on tightly to the polluting ions, they can be used to purify water in underground aquifers. This latter idea is being explored by Prof. Brian Berkowitz, who developed it originally with the late Prof. Warshawsky.
Plants growing by the Dead Sea do not extract water from their immediate surroundings, which are excessively saline. Instead, they extract only winter flood water from the Judean hills, which occasionally reaches the area. This finding, by Prof. Dan Yakir, suggests that the plants are able to distinguish between salty water and non-salty flood water.
An ultrasensitive detector, developed in a collaborative study by three Weizmann Institute teams that began as a basic research project, has a wide variety of potential applications, ranging from sensing minute amounts of biomolecules to detecting pollutants in water or air. Profs. David Cahen, Ron Naaman, and Avi Shanzer of the Materials and Interfaces, Chemical Physics, and Organic Chemistry departments, respectively, led groups of researchers who succeeded in tracing the path of electrons moving from custom-made molecules to a semiconductor surface. This was accomplished by grafting a single layer of molecules onto a semiconductor, thereby creating an ultrasensitive detector for electron transfer. Cahen’s and Naaman’s groups are exploring new directions of research made possible by the innovative device, called MOCSER (MOlecular Controlled SEmiconductor Resistor), including its use as a sensor for chemicals in the brain, a detector for DNA mutations, and possibly as a DNA chip.
Deep inside the earth, there are underground pockets of groundwater similar to oil traps. The water in these geological traps, resembling a sealed bubble, has been stored underground for millions of years, safe from pollution. Prof. Emanuel Mazor, after studying underground water pockets in Australia and Israel, has reached the conclusion that they could serve as emergency reservoirs in a pollution event such as a nuclear catastrophe.
Scarcity of water in the Middle East and vulnerability of groundwater, Israel’s major water resource, to pollution have placed the country at risk of severe water shortages in the near future. Prof. Emanuel Mazor has drawn up a number of strategies for environmentally savvy water management. He has outlined a comprehensive approach: clean management of all enterprises, a nationwide campaign of water saving, special legislation, and the establishment of a research institute for water and environmental conservation.
Industrial plants, municipal waterworks, and private homes throughout the world use water-softening equipment to remove calcium from water (calcium minerals build up on pipe walls and restrict flow). However, the softening sometimes damages water quality or produces waste products, including tons of sodium-rich wastewater that pollutes underground aquifers. A new, environmentally friendly water softening method has been developed by Prof. Emeritus Ora Kedem. In her “cake filtration” approach, commercialized by an Israeli company, calcium minerals are filtered out without pollutants being released into the environment.