"Man-made" Water Has Different Chemistry

As population growth, food production and the regional effects of climate change place greater stress on the Earth’s natural water supply, “man-made” water – created by removing salt from seawater and brackish groundwater through reverse osmosis desalination – will become an increasingly important resource for millions of humans, especially those in arid regions such as the Middle East, the western United States, northern Africa and central Asia.

But the introduction of this life-giving water will bring changes to the environment.

“Water that’s been desalted through reverse osmosis contains a unique composition which will induce changes in the chemistry and ecology of aquifers and natural water systems it enters,” says Avner Vengosh, associate professor of earth and ocean sciences at Duke University’s Nicholas School of the Environment.

A new study by Vengosh and colleagues in France and Israel provides tools to identify and trace this man-made water as it mixes with natural water supplies and, over time, replaces natural waters in areas entirely dependent on desalination.

The study, published this month in the peer-reviewed journal Environmental Science and Technology, details for the first time the isotope geochemistry – or chemical fingerprints – of the elements boron, lithium, strontium, oxygen and hydrogen found in reverse osmosis-desalted seawater and brackish groundwater.

Identifying these unique geochemical and isotopic fingerprints gives scientists and water-quality managers a new array of tools for tracing the presence and distribution of man-made fresh water in a region’s soils, surface waters and ground waters, Vengosh says.

“We studied the chemistry of water produced in several of the largest desalination plants on earth and found that that composition of the desalted water is totally different from those of natural waters,” he explains. “As this water leaks into the environment through poor infrastructure or enters it directly through irrigation, it will be possible to use our new tracers to track the water back to its origin

“It’s sort of like a detective who collects fingerprints at the scene of the crime and matches them to the guilty suspect,” he says.

Being able to trace water back to a desalinated source through its isotopic and geochemical fingerprints will allow local governments and water utilities to zero in on the problem of valuable water loss and correct it more quickly and efficiently. Moreover, because desalted wastewater can be recycled through the environment and reused as a drinking water source – a process already being used in southern California – the new tools would enable water authorities to trace the relative contribution of desalted water in their system, and to test the effectiveness of their water treatment processes.

“This will be especially beneficial in water-scarce regions like California or the Middle East, where natural water sources are diminishing and made-made waters are becoming the ultimate water sources,” Vengosh says. “Given the complexity and variety of man-made fresh water sources being used to replace natural recharge in these regions, traditional tests alone, such as testing for water salinity, cannot provide a single solution.”

Global capacities for producing freshwater through desalination are projected to double by the year 2015, he notes. In some regions, diminished natural water supplies already are problematic. In California, which is experiencing one of its worst droughts in decades, new housing and other development is being slowed or stopped under a state law that requires a 20-year water supply as a condition for approval before building can begin. Increased use of freshwater produced through desalination could help resolve this issue, Vengosh says.

Vengosh is a geochemist who is internationally cited for his expertise on the chemical and isotopic composition of water contaminants. His research has led to the development of new, more accurate methods for tracing contaminants in water supplies worldwide, from boron-laden surface and ground waters in the Middle East to radon-contaminated groundwater in the mountains of western North Carolina.

He co-authored the new study with Wolfram Kloppmann, Catherine Guerrot and Romain Millot of the Bureau de Recherches Geologiques et Minieres of France, and Irena Pankratov of the National Water Commission of Israel.

By Timothy D. Lucas
Nicholas School of the Environment and Earth Sciences

Possible New Approach To Purifying Drinking Water

A genetic tool used by medical researchers may also be used in a novel approach to remove harmful microbes and viruses from drinking water.

In a series of proof-of-concept experiments, Duke University engineers demonstrated that short strands of genetic material could successfully target a matching portion of a gene in a common fungus found in water and make it stop working. If this new approach can be perfected, the researchers believe that it could serve as the basis for a device to help solve the problem of safe drinking water in Third World countries without water treatment facilities.

The relatively new technology, known as RNA interference (RNAi), makes use of short snippets of genetic material that match -- like a lock and key -- a corresponding segment of a gene in the target. When these snippets enter a cell and attach to the corresponding segment, they can inhibit or block the action of the target gene. This approach is increasingly being used as a tool in biomedical research, but has not previously been applied to environmental issues.

"Pathogens, whether bacterial or viral, represent one of the major threats to drinking water in developed and undeveloped countries," said Sara Morey, a Ph.D. candidate� in the lab of Claudia Gunsch, assistant professor of civil engineering at Duke's Pratt School of Engineering. "Our data showed that we could silence the action of a specific gene in a fungus in water, leading us to believe that RNAi shows promise as a gene-silencing tool for controlling the proliferation of waterborne bacteria and viruses."

Morey presented the results of her experiments June 3, 2008, during the annual meeting of the American Society of Microbiology in Boston.

In addition to helping solve drinking water issues in underdeveloped countries, this new approach could also address some of the drawbacks associated with treated drinking water in more developed nations, Morey said. Methods currently used to treat water -- chlorine and ultraviolet (UV) light -- can be expensive to operate and the results of the treatment itself can affect the taste and smell of the water.

Although these methods have been employed for years, problems can emerge once the treated water enters the distribution system, where pathogens are also present. For this reason, water is often over-chlorinated at the plant so that it remains in high enough concentrations in the pipes to neutralize pathogens. This explains why people living the closer to a treatment plant will be more likely to taste or smell the chemical than those farthest away from the plant, the researchers said. Additionally, chlorine can react with other organic matter in the system, leading to potentially harmful by-products.

UV light, while also effective in neutralizing pathogens at the plant, has no effect once the water is pumped out of the plant. Gunsch said that many pathogens are developing a resistance to the effects of chlorine and UV light, so newer options are needed.

"We envision creating a system based on RNAi technology that would look from the outside just like the water filters commonly used now," Gunsch said. "This approach would be especially attractive in less industrialized countries without water treatment systems. This 'point-of-use' strategy would allow these countries to make safe water without the expense of water purification infrastructure."

The first prototypes would likely involve a filter "seeded" with RNAi that would eliminate pathogens as the water passed through it. These filters would likely need to be replaced regularly, Gunsch said, adding that she believes it would theoretically be possible to create a living, or self-replicating system, which would not require replacement.

The researchers are currently conducting additional experiments targeting other regions of the fungus' genome. For their proof-of-concept experiments, they tested RNAi on a non-essential, yet easy to monitor, gene. They are now testing this approach to silence or block genes essential to the viability of the pathogen.

They are also planning to test this strategy in water that contains a number of different pathogens at the same time, as well as trying to determine the optimal concentration needed in the water to be effective.

The experiments were funded by Duke's Pratt School of Engineering.

By Richard Merritt