by Robert Sanders
The artificial wetlands of the Santa Ana River in Orange County, are capable of reducing pollutants such as nitrates and chlorinated organic compounds, a team of researchers has determined.
The findings are so encouraging that the researchers, from Northwestern University and Berkeley, are now confident that they will be able to design wetlands systems that can purify the water through totally natural means.
The researchers reported their findings recently at a meeting at the Orange County Water District in Fountain Valley. Their study was funded by the water district and conducted in the Prado Wetlands, one of the largest constructed wetlands in Southern California. It is used for flood control, water storage, wildife enhancement and most recently nitrate removal.
"The challenge is to design systems where natural processes can eliminate the pollutants of the river," said Kimberly Gray, associate professor of civil engineering at Northwestern's Robert R. McCormick School of Engineering and Applied Science.
"This study shows that the dangerous versions of chlorinated organic compounds can be virtually eliminated in these kinds of wetlands," she said.
The study also determined that nitrate removal was much more efficient in marshes which contained cattails or in open water covered with duckweed than in bulrush stands.
"We had figured out reasonable methods for reducing just about every other kind of pollutant except for nitrate nitrogen," said Alexander J. Horne, professor of civil and environmental engineering here. "Now, almost by accident, we have determined that these wetlands are doing it very well. It's almost the perfect solution."
The researchers found that in a wetlands basin planted with cattails, microbial life removed 3.8 milligrams per liter of the nitrates. Denitrification was still high in the open water covered with duckweed, with a decline of 3.1 milligrams per liter. In the bulrush stands, the decline was only 1.3.
The findings concerning duckweed were purely accidental. What had been an area of open water became carpeted with a thick layer of the floating green plants at the time of the study, and the effectiveness of this system in removing nitrates from the water was totally unexpected, Horne said.
Horne noted that these results were obtained with artificial wetlands measuring only some 250 feet in length. Larger areas should be able to virtually eliminate the nitrate nitrogen, he said. Nitrate levels are particularly important, he noted, because when drinking water contains levels in excess of 10 milligrams per liter, it can pose health hazards to humans, particularly newborn babies.
Gray said that under the right conditions, chlorinated organic compounds were virtually eliminated from the water supply. Long term exposure to these chlorinated organic compounds in drinking water is believed to be associated with an elevated risk of cancer.
The Northwestern researchers used a sophisticated analytic technique called pyrolysis GC-MS to study how the plants and microorganisms in wetlands transform and remove pollutants. Pyrolysis involves heating samples, GS stands for gas chromatography and MS stands for mass spectrometry.