Riverbank filtration pulls pollutants from drinking water

Soil beside the stream can remove harmful microbes and organic material, researchers find

Harmful contaminants often taint drinking water drawn directly from a river, but a low-cost natural filter may lie just beyond the banks. Johns Hopkins researchers have found that the soil alongside a river can remove dangerous microbes and organic material as water flows through it. The cleaner water is then pumped to the surface through wells drilled a short distance from the river.

This technique, called riverbank filtration, has been used in Europe for more than 50 years to improve the taste and smell of drinking water and to remove some hazardous pollutants such as industrial solvents. But after studying these natural filtration processes for six years at three rivers in the Midwestern United States, Johns Hopkins researchers have determined that passing river water through nearby sediment can produce other health benefits and may cut water treatment costs.

Josh Weiss, a doctoral student in the university's Department of Geography and Environmental Engineering, presented the most recent research results on Aug. 25 in Philadelphia at the 228th national meeting of the American Chemical Society. He reported that riverbank filtration appears to significantly decrease the presence of bacteria and viruses. Water analyses also showed encouraging, though not definitive, signs that this technique can curtail Giardia and Cryptosporidium, two waterborne microorganisms that cause serious digestive ailments.

The latest results confirm the value of riverbank filtration, Weiss said. "It sounds counter-intuitive to drill wells nearby when water can be taken directly from a river," he said. "But our research indicates that riverbank filtration can naturally remove pathogens and organic material that can cause health problems, including some microbes that are able to survive conventional disinfection systems. If you think about how much it costs to build a full-scale treatment plant to make river water safe to drink, you can see how this could be very beneficial."

The research has been supported by Environmental Protection Agency grants awarded to a team led by Weiss' doctoral advisor, Edward J. Bouwer, a professor in the Department of Geography and Environmental Engineering. The team has been studying water drawn from commercial wells located beside the Wabash, Ohio and Missouri rivers near Terre Haute, Ind.; Louisville, Ky.; and Kansas City, Mo.

In several recent papers published in peer-reviewed journals, Weiss, Bouwer and their colleagues have reported that riverbank filtration helps remove organic material left behind by decaying plants. In its natural state, this material poses no health hazards, but exposure to common water treatment chemicals such as chlorine can transform the material into cancer-causing compounds called disinfection byproducts.

"For this reason, it's a good idea to remove as much of this organic matter as we can from the water before it's treated with chemicals," Bouwer said. "Our research indicates that with riverbank filtration, we wind up with fewer of these dangerous disinfection byproducts in the drinking water."

Bouwer added, "Riverbank filtration doesn't completely eliminate the need for water treatment. But it should lower the treatment costs and reduce the risks of mixing chlorine with the organic material that can become carcinogenic."

The researchers studied wells that had been constructed at varying distances – from 90 to 580 feet – from the three rivers. Over a period of days or weeks, river water moves outward toward these wells. As it travels through the sediment, the water is exposed to physical, chemical and biological processes that help remove impurities, the researchers say. Large particles may be pulled out by a straining process. Some of the chemical contaminants and microbes react with components in the sediment and remain behind, too. As a result, the water that reaches the wells is significantly cleaner than it was when it left the river.

In a campus lab, the Johns Hopkins researchers are trying to learn more about this natural filtration process by sending samples of river water through glass columns filled with sediment. They believe that soil characteristics and environmental factors such as the amount of river flow may also affect the natural filtration process.

Weiss, who is preparing his doctoral thesis on riverbank filtration, says the technique may not be appropriate in some areas, such as regions of the Western United States where rivers typically dry up in the summer. But in communities that depend on rivers for a year-round supply of drinking water, Weiss expects riverbank filtration to become more common in the coming years. "We definitely think riverbank filtration is worthwhile," he said. "We're letting nature maintain the system, minimizing the need for external maintenance and the associated costs."

25-aug-2004