Wetlands found to pollute coastline

A team of researchers says it may rewrite environmental textbooks after uncovering evidence that a saltwater marsh is a source of faecal bacteria contaminating one of California’s most popular beaches.


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The University of California-Irvine researchers’ conclusions contradict accepted environmental science theory that wetlands should purify contaminants flowing into the ocean. Their research shows that bacteria generated in Talbert Marsh, a man-made saltwater marsh near Huntington Beach in California may be partially responsible for faecal bacteria levels thousands of times over the legal limit, according to Stanley Grant, Ph.D., who led the research team. Grant identified droppings from resident sea gulls as the source of the contamination, which spreads from the marsh to the shallow ocean water near the beach.

The researchers found that Talbert Marsh regularly flushes millions of gallons of bird droppings into the Pacific Ocean. In Huntington Beach, enterococci bacteria levels up to approximately 5,700 bacteria per 100 millilitres of water were detected. Nearly 15% of approximately 2,000 samples taken ranged above the 104 parts per 100 millilitre standard that requires surf zone postings.

According to the study, the marsh’s rapid flow sweeps water into the ocean in less than 40 minutes. A slower flow rate, however, would be likely to prevent most contamination, since longer exposure to salt water and sunlight kills the bacteria.

Depending on a wetland’s size and water flow rates, some 4.6 million saltwater marshes in the continental United States could be affected similarly, Grant said. “One scenario is that anywhere along the coast in the United States, you might run into this problem. We thought there were multiple sources for the bacteria at Huntington Beach. What we’ve found is that the marsh is one of those sources. This beach is ground zero of what could be a national problem.”

Elevated faecal bacteria levels were discovered in California after a 1999 state law set uniform bacteria standards for the state’s beaches and required additional water testing and federal legislation calling for similar standards nationwide is currently under consideration in the US Congress, Grant said. Huntington Beach has now been closed due to the researchers’ tests.

While not ruling out the possibility of contamination from additional sources, Grant said pollution from the marsh alone was sufficient to close the beach. Paradoxically, the marsh, designed as a wildlife habitat, was expected to help clean the water.

Previous studies failed to find the cause of the contamination in Huntington Beach, and data from virtually all previous studies said wetlands should filter pollutants. Those studies were based on freshwater wetlands, however, which may function differently saltwater ones, Grant said, calling for more information on saltwater marshes.

“The irony is that we may be introducing a source of contamination where one didn’t exist before,” said Mark Sobsey, Ph.D., a professor of environmental microbiology at the University of North Carolina. “We’re trying to help the environment, but we may also be adding to a wildlife population that can lead to pollution where we want to swim. We have to be smarter about how we build these marshes. If they are designed so that the water goes through them very rapidly, like it does here, the chances are pretty high that you’ll have the same kinds of problems. When properly designed, though, these wetlands might improve water quality.”

The findings are reported in the 15 June issue of Environmental Science & Technology, published by the American Chemical Society.

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