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Milldams tell scientists about erosion, mercury contamination

Milldams tell scientists about erosion, mercury contamination

It's a picturesque early American image -- a gristmill complete with a water wheel perched on the banks of a swiftly flowing river or stream. Many of these mills are long gone today, but scientists are discovering that the dams associated with them can have lasting environmental effects.

“The dams may have played a role in trapping the mercury and their demise is key to getting it back into the river,” said Pizzuto, professor of geology.The University of Delaware's Jim Pizzuto and Michael O'Neal have documented those effects in Virginia, where they've been working to decrease the amount of mercury entering the South River. The College of Earth, Ocean, and Environment (CEOE) scientists are part of an interdisciplinary team that's trying to understand how mercury is still getting into the river even though a nearby former DuPont plant known to have caused the contamination stopped using the substance in 1950. The pair's research, published in Geology earlier this year, concluded that one of the mercury sources is related to milldams.

Historical documents show that about 14 dams were built during the first half of the 20th century in the 18-mile stretch between Waynesboro and Port Republic. The dams raised the water level upstream for power generation. Nearly all the dams in that 18-mile stretch were breached by the mid-20th century, increasing river flow and erosion rates in those areas.

Because the South River's clay and silt is contaminated with mercury, increased erosion means the toxic chemical is redistributed into the water.

Pizzuto and O'Neal's research, which also garnered an article inEarth magazine, published by the American Geological Institute, involved surveying historical documents and aerial photos to better understand erosion rates as well as talking with area residents to learn when the breaches occurred.

Today the researchers are working to figure out exactly where the most erosion is occurring and at what rates so they can find and remediate the main mercury sources. They plan to develop a conceptual model of how the river responded when the dams were breached in order to predict which sites are most susceptible to erosion.

“We think that some of these rates are going to be highest in some of the places where the old dams used to be,” said Pizzuto, who teamed up with O'Neal, assistant professor of geography, in 2005 to research the mercury problem.

Their findings don't impact the South River project alone, however. In 2008 colleagues of theirs published an article inScience documenting hundreds of thousands of milldams in the Mid-Atlantic. The authors argue that much of the behavior of rivers today is influenced by old dams, now disappeared. The research challenges standing beliefs, especially related to the multi-billion-dollar industry of river restoration.

“They claim that most of the contaminants and sediment that are reaching the Chesapeake Bay are not coming from farm fields, as previously thought, but in fact are caused by the erosion of these sediments trapped behind dams,” Pizzuto said. “But they didn't have data to show that. Our study points a finger in that direction pretty strongly.”