NEW BRUNSWICK/PISCATAWAY, N.J. – A Rutgers-led team of scientists has discovered that nature may be slowly scrubbing the lower portion of the Hudson River free of pollution in a "washing machine" of its own making. Their findings were presented today (Nov. 7) at the 113th meeting of the Geological Society of America in Boston.
In a yearlong study of the lower Hudson River estuary, the portion of the river where salt water and fresh water mingle from its mouth to the Tappan Zee Bridge about 40 miles upstream, scientists from Rutgers' Institute of Marine and Coastal Sciences found that dangerous organic and inorganic pollutants are not lying immobile in sediment on the river bottom.
Instead, tidal forces, storms, rain and spring runoff are powering a cycle in which polluted sediment is stirred up and suspended in the water column, then re-deposited on the river floor.
Repeated over and over again, the process ultimately releases out of the sediment many dangerous contaminants and moves them out to sea, the scientists said. Contaminants include mercury, zinc, chromium, cadmium, and lead.
The sediment continues to leak some pollutants back into the river each time it gets stirred up, but over the long term, say the researchers, the river may slowly clean itself.
"In some regions of the river, there's been, on average, about a 10-fold cut in pollutants over 30 years: the sediments are approaching the levels where they were 30 years ago," says Rutgers professor of marine sciences Yair Rosenthal, a principal investigator in the study.
"They are still not clean, but they are getting clean," he added. Rosenthal and fellow principal investigators Professor Rob Sherrell and researcher Paul Field, both of Rutgers, say that the long-term decrease in contaminant levels is due mainly to a number of control measures mandated by the federal Clean Water Act.
They cited in particular a strict permitting system for discharging into the river chemicals from factories, sewage treatment plants and other facilities near the Hudson River drainage basin. The scientists note, however, that the cleanup is significantly aided by natural processes occurring during the sediment resuspension events.
The researchers took samples of sediment and water at two strategic points in the estuary during the year. Their findings suggest that when pollutants get into the water, they dissolve.
But they don't remain that way, diffusing into the sediment or interacting with other metals or minerals to form particles that end up in sediment. There, microscopic organisms drive processes that tend to transform the metals into less toxic forms.
When storms, spring runoff and tidal forces stir up the sediment and suspend it in the river's water column, the already altered pollutants are again dissolved, and the process starts over. Field said that their findings pose a number of policy questions for local, state and national government agencies charged with environmental protection, as well as the general public, notably, "How safe do we want to be and how much are we willing to pay for it?"
He added, "Is it better to leave sediments as is, and let them leak slowly and ultimately clean themselves, or should we intervene and dredge to remove contaminated sediments?"
The next step for the researchers will be determining how fast the river is cleaning itself. "How many cycles does this giant laundry machine need to get the sediments clean?" asked Rosenthal. "When we know that, we'll be able to give the EPA and the public clearer choices."
Unfortunately, said the scientist, this natural cleanup is efficient only in the lower part of the river, in the mixing zone between the salty tides and fresh water outflow, and its effect on the contaminated sediments upstream is substantially smaller.
Other researchers include Robert Mason and Andrew Heyes, Chesapeake Biological Laboratory, University of Maryland-Solomons; Rocky Geyer, Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution; and Christopher Sommerfield, College of Marine Studies, University of Delaware.
The two-year research program is financed through a $225,000 grant from the Hudson River Foundation, which supports scientific and public policy research, education, and projects to enhance public access to the Hudson River.