Hudson River cleans itself

In a year-long 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 the Institute of Marine and Coastal Sciences at Rutgers, the State University of New Jersey, found that contaminants such as mercury, zinc, chromium, cadmium and lead, do not lie 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 many contaminants from the sediment and moves them out to sea, the scientists said.

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,” said Yair Rosenthal, a principal investigator in the study.

Rosenthal and fellow investigator Paul Field, 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, 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 and found that when pollutants get into the water, they first dissolve but then diffuse into the sediment or interact 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 again. Field said that their findings pose a number of policy questions for local, state and national government agencies charged with environmental protection. “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?” He said.

The next step for the researchers is to determine how fast the river is cleaning itself. “How many cycles does this giant laundry machine need to get the sediments clean?” Rosenthal said. “When we know that, we’ll be able to give the EPA and the public clearer choices.” Unfortunately, he added, 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.