8:35 a.m., Jan. 13, 2010----If you've ever stepped a bare foot on the bottom of the Delaware River, you've discovered what scientists already know -- there's gooey, gray muck down there, and lots of it. Each year, more than a million tons of sediment washes into the Delaware River estuary, which winds 134 miles from Trenton, N.J., to the mouth of the bay.
Not only is the sediment plentiful, it's crucially important in maintaining the natural balance of the estuary's ecosystem. Some of the sediment settles to the river bottom, where it adds to its stability and helps deter erosion. Some of it is deposited in neighboring marshes, where it helps maintain the marsh above sea level and provides nutrients that allow plants and animals to survive there.
But there's a lot that scientists are still learning about all that mud.
In a three-year, $1.1 million project funded by the National Science Foundation through the American Recovery and Reinvestment Act (federal stimulus), the University of Delaware's Christopher Sommerfield and Rutgers University's Robert Chant are determining just how efficient the estuary is at trapping sediment supplied by rivers.
The project builds on nearly a decade of Delaware Sea Grant and other research, in which Sommerfield and his colleagues have developed a “sediment budget” that accounts for the amount of sediment that is added to and removed from the Delaware estuary. Now the scientists are trying to understand how different processes affect the timing and movement of sediment from sources throughout the estuary watershed to resting places, or sinks, in the estuary seafloor and fringing tidal marshes.
The project's findings could shed light on how human or natural changes to the flow of sediment could affect the estuary. Natural changes include erosion by coastal storms and sea-level rise, whereas human changes include the construction of bulkheads and dredging.
“In an estuary, when you change the flow of water and sediment in any way, it feeds back into the condition of seafloor, shoreline, and estuarine fringe environments. This has major implications to the living ecosystem,” said Sommerfield, associate professor of oceanography in the College of Earth, Ocean, and Environment.
Previous work by Sommerfield's research group has shown that much of the muddy sediment delivered by rivers is temporarily stored within a region of the middle estuary that has high sediment concentrations, or turbidity, before being dispersed to other areas. But, exactly how the mud moves from place to place is a lingering question.
To answer this question, Sommerfield and Chant, associate professor of physical oceanography at Rutgers, are turning to UD's 146-foot research vessel Hugh R. Sharp. Aboard the Sharp, the researchers will monitor turbidity, salinity, currents, and temperature along the estuary over the next two years.
They'll also measure the distributions of naturally occurring radionuclides to track the movement of mud through the estuary. The radionuclides Beryllium-7 and Lead-210 are delivered to the estuary by precipitation and become attached to particles suspended in the water column, which means they can be used as sediment tracers.
“By comparing radionuclide levels in the estuary to rates of input and radioactive decay, we'll be able to establish the timing of sediment transport from source to sink,” Sommerfield said. “We really want to learn how long mud particles remain suspended in the water column before being buried permanently.”
The team also plans to install instrumented oceanographic moorings along and across the estuary to make continuous measurements of currents and turbidity. The data collected will allow the researchers to relate changes in sediment transport to tides, river discharge, and oceanic storms. Eventually, they will use the data to validate computer models developed to predict sediment transport in the estuary.
With this project, Sommerfield is satisfying his longtime interest in how natural and human factors influence sedimentation in coastal environments. But as human influences such as the Army Corps of Engineers' efforts to deepen the Delaware River and Bay shipping channel continue to make headlines, Sommerfield's research has broader impacts.
“Our project is hypothesis driven science; it's basic research relevant to estuaries worldwide,” he said. “But it so happens to have relevance to very specific issues in the Delaware River estuary.”
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