Sediment goals set, aimed at SAV comeback
The Bay Program has set its first-ever sediment reduction goals aimed at keeping sand, silt and dirt on the watershed—and along shorelines—so it doesn’t kill grass beds by muddying the Chesapeake’s water.
The sediment goals are aimed at helping the Bay Program meet its new 185,000-acre underwater grass goal by clearing the water in places where nutrient control alone may not be enough to do the job.
Officials acknowledge there is less certainty about the sediment goals than those for nutrients, which were set in March. This is because scientists are far from clear about how sediment moves through the Bay and affects its water clarity.
“We have confidence that the levels of reductions we are seeking are in the right direction,” said Rich Batiuk, associate director of science for the EPA’s Bay Program Office. “Are they the exact millions of tons? We’re not sure yet.”
The sediment goals, in fact, are being set in two stages.
The first step, completed in April, calls for a reduction from 5.05 million tons to 4.15 million tons annually in what washes into the Bay from the major Bay tributaries. That reduction is aimed at helping to clear the water in tidal freshwater areas of the upper Bay and its tributaries—the area where sediment from the rivers is thought to have its greatest impact.
Below those areas, where nearshore sources such as shoreline erosion are thought to be more important, reductions will be set on a case-by-case basis in coming months to restore historic levels of grass beds.
Since the 1980s, the Bay Program has focused mostly on reducing the amounts of nitrogen and phosphorus entering the Chesapeake. The nutrients fuel algae blooms that consume oxygen as they decay, blocking sunlight needed by important underwater grass beds.
Much less emphasis has been placed on sediment, either in control, or research. But in certain shallow water areas, sediment is now thought to play a more important role than nutrients in reducing the sunlight available for underwater grasses. Restoring those grass beds and the habitat they provide is considered a key part of bringing back a healthy Bay ecosystem.
But sediment is a murky issue, with many different sources that seem to affect different parts of the Bay.
Sediments enter the Bay through four main sources: material washed off the watershed that flows into the Chesapeake from its tributaries; material eroded from shorelines and other near-shore areas; material from the ocean; and material that is biologically produced in the Bay.
Ocean sediment is mostly sand, which tends to be heavy and stays on the bottom, so it is generally not a major factor in water clarity.
Sediment from the rivers—originating from agricultural runoff, developing areas and streambank erosion—is considered to be an important factor in clouding the water in tidal freshwater areas of the Chesapeake. That includes the uppermost areas of the Bay and its many tributaries.
Erosion from shorelines, marshes and other near-shore sources of sediment are considered to be major contributors of silt, sand and clay.
Biogenic sediment—bits of bone, fish scales and other organic material—is never the dominant source of material, but it is relatively more important in the open waters of the mid-Bay, where it may account for roughly 20 percent of the material in the water column.
But those sources of “new” sediment are still only part of the picture. Much of the sediment clouding the water may come from materials already in the Bay, which get stirred up during major storms. No one can say with certainty from place to place whether the most important factor is new sediment; old, stirred-up sediment; or some combination of sources.
Further, computer models used by the Bay Program to predict the impact of nutrient reductions on Chesapeake water quality are less accurate when it comes to predicting the impact of sediment reductions.
Nonetheless, Batiuk said, officials believe that the models did an adequate enough job of predicting broad-scale responses in freshwater-dominated portions of the upper Bay and its tributaries to set the Bay Program’s first sediment reduction goals for rivers.
None of those sediment goals exceed the levels of sediment reduction that could be achieved from phosphorus control actions already needed from those individual rivers. (Because phosphorus tends to bind to sediment, efforts to reduce phosphorus runoff also reduce sediment.)
But as more is learned about sediment, it’s possible that more stringent sediment goals will be set, if necessary, to clear the water for grass beds. “The Bay grass goal is the driver,” Batiuk said. “It’s the definition of what we consider to be a good resource.”
Sediment entering the Bay from rivers is typically trapped and buried in the zone where fresh and saltwater mix near the head of the Bay and its tributaries. Below those areas, known as turbidity maximum zones, near-shore sources of sediment are considered the most important sources of new sediment.
Impacts from sediment sources such as shoreline erosion vary dramatically from place to place in those areas, and current models cannot predict their impacts on water clarity. In those areas, reductions set in the coming months for individual segments will be aimed at restoring historic levels of grasses in that area, Batiuk said.
“That’s where the local perspective is most valuable,” Batiuk said. “It’s where our models and other tools don’t have the best level of specificity.”
The sediment goals are to be refined in coming years as greater emphasis is placed on researching sediment; monitoring is increased in near-shore areas; better local assessments are made of shoreline erosion rates; and the computer models are improved.
The uncertainty of the sediment issue is underscored in an upcoming Bay Program report, which calls excess sediment “one of the most important contributors to degraded water quality” but said that a better understanding of sediment processes “is critical for improving water quality and living resources in the Bay.”
One of the problems in establishing clarity-related sediment goals is that the movement of silt and other particles within local areas of the Bay is poorly understood. As a result, the draft report said, no one is certain how much of the sediment-related clarity problem is caused by new sediment flowing into the Bay, and how much is caused by sediment that has been around for years, but has been resuspended into the water column.
Heavier sediments, such as pebbles and sand, quickly sink to the bottom and do not cause water clarity problems. But fine particles, typically tiny bits clay or silt that are less than 1ž16 of a millimeter in diameter, can float in the water column for extended periods of time. Further, even after the small particles settle, they can be churned back into the water column during severe storms.
Estimates by Larry Sanford, a scientist with the University of Maryland’s Center for Environmental Science, suggest that, on a daily basis, much more sediment is resuspended into the water column than enters from the land, although much of that resuspension takes place in deep areas of the Chesapeake, far from the grass beds the Bay Program is seeking to protect. Much of the resuspended sediment quickly settles back to the bottom, but an unknown fraction remains in suspension where it can affect water clarity.
Historically, Sanford said, it’s possible that biological factors played a bigger role in keeping sediments out of the water column. The Bay’s once-vast, oyster bars filtered large amounts of fine sediment from the water, then excreted them in larger “packets” that were more likely to stay put on the bottom. Further, he said, the rough surface of oyster bars contains lots of cracks and crevices which serve as effective sediment traps.
“I think oysters may have played a significant role out there,” he said. “That has changed radically.” Other organisms, from zooplankton to menhaden, may be important in filtering sediments, but those connections have not been extensively studied.
Meanwhile, keeping new sediment from flowing into the Bay is an issue with its own sets of challenges.
Sediment erodes at different rates in different places and among different soil types. The speed at which it moves also varies from place to place, and among soil types. It can take years—if not decades or centuries—for sediment to complete its journey to the Bay.
“Future improvements in water clarity may take years to decades following implementation of land use changes in the watershed,” cautioned the draft sediment report.
By contrast, some close-to-the-Bay solutions, such as stemming shoreline erosion, can yield much quicker water clarity results. Unfortunately, those actions tend to be expensive. And, unlike the watershed, where cost-share programs help to underwrite the the cost of keeping dirt on farmland—the largest single source of sediment from the watershed—there are few similar programs to stem shoreline erosion.
Further, the cost of controlling shoreline erosion is huge: Breakwaters can cost hundreds of dollars per foot, while constructing oyster reefs can cost tens of thousands of dollars per acre.
As a result, most efforts to control shoreline erosion have focused on high-value property, said Tom Simpson, of the University of Maryland’s College of Agriculture and Natural Resources, who recently helped to organize a Bay Program workshop focused on finding innovative sediment control practices. Also, unless they are properly designed, efforts may just shift the problem somewhere else, he said.
“Historically, we have done shore erosion control on a property-by-property basis,” he said. “That in particular may not address the issue because you may just transfer the energy down shore.” Effective strategies in the future, he said, may need to address entire reaches of shoreline.
Despite the uncertainties, scientists say one thing is clear: Sediment control, as a rule of thumb, is a good thing.
In many rivers and streams throughout the watershed, sediment is the primary water quality problem as eroded soils degrade habitat, destroy spawning areas for fish and bury bottom-dwelling creatures. Excess sediment within streams changes their hydrology, causing the waterways to eat away at their own banks, adding even more sediment to the water.
And, although the Bay Program is seeking to reduce sediment primarily to improve water clarity for underwater grass beds, sediment creates a number of other problems when it reaches the Chesapeake, according to the upcoming sediment report.
Besides clouding the water, it can smother oyster bars and other bottom-dwelling organisms. Many toxic contaminants bind themselves to particles and enter the Bay attached to sediment. Clay, sand and soil from upland areas fill shipping channels in the Bay, requiring expensive annual dredging which, in turn, creates a pile of sediment that must be disposed of.
“Any sediment reduction is a good thing, no matter where it is,” said Mike Langland, a hydrologist with the U.S. Geological Survey, and co-author of the upcoming Bay Program report. “But it is going to have local benefits first, and then it is going to take some time before that translates itself down into the Bay.”
The report, “A Summary Report of Sediment Processes in Chesapeake Bay and Watershed,” is expected to be completed soon, and will be available on the Bay Program’s web site, www.chesapeakebay.net
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