Silt buildup at dam could endanger bay

Conowingo: As sediment piles up, bay officials are faced with unappealing options.

June 16, 2000|By Joel McCord | Joel McCord,SUN STAFF

CONOWINGO - Beneath the waters that slap against the wall of the dam here is a load of silt that, let loose, could choke much of the life out of Chesapeake Bay.

The mile-long dam across the Susquehanna River, connecting Harford and Cecil counties, has kept about 2.5 million tons of sediment from reaching the bay every year since it was built in 1928. But the lake behind the dam is filling fast and in 15 to 20 years is expected to reach the point where it will hold no more silt.

"There's no question that the sediment that's built up behind that dam is a time bomb just waiting to go off," said Mike Hirschfield, a senior vice president of the Chesapeake Bay Foundation.

Scientists and policy-makers trying to come up with a solution are faced with unappealing choices. .

Dredging the sediment would be prohibitively expensive and could stir up potentially dangerous silt. It is unlikely that tougher upstream erosion-control measures, no matter how strict, would stop enough silt to solve the problem. Doing nothing might allow the bay to reach an equilibrium naturally, but it also heightens the risk of devastation from what meteorologists call a 50-year storm, the kind so powerful that one tends to occur only twice in 100 years.

"I'm not sure I see an answer here," said Kent Mountford, senior scientist with the Environmental Protection Agency's Chesapeake Bay Program.

The Susquehanna River Basin Commission is organizing a conference for December to try to find an answer. "We're trying to identify what practical alternatives there are and what the most cost-effective alternatives are," said Paul O. Swartz, the commission's executive director.

The Susquehanna, which contributes more fresh water to the bay than its other tributaries combined, also carries nearly 66 percent of the nitrogen and 40 percent of the phosphorous from all nontidal areas in the bay's watershed. It also contributes about 25 percent of the sediment, which reduces light to submerged grasses and can smother shellfish and obstruct the gills of fin fish.

If the dam, which generates electricity for PECO Energy customers in Pennsylvania, New Jersey, Delaware and Maryland, stops trapping sediment, the amount of silt flowing into the bay would more than double, the nutrient load would increase by 2 percent and the phosphorous load would increase by 70 percent.

Everything that washes off parking lots in Binghamton, N.Y., and farm fields in Lancaster County, Pa., would flow right through the intakes and the giant turbines to the bay a few miles downstream. An event the magnitude of 1972's Tropical Storm Agnes or the flood brought on by melting snow and ice in January 1996 would scour tons more sediment from behind the dam and hurl it downstream.

It's difficult to predict what would happen if another Agnes blew through, but the 1996 flood sent 15 million tons of sediment - about 16 times more than the average annual load - through the dam in a few days, burying oysters in muck and coating the grasses that are essential to the health of the bay.

Ronald K. Smith, an environmental specialist for Susquehanna Electric, which operates the dam, said there is no "silver bullet" solution but that there might be a combination of ways to reduce the risk, including encouraging better erosion controls upstream, getting farmers to use less fertilizer on their fields and dredging.

Bill Matuzeski, head of the EPA's Chesapeake Bay program, said he is "pessimistic" that erosion controls will reduce the silt coming downstream enough to solve the problem. "On the other hand, if you don't do something about the upstream erosion, you make the problem worse," he said.

Dredging would increase the reservoir's capacity, Smith said., but would cost hundreds of millions of dollars, and the search for a place to put the spoil could be endless. Dredging would stir up silt that has lain on the bottom for decades. And no one knows what's in that silt.

The top layers, deposited since the federal Clean Water Act has been in effect, are relatively benign, but the lower levels were deposited when tanneries, steel mills and coal mines lined the river. "The first thing we have to do is figure out what's down there," said Swartz. "We need to know what's in this stuff. That determines where we put it."

The U.S. Geological Survey has analyzed the top four feet for nitrogen and phosphorous, but there have been no detailed analyses. "The concern is we may have some toxins down there," said Mike Langland, a hydrologist in the agency's Lemoyne, Pa., office. "Now, we're going down as deep as we can and sending the samples to labs for analysis."

The USGS has drilled 11 cores into the silt on the bottom of the lake, he said, and plans to drill more.

If the sediment is clean enough and if the transportation costs are affordable, the sediment could be used to fill in the mines.

"It would be good for them and good for the river," Swartz said.

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