Bay Journal

It’s time to get serious about Conowingo’s trapped sediment

  • By Tom Horton on April 30, 2012
The 100-foot-high by nearly a mile wide Conowingo Dam traps massive quantities of sediment, phosphorus and nitrogen from across Pennsylvania and New York that would otherwise be flushed by the Susquehanna River into the Chesapeake.  (Dave Harp)

It's been around a decade since the governments of the Chesapeake watershed last confronted the so-called "ticking time bomb" behind Conowingo Dam on the Susquehanna- and horrified by the costs of dealing with it, decided not to.

From its completion in 1928, the 100-foot-high by nearly a mile wide hydro dam began trapping massive quantities of sediment, phosphorus and nitrogen from across Pennsylvania and New York that would have otherwise been flushed by the Susquehanna River into the Bay.

Nowadays, with the estuary so polluted, this trapping is considered mostly a good thing. No one will ever know whether the Bay would actually be in worse shape had Conowingo never been built.

We do know the trapped sediment can come back to haunt the Bay when the rare storms big enough to infuriate the Susquehanna scour it from behind the dam.

Most notable was Tropical Storm Agnes, which blasted about 20 million tons of trapped sediment and nutrients through Conowingo's 53 flood gates in June 1972. Agnes triggered declines in resources that persisted for decades.

We also know that barring more huge, scouring storms, the reservoir behind Conowingo is filling to where it will begin passing a lot more pollution Bayward in little more than a decade.

This added pollution would more than offset all of the phosphorus reductions Pennsylvania expects to make and it will add a couple of million pounds a year of nitrogen and tens of millions of pounds of sediment.

What to do? Back in 2001, the Bay states took a cursory look at shipping the sediments buried behind the dam to abandoned coal mines in western Pennsylvania.

The price tag and prospect of 100 railcars a day laden with dredged material running from Conowingo, basically forever, was too much. Just dredging enough to keep up with the 1.5 million tons of new sediments brought down the river every year was $48 million annually (and may be double that now).

Going further, it would have cost astronomically more to dredge out enough of the 200 million tons now stored behind Conowingo so that big storms wouldn't scour it toward the Bay.

Recently, state and federal environmental agencies, led by the Army Corps of Engineers, have begun a more in-depth study of how the time bomb might affordably be defused.

During the next year or two, a number of options will be on the table, including assessing whether any of the options will work in the long run, when periodic, sediment-scouring storms are factored in.

One "do something" option seems certain to get serious consideration, because it is already getting very serious consideration as a solution to another related and enduring Bay problem.

It is what to do with the mammoth quantities of muck that must be dredged each year to keep Baltimore's port accessible to deep draft ships.

A promising solution in the final stages of evaluation by the Maryland Port Administration is to build a factory that would convert dredged sediments into a widely sought after construction material known as "lightweight aggregate."

Lightweight aggregate is to regular rock what popcorn is to a corn kernel. Think of pumice, or the lava rock in your outdoor grill. It's used to make lightweight building blocks, and the high temperature process meets all environmental standards, even with contaminated harbor spoils.

"It's a case where you've got huge sources of material and a huge demand for the product," says Jeff Otto, president of Harbor Rock, the Pennsylvania company that has made the lightweight aggregate plant proposal to the port.

"There's no doubt (Harbor Rock) can do it," says Frank Hamons, deputy director of port development for the MPA. He said the port is looking at the costs and financing of building the plant, but has made no decision.

A real attraction of this option, Hamons said, "is that it is a constant flow through of material, rather than just finding another disposal site and filling it up."

That would be the beauty of a similar plant constructed near Conowingo, said Michael Helfrich, the Lower Susquehanna Riverkeeper and an advocate of the lightweight aggregate solution.

Otto says Conowingo, like Baltimore, "is in the heart of a large marketplace for the product." A plant would cost at least $75 million and employ 65-70 people, with annual operating costs of $15 million to $20 million.

He doesn't tout such a plant as a huge profit center. "Our business model would have to include the savings from what it costs otherwise to dispose of the dredged material."

"There are not likely to be any simple solutions to Conowingo," cautioned Tom Beauduy, deputy executive director of the Susquehanna River Basin Commission.

But for the first time, we may soon have realistic options to compare.

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About Tom Horton

Tom Horton covered the Bay for 33 years for The Sun in Baltimore, and is author of six books about the Chesapeake. He is a freelance writer, splitting his time between Baltimore and Maryland’s Eastern Shore.

Read more articles by Tom Horton

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