Plans to dispose of 18 million cubic yards of dredged material in a deep part of the Bay is not likely to result in the serious long-term harm suggested by some of the project’s critics, according to a review by a team of university scientists.

But it would also not produce some environmental benefits claimed by the U.S. Army Corps of Engineers in its draft environmental impact statement, the scientists said.

“It’s not as rosy as the draft impact statement would suggest,” said Donald Boesch, president of the University of Maryland Center for Environmental Science. “The assertions about environmental improvements are speculative at best and have to be further demonstrated.

“On the other hand, the suggestions offered by others that it would have a devastating effect on the Bay that would set us way back don’t have a lot of support in terms of the scientific understanding of potential impacts.”

Boesch and a team of 15 scientists from the Center for Environmental Science reviewed the Corps’ impact study and other available information about the environmental consequences of placing some of the material that is dredged annually from Port of Baltimore shipping channels in a four-mile-long deep trench north of the Bay Bridge known as Site 104.

The scientists’ report neither advocated nor opposed the use of Site 104, but said their goal was to “inform debate and policy development as objectively as we are able.”

The Site 104 plan has been hotly debated since the Maryland Port Administration proposed using that — along with several other placement sites — to hold the roughly 3.5 million cubic yards of material that need to be dredged each year to maintain shipping channels.

No single site can hold all the dredged material, and the Port Administration has said it needs a low-cost “open water” placement option, such as Site 104, to help balance the high costs of other alternatives being used, such as the reconstruction of wetlands and uplands at Poplar Island.

The Corps’ draft environmental study was released in February. Its conclusion that the use of Site 104 would cause no serious environmental problems and suggestion that the placement of materials there would even produce some benefits created a storm of controversy.

The report was criticized by environmental and local activists, and federal agencies as incomplete and based on poor science. In July, Col. Bruce Berwick, commander of the Corps’ Baltimore district, announced plans to issue a revised study later this year.

The state’s congressional delegation is sharply divided on the issue.

One of the major criticisms of the project is that the dredging would “resuspend” nutrients which had settled to the bottom of the Bay back into the water — in effect, adding nutrients to the Chesapeake at a time when efforts are under way to reduce them. Nutrients can spur algae blooms which block sunlight to valuable underwater grasses. When the algae die, they sink to the bottom and decompose in a process that depletes the water of oxygen.

The scientists said that the nitrogen and phosphorus released by the dredging and placement of sediment would amount to “substantially less” than 1 percent of all nutrient inputs into the Bay.

In fact, little phosphorus is expected to leak at all, because it is so tightly bound with sediment particles. The scientists said some nitrogen — especially ammonia — would be released during the dredging and placement operation. By changing the dredging techniques used, they said the amount of nitrogen released could be reduced below the Corps’ estimate of 0.11 pounds of nitrogen per cubic yard dredged.

The effect of the released nutrients on dissolved oxygen problems in the Bay “would be so small as to be undetectable,” they said. While the Baywide impact is small, they agreed with critics that the amount of nutrients released at the site was large relative to discharges from individual wastewater treatment plants. But because the dredging would take place mostly during the winter when there is little algae growth, they said the likelihood of stimulating blooms is small.

“The phytoplankton is just not able to take it up and grow during that time,” Boesch said. “It’s not like a point source, because the point sources are releasing nutrients all the time.”

Nonetheless, when it comes to nutrient control, the scientists said “every little bit helps.” They suggested that channel dredging and a variety of other actions that can resuspend nutrients should receive attention for “control or mitigation” — including the possibility of supporting nutrient reductions elsewhere.

Critics of the project have also worried that a large amount of sediment placed in Site 104 would drift out, potentially smothering oyster bars or grass beds. But the scientists said the majority of sediments would most likely stay in the site. Most of what drifts out, they said, would ultimately be deposited in other deep water portions of the Bay, posing little threat to sensitive shallow water habitats.

Toxic contaminants are not likely to be a problem because the chemical concentrations in the sediments being dredged do not appear to be different from those in the rest of the Bay. But the scientists said the sediments should be tested regularly for any unusually contaminated sediment sources.

The scientists disputed a claim in the Corps’ study that there would be “significant improvement” of bottom habitat because the bottom at Site 104 would be elevated from a depth of about 70 feet to a depth of 40-45 feet where oxygen may be more plentiful during the summer. Even at that elevation, the scientists said, the bottom would be expected to suffer anoxia — depletion of oxygen — during summer months.

A bigger concern could be that, by filling in a deep part of the bottom that is usually warmer than surrounding waters during the winter, the Bay could lose an important refuge site for some species such as white perch, striped bass and sturgeon. Exactly how important the site is to those fish is uncertain because of a lack of information about habitat use for the fish.

But because Site 104 is located in one of the narrowest parts of the Bay and its deep water remains warm during the winter, it may be of “unusual importance” both as a migratory corridor and as a thermal refuge. If so, the filling of Site 104 would “permanently reduce its value” as a refuge, and force the fish to group more intensely in other areas, potentially affecting food resources, predation and survival. Because of a lack of information, the scientists said this was their most uncertain conclusion, and said Site 104’s value as a winter refuge should get more study.

Sediment placement would inevitably smother some fish and crabs living near or burrowing in the bottom when the placement takes place. But if the placement is confined to mid-October through March — instead of until April 15 as proposed — any impacts on larval fish could be minimized, the scientists said.

In the future, the scientists said a more holistic scientific assessment of various options for dredged material use or disposal is needed.

“It’s easy to suggest this is the worst alternative and we should do something else,” Boesch said. “The test of that premise is to have an equally hard look at the other alternatives. But the process [of selected dredge material placement sites] goes to the point where there is a crisis, and the debate gets focused over a particular alternative rather than over multiple options.”

The scientists said there is no single best solution, and decision-makers need to better understand the tradeoffs involved in all alternatives.

Although some consider any open-water placement of dredged materials as “inherently the most harmful alternative,” the scientists said all disposal options have drawbacks. Nitrogen losses could actually be greater in some beneficial use projects than in deep-water placement. In addition, projects that create islands also eliminate important shallow water habitats.

Other options could be examined, they said, such as the spray application of dredged material to help rebuild tidal marshes that are disappearing because of sea level rise.

In an ecosystem context, the scientists said the concept of requiring mitigation — such as funding for nutrient removal elsewhere or the creation of oyster reefs — could be considered as part of any dredged material disposal option.