After 87 years, the ability of the 14-mile-long reservoir behind the Conowingo Dam to trap sediment and nutrients coming down the Susquehanna River has largely ceased, threatening the region’s ability to meet Bay cleanup goals, a multi-year study concludes.

Offsetting that impact may require millions of pounds of additional nutrient reductions beyond what was anticipated when the Bay cleanup plan was released in 2010, according to the Lower Susquehanna River Watershed Assessment report, the final version of which was released Thursday.

The $1.4 million study, led by the Army Corps of Engineers and the Maryland Department of the Environment, comes at a time when the state-federal Bay Program partnership has embarked on a multi-year effort to assess and update cleanup goals by the end of next year — a task made harder by the Conowingo situation.

The report “puts more onus on all of us to recognize that Conowingo needs to be addressed, and with a sense of urgency,” Maryland Environment Secretary Ben Grumbles said. The contributions of nitrogen, phosphorus and sediment pollution from behind the dam and upriver are “important,” he said.

When running for office in 2014, Gov. Larry Hogan had charged that federal and state partners in the Bay restoration effort were neglecting the pollution threat posed by Conowingo, and he later expanded his criticism to accuse Pennsylvania and New York of not doing their share to help clean up the estuary. “This is going to continue to be one of the governor's priorities on the Chesapeake Bay,” Grumbles said.

For decades, the 100-foot-high dam had helped Bay cleanup efforts by trapping a portion of the sediment and nutrients coming down the Susquehanna River, and keeping them from reaching the Chesapeake. While it has long been known that the dam’s reservoir was reaching its storage capacity, that day was thought to be further in the future. As a result, the 2010 Bay cleanup plan did not anticipate the impact of the reservoir filling on Chesapeake water quality.

But the new report concludes that Conowingo and two upstream dams, Safe Harbor and Holtwood, “are no longer trapping sediment and the associated nutrients over the long term.” Instead, the dams delay a portion of the sediment and nutrients coming down the river during dry years, only to have that material flushed into the Bay during years with higher than average rainfall.

Dealing with the Conowingo issue is one of the key issues for consideration in a midpoint assessment under way of the cleanup plan, formally known as the Chesapeake Bay Total Maximum Daily Load, or TMDL. The plan established nutrient and sediment reduction goals for each state.

The midpoint assessment, conducted by officials from the Environmental Protection Agency and all of the states in the watershed, will use information gained since 2010 to update and revise goals through 2025, when all needed cleanup actions are to be in place. “This report gives us even better clarity into the water quality impacts of pollutants that flow through the dam,” the EPA said in a statement.

But the report also indicates that the filling of the reservoirs will make attaining the 2025 cleanup goal more difficult.

Much of the public has focused on the impact of sediment from the dam, Extreme events like Tropical Storm Lee in 2011 can “scour” built-up sediment from behind the dam, resulting in stunning images of brown sediment-laden plumes stretching far down the Bay.

But the report said the greatest threat to Chesapeake water quality comes not from sediment scoured during large events, but rather from nutrients coming down the Susquehanna that are no longer being trapped. When they reach the Bay, those nutrients spur algae blooms that cloud the water and — when they die — rob it of oxygen needed by aquatic life and contribute to summertime dead zones.

Even if all currently planned nutrient control efforts are in place by the 2025 deadline, the report found that parts of the mid-Bay — the area with the most severe “dead zone” — would not meet water quality goals because of the dam’s diminished nutrient-trapping capacity.

Computer modeling done for the report showed that meeting water quality goals without the dam's help would require an additional 2.4 million pounds of nitrogen reductions and an extra 270,000 pounds of phosphorus reductions from the Susquehanna each year.

That’s problematic because Pennsylvania —which supplies the vast majority of nutrients in the Susquehanna basin — is also lagging far behind in meeting its Bay cleanup goals, making additional nitrogen and phosphorus reductions from the river even more difficult.

In the last nearly three decades, Pennsylvania reduced nitrogen pollution to the Bay by about 7.3 million pounds, according to the Bay Program; it needs to increase reductions by nearly five-fold over the next 11 years to meet its 2025 goal. Pennsylvania officials have already said they will not achieve interim goals set for the end of next year.

But trying to offset Pennsylvania’s impact by reducing nutrient pollution from other rivers is equally problematic, because improvements elsewhere have less impact on the mid-Bay dead zone. Modeling estimates in the report suggest that meeting the water quality goals would require much greater reductions of 4.4 million pounds of nitrogen and 410,000 pounds of phosphorus if they had to come from the rest of the watershed.

Charles “Chip” MacLeod, an attorney who represents the Clean Chesapeake Coalition, a group of mostly rural Maryland counties, said the report affirms the coalition’s long-held position that the issue of the Conowingo reservoir must be addressed to meet Bay goals.

“This justifies a sense of urgency; as our efforts and expenditures in Maryland to improve Bay water quality are otherwise being inundated by upstream pollution,” he said.

But the report discounted the potential of dealing with the problem by dredging the reservoir, as some including the coalition have suggested. When it comes to dredging, the report said, “ecosystem benefits are minimal and short lived, and the costs are high.” Dredging to roughly keep pace with what the dam historically trapped would cost between $15 million and $270 million annually, it said.

Exelon Corporation, which owns the hydroelectric facility, is funding $3.5 million in additional studies to gain more detailed information about how sediment and nutrients from behind Conowingo are impacting the Bay. The results of those studies, which will inform the TMDL midpoint assessment, are expected later this year.

Exelon had applied in 2014 for a new 46-year license from the Federal Energy Regulatory Commission to generate power at Conowingo. Maryland environmental officials, concerned about the dam's impact on Bay water quality, signaled their intent to deny the company the certification it needed before FERC could issue the license, and the company withdrew that request pending the outcome of the additional studies.

Exelon last month indicated it plans to file another application soon. Maryland would then have a year to decide whether to grant the company a water quality certification it needs to gain the operating license.

In an emailed statement, Exelon spokesman Marshall Murphy emphasized the dam's importance to Maryland, calling it the state's second largest generator of renewable energy. In the statement, the company vowed to "continue to work with key stakeholders" to ensure the health of the lower Susquehanna and the Bay. Exelon is also negotiating with federal and state agencies over how to improve fish passage over the dam - another potential hurdle to the facility's relicensing.