The Water Quality Certification would require Exelon to build best management projects (BMPs)—such as constructed wetlands or infiltration trenches—to cut annual levels of pollutants such as nitrogen, phosphorus and sediment. Alternatively, the utility could fund BMP construction or perform dredging. The projects are necessary to reduce high levels of nutrients and sediment that have built up in the bay estuary during the dam’s existence, state officials say. 

But Exelon said in a statement that the dam itself “does not produce any pollution.” It said that “the science clearly shows that the pollutants that travel down the Susquehanna River, from New York and Pennsylvania, are the source of the nutrients and sediment that flows into the bay.”

Betsy Nicholas, executive director of Waterkeeper Chesapeake, says Exelon has taken an adversarial stance since 2012, when the relicensing process began. “Maryland and other federal parties have had to continually push Exelon to provide even the most basic of information on environmental studies and impacts of dam operations,” Nicholas says.

The certification is a required state action under Section 401 of the Clean Water Act. Doug Myers, Maryland senior scientist for the Chesapeake Bay Foundation, says the water certification process is the first under the more stringent “pollution diet”—the total maximum daily load established by the U.S. Environmental Protection Agency in 2010.

The Maryland Dept. of the Environment said via email that it would “vigorously defend” the plan. “The Conowingo Plan is at the heart of our multi-state strategy” to restore the bay’s water quality, it added.

Myers says the dam has changed the way sediment and nutrients come down the river. “There’s been no offsetting” of how pollutants are discharged into the bay by the dam, he says.