Diana Muller, director of scientific research at the South River Federation, flicks "black muck" from her hands after obtaining pollution samples on Church Creek, a tribute to the Chesapeake Bay in Annapolis, Md., in May 2015. (Linda Davidson/The Washington Post)

“SWIMMABLE AND fishable” — these are the modest yet unmet standards to which the nation holds some of its greatest bodies of water, including the Chesapeake Bay, a precious resource that has been long abused. After decades of environmental mismanagement, the six states in the bay’s watershed and the District struck an agreement with the Environmental Protection Agency to improve conditions — at least to the point where Americans no longer have to wonder if they really want to dip a toe into the largest estuary in the lower 48 states.

In an otherwise unimpressive performance before the Senate Environment and Public Works Committee this month, Scott Pruitt, Oklahoma’s Republican attorney general and President Trump’s pick to run the EPA, seemed to embrace the bay cleanup effort. He said the agreement “should be commended and celebrated,” promising to enforce it and to preserve funding for federal bay efforts. This was surprising because Mr. Pruitt had earlier brought Oklahoma into a lawsuit against the EPA’s bay program.

It turns out that, when Sen. Ben Cardin (D-Md.) pressed him further in written questions, Mr. Pruitt’s commitment to the cleanup deal got murkier.

The plan relies on two crucial elements that require federal oversight. First is the establishment of targets for various types of bay-killing pollution: phosphorous, nitrogen and sediment. Among other things, these pollutants contribute to algae blooms, murky water and oxygen-deficient zones, which harm all sorts of bay life. So the goal is to cut them by 20 to 25 percent. This “pollution diet” and each state’s share in meeting it came after extensive federal expert study.

But when asked how he would “promote effective numeric standards,” Mr. Pruitt wrote back to senators that he prioritized “state action to encourage on the ground activities over establishment of numeric nutrient limits.” Asked if he would defend the bay watershed’s pollution limits in court, Mr. Pruitt only promised to “consult with the States and other interested stakeholders about the issues raised in such litigation.”

The EPA is also responsible for holding states accountable for meeting their targets. Mr. Pruitt pledged to “continue to enforce the law” so long as the courts upheld the pollution diet. But he also downplayed the significance of the plan, arguing that actions predating it may have resulted in some recent gains. Pressed on the importance of EPA policing states’ progress, Mr. Pruitt responded vaguely that “EPA has a role in overseeing its implementation.” This statement could mean practically anything.

Past bay cleanup efforts have not done enough. Without a strong federal hand that keeps states moving in the same direction, cross-state pollution-cutting efforts can all too easily collapse, which is why the current plan promises more success. We will hold Mr. Pruitt to his encouraging tone during his in-person questioning. If, as EPA chief, his approach to bay cleanup is as distressingly tepid as his written answers were, it may take much longer to right the national disgrace that the Chesapeake Bay has been for so long.