In this July 10, 2011 file photo, oil-fouled islands and a sheen on the water are seen downstream of an Exxon Mobil crude pipeline that broke beneath the Yellowstone River near a CHS, Inc. refinery, background, in Laurel, Montana. Credit: Julie Jacobson | AP

The administration of President Donald Trump said Thursday it plans to scrap a 2015 definition of what qualifies as “waters of the United States,” a move one environmental group said risks protections for half of Maine’s drinking water.

The act, also called the Waters of the United States regulation, has been changed a number of times since it was first enacted in 1972. The 2015 modification put in place under former President Obama clarified the federal government’s authority to oversee a wide array of lakes, streams, wetlands, storm-water controls and ditches that feed into larger waterways that are clearly protected under the 1972 Clean Water Act.

Critics say the rule gave the federal Environmental Protection Agency too much power. Supporters countered that it would prevent the loss of swaths of wetlands. Court rulings have delayed the regulation in 28 states. It is in effect in Maine and 21 other states.

The Natural Resources Council of Maine, a liberal environmental group, said in a news release the change would roll back protections for about half of Maine’s drinking water supply fed by small streams protected under the Clean Water Act.

Heather Spaulding, policy director for the Maine Organic Farmers and Growers Association, said part of the change would take away Maine’s ability to impose its own regulations on top of federal rules, something she said is integral to the state’s culture.

She said the rollback could potentially undo decades of efforts to clean the state’s waters, particularly its rivers, where industrial plants used to dump waste until regulations were put in place. HoltraChem was one such plant — and the defunct company is still involved in a multi-year effort to clean up mercury it dumped into the Penobscot River while in operation.

“It’s a scary thing that the federal government could come in and ride roughshod over all our progress,” she said.

EPA Administrator Andrew Wheeler argued that differences between state and federal laws have created “a patchwork across the country” and a more uniform regulatory approach is needed. Doing so would end any lingering uncertainty and return more oversight to states, he said.

Critics say the rollback will speed the conversion of wetlands and headwaters, which provide critical habitat for wildlife and support the nation’s drinking water supply. Americans drained about half of the in the contiguous U.S. between the 1780s and 1980s, most of it to expand farmland. That rate began to slow after George H.W. Bush took the White House, pledging to stem the tide of wetlands loss.

The Clean Water Act makes it unlawful to pollute a “water of the United States” without a permit, but what constitutes such water has been the subject of lengthy litigation.

In a 2006 decision, Rapanos v. United States, the Supreme Court’s four most conservative justices at the time offered a constrained view that only “navigable waters” met this test. But Justice Anthony Kennedy, who refused to join either the conservatives or the liberals, said that the government could intervene when there was a “significant nexus” between large water bodies and smaller ones.

Trump’s executive order said federal officials should rely on the opinion of the late U.S. Supreme Court Justice Antonin Scalia, who argued that the law should apply only to “navigable waters.” The change would return the U.S. to standards last used in 1986.

The move was a culmination of an executive order the president made in 2017 directing the Environmental Protection Agency and the Department of the Army to rescind or revise the rule. It is part of a broader effort by the administration to roll back regulations affecting the private sector by the end of Trump’s first term.

Bangor Daily News writer Caitlin Andrews contributed to this report.