The Supreme Court on Thursday sided with environmentalists by giving a broad reading to the types of water-borne pollution covered by the Clean Water Act.

In a 6-3 decision, the justices held that a permit is required for either a direct discharge of pollutants into federally regulated rivers and oceans or its “functional equivalent.”

“Suppose, for example, that a sewage treatment plant discharges polluted water into the ground where it mixes with groundwater, which, in turn, flows into a navigable river, or perhaps the ocean,” Justice Stephen BreyerStephen BreyerREAD: Supreme Court justices mourn death of Ginsburg, 'an American hero' Ginsburg death sets up battle over future of court Trump's Supreme Court list reveals influence of Clarence Thomas MORE wrote for the majority.

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“Must the plant’s owner seek an EPA permit before emitting the pollutant?” he continued, referring to the Environmental Protection Agency. “We conclude that [a permit is required] if the addition of the pollutants through groundwater is the functional equivalent of a direct discharge from the point source into navigable waters.”

At issue in the case was whether Maui County in Hawaii violated the Clean Water Act, the landmark 1972 environmental law, by injecting wastewater underground without a permit that then seeped into the Pacific Ocean.

In siding with environmental groups, Breyer was joined by his fellow liberal justices Ruth Bader Ginsburg Ruth Bader GinsburgGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE, Sonia Sotomayor Sonia SotomayorNames to watch as Trump picks Ginsburg replacement on Supreme Court READ: Supreme Court justices mourn death of Ginsburg, 'an American hero' READ: Supreme Court justices offer tributes to Ruth Bader Ginsburg MORE and Elena Kagan Elena KaganREAD: Supreme Court justices mourn death of Ginsburg, 'an American hero' Democrats, advocates seethe over Florida voting rights ruling Supreme Court denies push to add Green Party candidates to Montana ballot MORE, as well as more conservative Justice Brett Kavanaugh Brett Michael KavanaughGraham: GOP will confirm Trump's Supreme Court nominee before the election Tumultuous court battle upends fight for Senate Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' MORE and Chief Justice John Roberts.

The decision returns the case, County of Maui, Hawaii v. Hawaii Wildlife Fund, to the 9th Circuit Court of Appeals to apply the new “functional equivalent” test.

David Henkin, an attorney with Earthjustice who argued the case on behalf of environmental groups, celebrated the win.

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“This decision is a huge victory for clean water,” he said in a statement. “The Supreme Court has rejected the Trump administration’s effort to blow a big hole in the Clean Water Act’s protections for rivers, lakes, and oceans.”

Justice Clarence Thomas Clarence ThomasRush Limbaugh encourages Senate to skip hearings for Trump's SCOTUS nominee Names to watch as Trump picks Ginsburg replacement on Supreme Court READ: Supreme Court justices mourn death of Ginsburg, 'an American hero' MORE wrote a dissent that was joined by Justice Neil Gorsuch Neil GorsuchConservative group launches .2 million campaign urging GOP senators to fill Ginsburg's seat Names to watch as Trump picks Ginsburg replacement on Supreme Court READ: Supreme Court justices mourn death of Ginsburg, 'an American hero' MORE, and Justice Samuel Alito Samuel AlitoREAD: Supreme Court justices mourn death of Ginsburg, 'an American hero' Ginsburg death sets up battle over future of court Trump's Supreme Court list reveals influence of Clarence Thomas MORE wrote a separate dissenting opinion that accused the majority of going beyond the text of the Clean Water Act.

"If the Court is going to devise its own legal rules, instead of interpreting those enacted by Congress, it might at least adopt rules that can be applied with a modicum of consistency,” Alito wrote. “Here, however, the Court makes up a rule that provides no clear guidance and invites arbitrary and inconsistent application.”

Michael Kimberly, an attorney at McDermott Will & Emery who co-authored an amicus brief in support of the Maui County, criticized the majority opinion as setting an “amorphous” new environmental standard.

“Not only is the decision vague, but it leaves countless responsible landowners potentially liable for discharges from ‘point sources’ to ‘navigable waters’ that aren’t actually anything of the sort,” he said.

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The case arose in the spring of 2012, when four Hawaii environmental groups sued Maui County to stop a municipal water treatment plant from pouring millions of gallons of wastewater each day into wells running hundreds of feet deep, where the treated sewage combined with groundwater.

A study showed some of the wastewater later surfaced at popular beach areas, and the environmental groups said pollutants contained in the discharge had interfered with nearby coral reef and triggered invasive algae to bloom. They argued the county was operating in this way without a federal permit, in violation of the Clean Water Act.

The 9th Circuit Court of Appeals eventually sided with the environmental groups, prompting an appeal to the Supreme Court.

Updated at 12:14 p.m.