The Supreme Court on Wednesday appeared to side with the City of San Francisco in its unusual challenge of federal water regulations that it said were too vague and could be interpreted too strictly. The outcome could have sweeping implications for curtailing water pollution offshore and would deal another blow to the Environmental Protection Agency, which has faced a string of losses at the court over its efforts to protect the environment. The case has given rise to unusual alliances, with the city joining oil companies and business groups in siding against the E.P.A. In arguments on Wednesday, it was the conservative justices who seemed the most aligned with a city best known as a liberal bastion. At its core, the case is about human waste and how San Francisco disposes of it — specifically, whether the Clean Water Act of 1972 allowed the E.P.A. to impose generic prohibitions on wastewater released into the Pacific Ocean and to penalize the city.

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