Supreme Court justices, both conservative and liberal, appeared skeptical Wednesday of a Trump administration argument that the federal Clean Water Act should not apply to sewage plant wastewater that flows into the ground and eventually seeps into federally protected waters, such as rivers or oceans.

The case from Hawaii has emerged as a major test of the federal anti-pollution law’s scope even as the Environmental Protection Agency under President Trump cuts back on enforcement. If justices side with environmentalists, their ruling could extend federal regulation to water treatment plants across the country.