In this case, environmental groups sued the Army Corps of Engineers (Corps) to stop the permitted filling of a lake in Alaska with mining waste by arguing that authority to issue CWA Section 404 permits is vested in EPA, not the Corps. They argued that the mining company should have sought a CWA Section 402 permit from the EPA, just as it did for the lake water discharge. The question at hand was whether federal permitters should consider mine tailings (the ground-up waste rock that's left after the ore is removed) to be fill in the lake or wastewater from the mine. If it's fill, the Corps is in charge. If it's wastewater, EPA's standards would apply precluding the discharge of the tailings into the lake. Writing for the six-judge majority of the U.S. Supreme Court, Justice Kennedy found both agencies' regulations to be ambiguous. He held that the Corps' interpretation of CWA regulations was entitled to deference under Chevron principles. The decision affirms that discharges allowed by the Corps under Section 404 "do not require permits from the EPA" under Section 402 of the CWA. Now, for the first time in 30 years, a U.S. mine will be allowed to transform a natural lake into a tailings pond by acquiring a Corps' permit.