The district court erred in holding Dominion liable for arsenic that leached from coal ash at its Chesapeake facility into groundwater. The plant’s landfill and settling ponds that contained the coal ash were not “point sources” under the Clean Water Act, so § 1311(a) of the Act did not apply. However, the district court properly deferred to a Virginia agency determination that the groundwater contamination didn’t amount to a breach of Dominion’s discharge permit, contrary to Sierra Club’s complaint.