AOC stands for Area of Concern. It's defined in the United States-Canada Great Lakes Water Quality Agreement as "a geographic area designated by the Parties where significant impairment of beneficial uses has occurred as a result of human activities at the local level." It is an area that has experienced environmental degradation. When AOCs were originally identified, there were 43 areas in the Great Lakes region. As of August 2016, 7 AOCs (4 in the US and 3 in Canada) have been delisted.

There are currently three Areas of Concern (AOCs) in Ohio: Black River, Cuyahoga River, and Maumee River. The Ashtabula River was delisted in 2021.

The Black River was originally the only river system in Ohio where the entire watershed was designated an Area of Concern (AOC). It was designated an AOC because of the following impairments: poor water quality due to toxic substances and bacterial contamination, loss of biodiversity due to water contamination, habitat degradation and sedimentation, and land use changes that altered the river and its tributaries. In June 2015, the boundary of the AOC was modified to include the entire mainstem, the French Creek watershed, the Outer Harbor, and the nearshore of the Lake Erie area.

The Black River AOC Advisory Committee adopted the Ohio EPA Delisting Guidance and Restoration Targets for Ohio Areas of Concern, which provides statewide targets for restoring beneficial use impairments (BUIs) and identifies steps required for BUI removal and AOC delisting. The Black River AOC Advisory Committee will work collaboratively with local agencies and government to reach the targets. The delisting guidance document can be found here: Ohio EPA Delisting Guidance and Restoration Targets for Ohio Areas of Concern