Dissolved Oxygen and Nutrients
Delaware Estuary Dissolved Oxygen: Historical Problems & Recovery

The limited treatment of human and industrial wastes through the mid-1900s caused severe water pollution problems in many areas of the Delaware Basin, with the urban Delaware Estuary corridor surrounding Philadelphia the most notorious of the problem areas. 

In fact, these water pollution problems were a key factor in the formation of the DRBC in 1961 and in federal and state water pollution control laws of the 1960s and 1970s.

Slaughterhouse waste discharge in Philadelphia, Pa., 1928. Photo courtesy of the Philadelphia Water Department Historical Collection. A fishkill in Philadelphia, Pa., circa 1929. Photo courtesy of Temple University Archives.
Slaughterhouse waste discharge in Philadelphia, Pa., 1928. Photo courtesy of the Philadelphia Water Department Historical Collection. A fishkill in Philadelphia, Pa., circa 1929. Photo courtesy of Temple University Archives.

The pollution in the Philadelphia section of the Delaware Estuary was so severe that in the summer and early fall (May-November), there was essentially no dissolved oxygen (DO) in the Delaware River on a typical day. 

This zone of "anoxia" (a lack of DO) and the surrounding zones of "hypoxia" (severe depression of DO) eliminated the fish and other aquatic organisms from this zone of the Delaware River. It also prevented migratory fishes, such as American shad, from completing their runs to the upstream spawning grounds and the return migration of juvenile fish back to the sea. This stretch of the Delaware River was considered a "dead zone," void of aquatic life.

DRBC Formed in 1961

After the DRBC was created in 1961, it went right to work to address Delaware Estuary water pollution.

Its first significant accomplishment was the adoption and implementation of water quality standards in 1967 (pdf).

To address an important oxygen-consuming pollutant known as CBOD (carbonaceous biochemical oxygen demand), a waste load allocation was developed in 1968. 

As a result of these DRBC regulations, and with the significant help in subsequent decades from Clean Water Act grants and the diligent work of state and federal agencies, the DO levels in the Delaware Estuary steadily improved, to the point where oxygen levels now meet water quality criteria (e.g., 3.5 mg/L average DO concentration around Philadelphia's Ben Franklin Bridge). Fish populations in this region of the estuary, as well as migratory fish, have returned.

The amazing DO restoration seen in the Delaware Estuary - from essentially no oxygen for 6 months each year to achieving levels such as the 3.5 mg/L standard - is hailed as one of the world's top water quality success stories.

How About Now?

DRBC has been monitoring DO in the Delaware Estuary for over 50 years. While there continues to be a summertime DO sag in the most urbanized part of the estuary, DO levels have greatly improved since the 1960s.

Graphic of how DO levels in the Delaware River at the Ben Franklin Bridge have Improved since 1965 (jpg)

To get a closer look, here's an animated graphic showing summertime (July and August) DO improvement from the late 1960's through 2019. Please note: In the graphic, the criteria line is 24-hour mean DO, while the measurements are daytime spot measurements near surface.

Created by John Yagecic, P.E., DRBC Water Quality Assessment Manager

DRBC's Current Effort to Further Improve DO

Dissolved Oxygen and Nutrients

Aquatic Life Designated Use Study

Additional Resources

Fact Sheet: DO (pdf)

Map: DRBC Water Quality Zones for the Main Stem River (pdf)

Presentation: Continuing Restoration of DO in the Delaware Estuary - Historical Data and Current Efforts (pdf; 2010)

Article: Estuarine oxygen dynamics: What can we learn about hypoxia from long-time records in the Delaware Estuary (pdf; U. Del.'s Jonathan Sharp, Limnol. Oceanogr., 55(2), 2010, 535-548)