Dissolved Oxygen

Oxygen is a fundamental requirement for nearly all animal life on our planet. In aquatic environments, organisms ranging from fish to insects to small protozoans utilize the gaseous oxygen that is dissolved in water in order to respire or "breathe," and scientists measure this needed resource as Dissolved Oxygen, also known simply as DO. 

Oxygen enters water both by direct absorption from the atmosphere and as a by-product of photosynthesis from algae and aquatic plants. Oxygen levels in water bodies can be depressed by the discharge of oxygen-depleting materials (measured in aggregate as biochemical oxygen demand, BOD, from wastewater treatment facilities), from the decomposition of organic matter including algae and aquatic plant biomass generated during blooms, and from the oxidation of ammonia and other nitrogen-based compounds.

Historical Problems and 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 key to motivating the Delaware River Basin Commission's formation and the historical state and federal laws to control water pollution in the 1960s and 1970s.

The pollution in the Delaware Estuary was so severe that, among many symptoms, there was essentially no dissolved oxygen in the Delaware River on a typical day from May through November of every year in the areas around Philadelphia. This zone of "anoxia" (a lack of dissolved oxygen) and the surrounding zones of "hypoxia" (severe depression of dissolved oxygen) eliminated the fish and other aquatic organisms from this zone of the Delaware River and 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.

Attempts to restore oxygen to the Delaware Estuary and to other impacted areas of the Delaware Basin pre-date the formation of the DRBC. But a significant accomplishment for the DRBC has been the adoption and implementation of water quality standards (pdf 4 MB) in 1967 and a waste load allocation in 1968 for one form of oxygen-consuming pollution known as CBOD (carbonaceous biochemical oxygen demand). 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 dissolved oxygen levels in the Delaware Estuary steadily improved through the 1970s, 1980s, and into the 1990s, to the point where oxygen levels now meet the targeted goals (such as 3.5 mg/L average dissolved oxygen concentration around the Ben Franklin Bridge) and the fish populations in this region of the estuary have been partially restored. This amazing restoration of dissolved oxygen 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.

Dissolved oxygen conditions have likewise improved in other streams and rivers of the Delaware Basin as a result of the broader efforts to protect and restore water quality. From areas surrounding Port Jervis, N.Y., on the Delaware River to areas on the Lehigh River in its urban and industrial corridor, dissolved oxygen conditions are one of the primary ways that water pollution control technologies and regulations have proven successful over the past 50 years.

Continuing Efforts

Although the worst of the dissolved oxygen problems have been addressed over the past 50 years, dissolved oxygen conditions remain a concern even today. Because the pollution problems in the Delaware Estuary were so severe back in the 1960s, a compromise solution was selected for some areas of the estuary wherein the goal of the 1968 wasteload allocation was to restore dissolved oxygen concentrations to a level below what was needed for all forms of fish and aquatic life during all stages of their life cycle. As a result, although current conditions typically meet the 1967 dissolved oxygen criteria, mid-summer dissolved oxygen is, at times, only 50% or less of full saturation levels in the areas around the Ben Franklin Bridge. At these depressed oxygen levels, studies have shown that significant mortality can occur for juvenile fishes native to the Delaware Estuary (e.g., Atlantic sturgeon).

The current nation-wide focus on elevated nutrients (often termed "eutrophication") and the depletion of dissolved oxygen in many aquatic environments subjected to high nutrient loading has re-focused efforts on the remaining dissolved oxygen issues in the Delaware Estuary. To address both nutrients and dissolved oxygen, the DRBC and its partners in state and federal agencies, the regulated community, and the environmental community, have initiated a process to measure the sources of nutrients and oxygen-depleting materials and to build a water quality model to integrate this information and forecast future scenarios for the Delaware Estuary. Although the results from such efforts are not yet available, the goal is to select an appropriate path towards a healthy, functioning ecosystem in all parts of the Delaware Estuary.