The incidence of fin erosion, ulcers and abnormal growths in fish are generally low...about 5 per 1,000 fish, EPA says. Still, this rate is nearly double other estuaries in the region, including Chesapeake Bay.

Some of the river's most common pollutants:

PCBs (Polychlorinated biphenyls)
This class of chemicals is suspected of causing cancer, liver damage, and respiratory distress. PCBs were once widely used in cooling electrical equipment, plasticizers, inks and dyes. They are slow to degrade in the environment and accumulate in the aquatic food chain. A 1994 study by Arthur D. Little Inc. of Cambridge, Mass., found PCBs in river sediments to be higher than previously thought. They are also turning up in the water, leading officials to suspect PCBs may still be getting into the river by way of leaking landfills, improper storage and illegal dumping.

DDT (Dichlorodiphenyl trichloroethane)
DDT was originally used to control mosquitoes that caused malaria, typhus and other diseases. It has been banned in the United States since 1970, after it was recognized as an environmental hazard because it accumulated in the tissue of fish and birds that prey on fish. DDT causes eggshell thinning in birds of prey. It's also suspected of causing breast cancer, birth defects and premature labor. DDT and its breakdown constituents, carried into river sediments by way of stormwater runoff, take decades to decompose. Levels in fish appear to be declining.

Metals
Dozens of kinds of toxic heavy metals including chromium, copper, lead, mercury and arsenic can have a wide range of health impacts, including causing neurological problems and cancer. These metals are found in sediments from past industrial pollution. Industries continue to discharge metals into the river. Urban runoff and airborne pollutants also contribute to metals in the river.

Nutrients
Nitrogen, phosphorous and ammonia from the discharge of treated municipal and industrial wastes contribute to algae blooms that can result in reduced dissolved oxygen. The Delaware River has one of the nation's highest concentrations of nutrients. But these nutrients haven't been a big problem, possibly because sediments suspended in the water may block out sunlight that algae need to grow.

Volatile Organic Compounds
Cancer-causing chemicals used by industry including 1,2-dichloroethane and tetrachloroethene have turned up in the river at levels exceeding federal drinking water criteria. Concerned about the risk of these chemicals ending up in drinking water supplies, officials are taking steps to set maximum industrial discharge limits. They are also tracking sources that discharge these chemicals into municipal wastewater treatment plants that discharge effluent into the river.