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The effects of urbanization on water quality:
PCBs and chlordane in fish

Many chemicals and substances that were once deemed safe for use are now known to be harmful. Even though their usage may have been banned years ago, they still persist in the environment. PCBs and chlordane are two such substances. PCBs are synthetic oils, banned from use in 1976, that were used in electrical transformers, hydraulic oil, and lubricants. Chlordane is a banned pesticide that was used to control termites.

Despite being banned since the 1970's, polychlorinated biphenyls (PCBs) continue to pose a threat to the environment because of their persistence and toxicity to organisms ranging from minute algae to fish, waterfowl, and human beings. PCBs, a set of 209 related chlorinated organic compounds, had various industrial uses such as in hydraulic fluids, cutting oils, sealants, and pesticides. Despite the manufacturing ban in the mid-1970's, PCBs remain ubiquitous in the environment.

In some urban areas these substances are still detected in fish, where they tend to build up in the fatty tissue. Eating too much of these fish might lead to an increased risk of cancer.

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Effects of urbanization  USGS Fact Sheet FS-116-96