Description of  Contaminating Compounds

Polyaromatic hydrocarbons, or PAHs are the heavier compounds left from fuels and motor oil after lighter compounds such as benzene and toluene have evaporated. PAHs are resistant to bacterial degradation, they are quite toxic, and they are not very soluble in water. But they are lipid (fat) soluble, and animals take them up in their fat. PAHs concentrate in marine organisms when those organisms are almost pure fat and at their most vulnerable -- the egg and larval stages.

Polychlorinated biphenyls, or PCBs, and dioxin are long-lasting compounds that are solid in their pure state but easily soluble in fats, oils or solvents.

Polychlorinated biphenyls were widely used for insulation in electrical transformers and capacitors. Soils have been contaminated with PCBs in myriad spots around the Bay Area, and the compounds wash easily into rivers and estuaries. PCBs are a real concern because they're slow to break down.

Like PAHs, PCBs have been associated with decreased fertility in fish.

Dioxin is one of the most toxic compounds known, minute amounts can cause cancer and reproductive disorders. Dioxin is a byproduct of the manufacture of certain pesticides. It is also formed in combustion processes involving fuel that contains both chlorine and carbon. The burning of diesel fuel, coal or wood results in the formation of dioxin. In the past, much of the dioxin came from now-banned pesticides and industrial waste discharges. Today, most comes from diesel exhaust.

Mercury is a contaminant that is profoundly harmful to both human beings and wildlife. It's released from fuel combustion in cars and at refineries, cement kilns and power plants. It's in silver amalgam fillings, so it gets into the sewage system from dentists' offices. It's in fluorescent lights, camera and watch batteries and certain electrical switches.

DDT and dieldrin, which have been banned for decades, continue to pollute. Both compounds -- known as chlorinated hydrocarbon pesticides -- are extremely stable and take decades to degrade.

Organophosphates replaced the earlier pesticides, promoted as short-lived compounds that would break down quickly when exposed to sunlight and soil. But some of the organophosphates don't break down as quickly as promoted. Organophosphates kill insects by interfering with an enzyme that aids in the transmission of nerve impulses. Shrimp and numerous other marine animals are biologically similar to insects, so it affects them as well.

The problem is not simply PAHs or PCBs or dioxin, say scientists. It is probably all of them combined, each working in malign concert with the other.