Fate & Transport of PCBs
PCBs can enter the environment in many ways. PCBs may be contained in direct discharges of industrial wastes to water, or they may be carried in landfill leachate eventually discharging to surface or ground water, or they may be emitted to the atmosphere, either by incinerators, or as recent studies show, they may also become airborne without burning. Once in the air, PCBs are readily adsorbed onto particles of dust where they may be widely dispersed. The exact fate and transport of PCBs is directly related to the specific structure of individual PCB compounds. In general, PCBs have a low solubility in water. Since they are heavier than water and do not readily dissolve, they are adsorbed onto particles of sediment. These particles are transported in suspension and eventually settle in the river bottom or floodplain sediments. The finer the sediment particles, the more easily PCBs are adsorbed.
Within river systems such as the Housatonic, PCBs are found in accumulated bottom and floodplain sediments. Concentration in sediments tends to be greater behind surface impoundments (dams) and in slow moving river sections. Storms, or other high flow events, may cause resuspension of the PCB laden sediment and result in sediment transport and deposition further downstream.
PCBs are very stable, or persistent, when released to the environment, and therefore, are slow to degrade. They persist because the organisms which degrade such wastes are unable to break the strong bond made between the chlorine atom and the biphenyl structure. PCBs are highly soluble in fat (lipids) and tend to accumulate in adipose, or fatty tissue and further tend to become more concentrated through the food chain. For example, PCB uptake by freshwater fish, either from direct water contact (through gills or skin) or through ingestion of PCB contaminated organisms, can bioaccumulate in tissue to many thousands of times the concentration of the PCBs the fish were exposed to in the water or food source. The birds and mammals which then feed on these fish take in greater amounts of PCB residues, becoming subject to their toxic effects.