9.3. ENVIRONMENTAL FATE

Despite a growing body of literature from laboratory, field, and monitoring studies examining the environmental fate and environmental distribution of CDDs and CDFs, the fate of these environmentally ubiquitous compounds is not yet fully understood, and the following represents our best understanding, based on available data. In soil, sediment, the water column, and probably air, CDDs/CDFs are primarily associated with particulate and organic matter because of their high lipophilicity and low water solubility. They exhibit little potential for significant leaching or volatilization once sorbed to particulate matter. The available evidence indicates that CDDs and CDFs, particularly the tetra- and higher chlorinated congeners, are extremely stable compounds under most environmental conditions, with environmental persistence measured in decades. The only environmentally significant transformation process for these congeners is believed to be photodegradation of chemicals not bound to particles in the gaseous phase or at the soil- or water-air interface. Brominated congeners are significantly more readily transformed by photodegradation. CDDs/CDFs entering the atmosphere are removed either by photodegradation or by dry or wet deposition. Although some volatilization of dioxin-like compounds on soil does occur, the predominant fate of CDDs/CDFs sorbed to soil is to remain in place near the surface of undisturbed soil or to move to water bodies with erosion of soil. CDDs/CDFs entering the water column primarily undergo sedimentation and burial. The ultimate environmental sink of these CDDs/CDFs is believed to be aquatic sediments.