Long-Term Studies on the Persistence and Movement of TCDD in A Natural Ecosystem

Field studies of the persistence and movement of 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) were conducted during 1973- 1979 on a unique 3.0 km2 military test area (Test Area C-52A, Eglin Air Force Base, Florida) that was aerially sprayed with 73,000 kg 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) during the period 1962–1970. Analysis of archived samples of the formulations indicated that approximately 2.8 kg TCDD were applied as a contaminant of the herbicide. However, 2.6 kg of this TCDD were applied to a 37 ha test grid (Grid I) from June 1962 through July 1964. Levels of 10 to 1,500 parts-per-trillion (ppt) could be found in the top 15 cm of soil 14 years after the last application of herbicide on this site. Nevertheless, analysis of 61 soil samples suggested that less than one percent of the TCDD remained on the test area. Photodegradation at the time of and immediately after aerial application probably accounted for much of the disappearance of TCDD, although volatilization, wind and water erosion, and biological removal may have also contributed to its disappearance.