Assessing dioxin precursors in pesticide formulations and environmental samples as a source of octachlorodibenzo-p-dioxin in soil and sediment.

An as yet unidentified origin of elevated concentrations of polychlorinated dibenzo-p-dioxins (PCDDs) in soil and sediment has repeatedly been described from different locations around the world, including Australia. Natural sources have been hypothesized to account for such contamination, which is characterized by a distinctive dioxin profile, in particular, elevated levels of octachlorodibenzo-p-dioxins (OCDD) as well as relatively low contributions of polychlorinated dibenzofurans (PCDFs). The present study investigated whether OCDD formation via anthropogenically derived precursors represents a possible source in such samples. Soil and sediment from Australia and Hawaii were screened for known pesticide derived dioxin precursors. Two pesticide formulations containing pentachlorophenol (PCP), which are well-known to contain predominantly OCDD impurities, were also analyzed. Polychlorinated phenoxyphenols (PCPPs), common byproducts of pesticide production, were detected at parts-per-billion (ppb) levels in two PCP formulations and in five environmental samples. Of particular interestwasthe presence of the PCPP isomer 3,4,5,6-tetrachloro-2-(2,3,4,5,6-pentachlorophenoxy)phenol (nonaC2PP), often also termed predioxin, in these samples. This compound readily undergoes ring closure to form OCDD under a range of conditions and environments. In addition, the pesticide PCP itself, which also represents a potent precursor to OCDD formation and is known to contain OCDD impurities, was detected in some environmental samples. The evidence from this study indicates that pesticides and their impurities play an important role in the dioxin contamination of Australian soils and sediments, as well as other locations with similar PCDD/F patterns. The results further suggest that formation of OCDD from pesticide derived precursors may be a possible past, present, and future pathway for contamination of environmental samples.