2,3,7,8-dibenzo-p-dioxins in mined clay products from the United States: Evidence for possible natural origin

Ball clay was the source of dioxin contamination discovered in selected chi ckens analyzed as part of a joint U.S. Department of Agriculture/U.S. Envir onmental Protection Agency national survey of the U.S. poultry supply condu cted in 1997. The affected animals, which had been raised in the southern U nited States, represented approximately 5% of the national poultry producti on. All of these chickens and other animal food sources (i.e., farm-raised catfish), similarly contaminated, were fed a diet of animal feed containing ball clay as an anti-caking additive. The clay was mined in northwestern M ississippi within a geological formation referred to as the Mississippi Emb ayment. Individual raw and processed ball clay samples were analyzed for th e presence of the 2,3,7,8-PCDDs/ -PCDFs. The average toxic equivalents (TEQ s) for the raw and processed samples were 1513 and 966 ppt dw, respectively . Other mined clay-based products used in animal feeds revealed lower TEQs. All of the products exhibited either an absence of detectable concentratio ns of 2,3,7,8-PCDFs or concentrations 2-3 orders of magnitude lower than th e PCDDs. The isomer distribution, specific isomer identification, and conge ner profile of the PCDDs in the clay were established and compared to known sources of dioxin contamination. Several unique features of this isomer di stribution are characteristic of the clays and are distinguishable from tho se of other known sources. These characteristics found in prehistoric clay deposits in the United States and Germany have led to speculation attributi ng their presence to natural geologic processes.