| Abstract
| - Ball clay was the source of dioxin contaminationdiscovered in selected chickens analyzed as part of ajoint U.S. Department of Agriculture/U.S. EnvironmentalProtection Agency national survey of the U.S. poultry supplyconducted in 1997. The affected animals, which hadbeen raised in the southern United States, representedapproximately 5% of the national poultry production. All ofthese chickens and other animal food sources (i.e., farm-raised catfish), similarly contaminated, were fed a diet ofanimal feed containing ball clay as an anti-cakingadditive. The clay was mined in northwestern Mississippiwithin a geological formation referred to as the MississippiEmbayment. Individual raw and processed ball clay sampleswere analyzed for the presence of the 2,3,7,8-PCDDs/-PCDFs. The average toxic equivalents (TEQs) for the rawand processed samples were 1513 and 966 ppt dw,respectively. Other mined clay-based products used inanimal feeds revealed lower TEQs. All of the productsexhibited either an absence of detectable concentrationsof 2,3,7,8-PCDFs or concentrations 2−3 orders ofmagnitude lower than the PCDDs. The isomer distribution,specific isomer identification, and congener profile ofthe PCDDs in the clay were established and compared toknown sources of dioxin contamination. Several uniquefeatures of this isomer distribution are characteristic of theclays and are distinguishable from those of other knownsources. These characteristics found in prehistoricclay deposits in the United States and Germany have ledto speculation attributing their presence to natural geologicprocesses.
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