Derivation of Soil Clean-Up Levels for 2,3,7,8-Tetrachloro-dibenzo-p-dioxin (TCDD) Toxicity Equivalence (TEQ_D/F) in Soil Through Deterministic and Probabilistic Risk Assessment of Exposure and Toxicity

ABSTRACT

While risk assessments are extensively used for guiding critical and resource intensive decisions, assessments that rigorously integrate key exposure and toxicity terms are less often published. This article derives residential soil clean-up levels accounting for ingestion and dermal contact (direct contact criteria [DCC]) for a chlorinated dibenzo-p-dioxin and furan (PCDD/F as toxicity equivalence TEQ_D/F) impacted site using site-specific information and deterministic and probabilistic methods. In addition, TEQ_D/F risk assessment has been the subject of extensive scientific and regulatory debate including in-depth comments from two USEPA Science Advisory Boards and the National Academy of Sciences (NAS) on the proposed USEPA Draft Dioxin Risk Assessment. This article presents and applies toxicity values seeking to address the NAS recommendations regarding cancer risk assessment. Deterministic DCC estimates ranged from 19 to 250 ppb through application of linear and nonlinear cancer toxicity values, and a DCC of 5.3 ppb was estimated based on the World Health Organization's Joint Exposure Committee on Food Additive's assessment value for noncancer and cancer endpoints. A wide range of DCC estimates were calculated using probabilistic methods, with the prior USEPA 1 ppb clean-up value falling below the first percentile of estimates, suggesting that the 1 ppb value is health protective.

Key Words:

• dioxin
• risk
• Monte Carlo
• toxicity values
• soil
• clean-up

ACKNOWLEDGMENTS

This work is dedicated to the late Thomas F. Long, a valued colleague and dear friend. The Dow Chemical Company funded our work. The authors gratefully acknowledge technical contributions from Dr. Edmund Crouch, Cambridge Environmental, Cambridge, Massachusetts. Dr. Crouch prepared aspects of the underlying calculations used here in support of the soil ingestion and exposure duration assumptions and provided technical input into numerous other aspects of this work as well as detailed comments on the manuscript.