Document rBmB957R6LLRedZm4R9Vq667a

t ,\tOSr*, f.o S y po't0 UNITED STATES ENVIRONM ENTAL PROTECTION AGENCY W A S H IN G T O N , D.C. 20460 NOV I 8 1900 <> \ 0 ^ Cl/C U> OF FIC E OF S O L ID WASTE A N D E M E R G E N C Y RES f MEMORANDUM SUBJECT: Diox in Ri k Assessment From: Eileen Cl ussen, Director Character zation and Assessment Division To : Marcia E. Williams, Director Office of Solid Waste (r-' > L ^r ' * We've looked fairly closely at the dioxin risk assessment paper prepared by Don Barnes. In general, we feel that the paper does a good job of assessing where there is adequate uncertainty in the risk assessment procedure to invite flexibility in a risk management situation. However, it might be useful for Don to provide more discussion of several points that we believe are important. The area where there is the greatest room for movement is in the exposure assessment. This is really comprised of 2 areas that are important. The data suqgest that weathered, contaminated soil has a different level of bioavailability of dioxin than other soils. This, combined with changes in the appropriate fish consumption scenario (in other, words, better sampling of fish, etc.) may influence the final risk assessment for wastewaters applied to the land and discharged to rivers. (The bottom line here is perhaps 1 order of magnitude change.) Another pos sible avenue for change lies in examining what inatural," backg round levels of dioxins are in humans, and using this as a point of reference from which to compare specific sources of contamination. If we can move in these areas, and use them to change the specific exposure scenario that we ourselves used to arrive at the delisting levels, we may be an excellent position to be sensible.