Bioavailability coming of age? For many soil contaminants there is ample evidence that only a portion of the total contamination is available to living organisms. This observation holds true for some organic contaminants and for some metals. For example only a part of aged phenanthrene in soil is assimilated by earthworms;1 in some soils only a little of the 2,3,7,8-tetrachlorodibenzo-p-dioxin is found to affect rabbits that are fed the compound;2 and adsorption varies between different lead compounds.3 one thing applying the concept to the remediation of contaminated sites is another. One of the most pressing questions is how to measure the bioavailable fraction rapidly and economically? Animal toxicity testing might be seen as an obvious solution but such tests have many limitations according to Alexander.Testing on organisms takes a long time and the precision is not great enough to provide a clear answer. In addition such testing can be prohibitively expensive he said. As a result the search is on for chemical tests that could estimate the bioavailability of contaminants in soils. Such evidence has been accumulating for many years. Over three decades ago Nash and Woolson reported that for the Ærst ten years after it was applied DDT (1,1,1-trichloro-2,2-bis[p-chlorophenyl]- ethane) disappeared slowly and steadily. But after that time little or none of the compound was lost.4 Similar curves each with an initial phase of loss followed by a period of little or no detectable loss have been reported for a variety of other chlorinated hydrocarbons.5 Opinion is divided over how soon it might be possible to account for bioavailability.The factors that control the bioavailability of metals are much better understood than those that affect organic contaminants according to Alexander. In fact for lead whose bioavailability is best understood many researchers believe that a relatively cheap rapid chemical test for determining bioavailability will be available soon. The method involves a buffered simulated stomach solution. Soil added to the solution is rotated end-over-end for a Æxed time at a Æxed temperature. It is then Æltered and analyzed. For some heavy organic contaminants ``The science is now very clear''; according to Martin Alexander an environmental toxicologist at Cornell University in Ithaca New York who has studied bioavailability for over ten years.``There are major changes in bioavailability which occur as soils age. This applies to polycyclic aromatic hydrocarbons (PAHs) and other classes of organic compounds.'' Acceptance of the idea that some contaminants become sequestered in soil over time so that they are unavailable to living organisms is growing according to Raymond Loehr a civil engineer at the University of Texas Austin who formerly chaired EPA's Science Advisory Board. Initially bioavailability was the darling of industries faced with contaminant liability. It has become an accepted phenomenon among those involved in remediation as a result of accumulating evidence that it occurs.In addition the shift to risk-based cleanup standards that explicitly ask when is a site clean enough has turned the spotlight on bioavailability in soils. Risk-based cleanup regulations that are designed to incorporate bioavailability have been adopted by some states including Washington and EPA has created an internal working group to develop guidance on the subject. But recognizing that contaminant sequestration is a real phenomenon is such as PAHs and wood preservatives and some soils measures of bioavailability are also in the pipeline. Loehr's group is trying to correlate soil characteristics with the results of soil extraction experiments.Alexander agrees that for some organic compounds for example DDT and some PAHs and for some soils there is a correlation between the extraction tests and bioavailability. In these cases the fraction of contaminants that is easily extracted is a good approximation of the bioavailable fraction.6 However it is also known that bioavailability is species dependent. Bacteria for example excrete a substance that can mobilize contaminants and earthworms can access an even greater proportion than bacteria according to Alexander. ``We don't know the mechanisms and we don't have enough information to predict which organisms access the most,'' he said. But it is clear that these differences in bioavailability between 38N J.Environ. Monit. 2000 2 This journal is # The Royal Society of Chemistry 2000 US Focus species are real differences that are not related to acclimation or adaptation. To explain the observation that contaminants undergo an initial phase of steady loss followed by a period of little or no detectable change researchers envision that the processes of chemical photochemical and biological degradation which are responsible for these losses stop. They stop researchers believe because the contaminant molecules diffuse into the soil matrix where some of them become sequestered in remote pores. Researchers are now using ultramicroscopic techniques to study the molecular-scale locations where organic compounds accumulate.Bioavailability has the potential to reduce uncertainty in cleanups and to make cost savings. But regulators are also cautious. Many other factors must be considered if reduced bioavailability is to be a factor in determining soil cleanups according to EPA ofÆcials. These include past and future uses of the site the organism that can access the greatest amount of contaminant and the pathway by which this exposure occurs. With clear evidence that factors such as soil composition and site history can signiÆcantly reduce or modify bioavailability there is now great pressure to deliver the tools that could reduce the enormous expense of site cleanups without compromising public or environmental health. But many scientists involved in this research say that it is important to proceed slowly. For many questions about bioavailability the answers are still a long way off. Notes 1 J. W. Kelsey and M. Alexander Environ. 2 A Bonaccorsi et al. Arch. Toxicol. Suppl. 3 D. Barltrop and F. Meek Postgrad. Med. 4 R. G. Nash and E. A. Woolson Science 5 M. Alexander Environ. Sci. Technol. 6 J. Tang and M. Alexander Environ. Toxicol. Chem. 1997 16 582. 1984 7 431. J. 1975 51 805. 1967 157 924. 1995 29 2713. Toxicol. Chem. 1999 18 2711. Rebecca Renner Science Writer and Editor Tel z1 570 321 8640 Fax z1 570 321 9028 E-mail applepie@sunlink.net