contaminants can have acute toxic effects on benthic organisms, or 

 accumulate slowly in the organisms until some toxic threshold is 

 reached. 



The toxicity to aquatic organisms is known for only a fraction 

 (<1%) of the approximately 50,000 compounds manufactured in the U.S. 

 (Martell et al . 1990). This situation is further complicated by the fact 

 that organisms usually are simultaneously exposed to a number of chemi- 

 cals (Giesy et al . 1990). The toxic responses associated with these 

 mixtures of compounds depends on their bioavailability--some contami- 

 nants are bound to sediment particles or otherwise unavailable to organ- 

 isms. For instance, the bioavailability of non-ionic organic compounds 

 depends on the total organic carbon content (TOC) of the sediment 

 (Nebeker et al . 1989, Swartz et al . 1990, Di Toro et al . 1991) and the 

 bioavailability of certain cationic metals depends on the acid-volatile 

 sulfide (AVS) content of the sediment (Di Toro et al . 1990, Ankley et 

 al. 1991, Carlson et al . 1991). Due to the complex mixtures of contami- 

 nants present in most toxic sediments, as well as the effects that 

 sediment matrices may have on the bioavailability of compounds, it has 

 been difficult to link specific compounds with toxicity. The tradition- 

 al approach to identifying toxic agents has been to correlate toxicity 

 with the concentrations of chemicals in the bulk sediment sample (Carr 

 et al . 1989). This approach does not work well with complex mixtures 

 and does not address the question of bioavailability. The dose response 

 curve for biological effects from certain chemicals is not correlated to 

 the bulk sediment concentration but rather to the porewater concentra- 

 tion (Di Toro et al . 1991). The recent development of Toxicity Identi- 

 fication and Evaluation (TIE) methodology has made it possible to iden- 



