The "equilibrium partitioning theory" is one of the most widely used models to evaluate the bioavailability of sediment-associated, nonpolar, organic contaminants and it makes specific assumptions regarding the factors that influence this bioavailability. The objective of this research was to test two assumptions of this theory: (1) that benthic organisms are exposed to a constant, equilibrium-predicted concentration of a contaminant in interstitial water, regardless of the behavior of the organism; and (2) that exposure to interstitial water in a sediment exposure system is equivalent to the exposure in a water-only exposure system. The effect of behavior on the exposure to sediment-associated phenanthrene was tested by exposing three marine amphipod species (with different burrowing behaviors) to the polycyclic aromatic hydrocarbon (PAH) phenanthrene under two exposure conditions, one with spiked sediment and clean overlying water and the other with spiked sediment and contaminated overlying water. This was done to evaluate the extent to which the burrow irrigating behavior and the different tube or burrow building behavior exhibited by the amphipod species could effect the accumulation of sediment-associated phenanthrene. The assumption of equivalent exposure between sediment and water systems was tested by exposing the amphipods to the same concentration of phenanthrene in a water-only versus sediment exposure system. In both series of experiments, the bioaccumulation of phenanthrene by the amphipods was followed over 72 hours and bioaccumulation kinetics calculated for each species and exposure treatment. The results indicated that the burrow irrigating behavior of benthic marine amphipods can significantly affect the exposure of these amphipods to sediment-associated contaminants by diluting the concentration of contaminant in the interstitial water surrounding the organisms with overlying water. Additionally, there was a species dependent decrease in exposure based upon the tube or burrow building strategy used by the amphipod species. The results also indicated that exposure in a sediment system was not equivalent to exposure in a water-only system. The bioaccumulation of phenanthrene was significantly higher for all three species in water versus sediment. However, the interpretation of the results from this second series of experiments was complicated by the degradation of phenanthrene in the sediment-only exposure. / Graduation date: 1998