Narcosis

The model uses a regression, based on the relationship between concentrations of

PAHs in interstitial water and toxicity to estuarine amphipods in spiked sediment

bioassays, to determine toxic units for PAHs. Toxic units for each compound,

which are equal to the concentration in the interstitial water of the contaminated

sediment divided by the interstitial water 10-d LC50, are summed. The sum of the

toxic units is used to predict the probability of significant acute sediment toxicity

to marine and estuarine amphipods, where significant mortality was defined as

>24 percent mean mortality in field-collected sediments. The model was verified

by comparison to mortality observed in sediment toxicity tests in field-collected

samples from 13 investigations. The Sum-PAH (Σ-PAH) level of acute toxicity

(Sum of Toxic Units = 3.291, p of >24% mortality = 1.0) was determined as the

toxic-unit concentration above which acute toxicity is always expected to occur.

The Σ-PAH threshold of acute toxicity (Sum of Toxic units = 0.186, p of >24%

mortality = 0.05) is the toxic unit concentration below which mixtures of PAHs

are unlikely to contribute to sediment toxicity above background. The 50 percent

probability of acute toxicity (sum of toxic units = 0.725, p>0.050) is the

concentration expected to cause acute toxicity in 50 percent of the cases.

Narcosis

Narcosis due to organic contaminants in aquatic organisms is defined as a

nonspecific reversible disturbance in the functioning of the membrane, caused by

the accumulation of contaminants in the hydrophobic (lipid) phases of the

organism (van Wezel and Opperhuizen 1995). Experimental work has

demonstrated that the critical body residue (CBR) for the acute lethal effect of

nonpolar narcotic chemicals is fairly constant at 2 to 8 mmol chemical/kg wet wt

tissue (McCarty et al. 1992). In addition, the effects of mixtures of nonpolar

chemicals that act by a narcotic mode of action appear to be generally additive

(McCarty and Mackay 1993).

Appendix B Food Chain and Toxicity Models