EPA's Vessel General Permit and Small Vessel General

PAH and MAH
Because MAHs and PAHs are Type I narcotic compounds, the toxicity of these compounds in
water (and sediments) is most easily assessed using what is called the target lipid model or TLM
(DiToro and McGrath, 2000). Under the TLM, acute and chronic toxicity of MAHs and PAHs
to aquatic animals is predicted as single pollutants and as mixtures by calculating the critical
body burden in the target lipid of an organism. EPA used the critical body burden of
benzo(a)pyene and benzene in lipid to develop its response thresholds against which modeled
environmental concentrations would be compared. Under EPAs estuarine harbor model, average
post permit PAH and MAH concentrations were unchanged for all analytes but benzene, which
was reduced between 17 and 53 percent under worst case and best case conditions, respectively.
For the river harbor model, average post permit PAH and MAH concentrations were unchanged
for all but benzene, which was reduced between 6 and 9 percent, depending on scenario.
The TLM guidelines proposed by McGrath and DiToro (2009) are equivalent to a HC5, which is
the hazard concentration considered to protect 95% of the tested species. Thresholds for
individual analytes ranged from 0.286 ug/L for bezo(a)pyrene to 2,433 for benzene ug/L.
Average RQs calculated by EPA for individual analytes in modeled estuarine/marine harbors
ranged from