EPA's Vessel General Permit and Small Vessel General

For the fraction of freshwater model, EPA identified two types of estuarine systems in
commonly found in the port areas:
� Estuaries where tidal influence is the primary driver for mixing in the estuary as indicated
by a lower river flow and higher salinity value; and
� Estuaries where the freshwater inflow is the primary driver for mixing in the estuary as
indicated by a higher river flow and lower salinity value.
For the river harbor model, the dilution equation requires the average annual river flow rate to
calculate a receiving water concentration. EPA selected the minimum and maximum river flow
rates from the nine river harbors to represent the bounding conditions for the river harbors,
resulting in two river harbor scenarios for the BE effects analysis.
A range of water body flow, salinity, vessel populations, and loading assumptions were applied
to the model to explore different “likely” and “worst-case” scenarios pollutant exposure
concentrations resulting from vessel discharges covered under the VGP and sVGP. Vessel
population scenarios were developed for each RAA based on data provided by the RAA port
authorities, available Department of Transportation port call data, and other vessel related
literature to characterize the average daily vessel equivalents present in the RAA by vessel class.
Vessel discharges were expressed as vessel equivalents where a vessel equivalent represents a
single vessel discharging 24-hours per day in the harbor. For example, 1 vessel equivalent could
represent 24 vessels that each operates (and discharges) for 1 hour in the modeled harbor each
day. EPA also collected receiving water characteristic data from a variety of estuaries and rivers
where vessels covered under VGP and sVGP are known to travel.
EPA identified 32 pollutants to include in the modeling analysis based on pre-permit frequency,
vessel discharge flow rates and pollutant concentrations as described in Discharges Authorized
by the VGP and sVGP. EPA then estimated reductions in vessel discharge pollutant
concentrations and flow rates anticipated to occur as the result of issuance of the VGP and sVGP
for the pollutants, discharges and vessel types evaluated. Harbor specific pollutant loads were
estimated for each of the seven RAAs by multiplying vessel and discharge specific pollutant
loads by the number of vessel equivalents present in the RAA harbors. EPA used this
information to model 14 scenarios addressing harbor specific pollutant loads for 7 RAAs and
receiving water characteristics representing minimum and maximum dilution conditions for
estuary and river ports (Table 25).
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