EPA's Vessel General Permit and Small Vessel General

concentration calculated for the hypothetical river harbor. Given this assumption, EPA believes
that the calculated river EC values and subsequent risk conclusions for freshwater species
adequately capture the potential “worst case” scenario pollutant concentrations and risks in a
lake environment.
Services Assessment of Uncertainty in the Exposure Analysis
In order to determine if EPA’s decision process in evaluating its action has insured against
jeopardizing listed species, the Services must evaluate EPA’s acknowledged uncertainties and
how they were addressed by EPA along with additional uncertainties identified by the Services.
The Service took EPA’s acknowledged uncertainties into consideration while identifying
discharges and stressors that posed the greatest risk to listed species. Additional monitoring
agreed to by EPA is expected to provide mechanisms for identifying unacceptable risks and
implementing corrective measures.
Perhaps the greatest source of uncertainty in the effects analysis lies in EPA’s ability to predict
exposure to pollutants. EPA identified three major uncertainties in their exposure analysis,
including: model pollutant load inputs reflect the data limitations in available vessel discharge
data (discharge flow, discharge concentrations, and vessel populations); model inputs assume no
background concentrations of pollutants are present in the receiving water; and the model is not
designed to evaluate localized impacts or stressors.
Uncertainties regarding EPA’s estimation of pollutant loadings from vessel discharges arise
primarily for the limited amount of data for discharges, i.e., discharge volumes and
concentrations of pollutants in those discharges for different discharge/vessel types. In terms of
pollutant concentrations, where sample data were available EPA based their estimates on median
values (EPA’s report to Congress). Predictions based on medians may underestimate the full
breath of potential exposures. Data collected as part of the monitoring requirements proposed in
VGP and sVGP should improve the quality of information used for EPA’s future analyses.
Models used to predict concentrations of pollutants in water assumed no background
concentrations in the receiving water. While these predictions may capture pollutant
concentrations attributable to vessel discharges, they likely underestimate actual exposures
because other sources of pollution are not considered. In addition, the models assume complete
mixing throughout the harbor and are not designed to evaluate localized impacts or stressors.
Organisms that reside in close proximity to vessels may be exposed to higher pollutant
concentrations. Risks associated with these acute exposures will be dependent on the overlap
between species and vessel locations, proximity to discharges, and duration of exposure. Sessile
species will likely be most vulnerable to these types of exposures. Because of the scope of the
VGP and sVGP, EPA evaluated exposure using Reference Action Area (RAA) harbors. The
RAA harbors are expected to represent areas where the greatest numbers of vessels are located.
EPA modeled several different harbor scenarios and based their risk assessment on the maximum
predicted pollutant concentration (among all harbor scenarios). The RAA harbor approach and
use of maximum predicted pollutant concentrations should help balance the uncertainties
mentioned above and reduces the likelihood that pollutant exposures used in the effects analysis
are underestimated.
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