EPA's Vessel General Permit and Small Vessel General

Impacts on a local scale potentially occurring in stagnant side embayments, coves within the
larger water bodies, or occurring as a result of large volume discharges are not addressed in
EPAs assessment because screening models assume instantaneous and universal mixing within
the modeled harbor. EPA concluded that by using model parameters to bound in the analysis
and reflect a “worst case” scenario (e.g., minimum river flow from the nine rivers considered in
the analysis), the screening-level models appropriately characterize the range of potential
exposure concentrations and subsequent pollutant risk.
EPA believes that the analysis also represents vessel discharges to intracoastal waterways
because several of the major water bodies (e.g., Elizabeth River, Charleston Harbor, Biscayne
Bay) that are part of the Atlantic and Gulf intracoastal waterway were considered in developing
the receiving characteristics for the estuary model. Even though a significant amount of vessel
traffic travels these waters on a daily basis, the daily vessel equivalent population is assumed to
be significantly lower in these waters than in a harbor environment. EPA acknowledged that
listed species present in intracoastal waterways may be directly exposed to a vessel discharge
plume, but they also stated that the exposure duration is expected to be brief such that the chronic
effects thresholds considered in the BE are not of concern.
Although the environmental concentrations calculated for the effects analysis do not specifically
account for localized effects, EPA considered the potential risk posed by vessel discharges in
other unique environments and determined vessel discharges are unlikely a concern. For
example, in the case of localized changes in buffering capacity due to climate change, exhaust
gas scrubber discharges have the greatest potential to depress pH values in receiving waters as
the discharge can have pH values as low as 3(USEPA 2011b). EPA originally relied on its
economic and benefit analysis for the 2013 VGP and concluded that that a maximum of 5 to 10
percent of affected vessels would install this treatment by the end of the permit period. EPA has
since revised this estimate to reflect far fewer vessels plying this technology. EPA assumed that
the concurrence of site-specific changes in buffering capacity with a significant population of
vessels discharging exhaust gas scrubber water is highly unlikely to occur. Similar to the
assessment of intracoastal waterways, EPA assumed that any localized effects caused by direct
exposure to an exhaust gas scrubber discharge plume would be brief such that risk effects to
listed species considered in the BE are not of concern. However, the discharge volume and
concentrations of selenium associated with this discharge was identified as a major contributor to
existing selenium issues in San Francisco.
Lakes and reservoirs isolated from the major rivers navigable by vessels covered under the VGP
and sVGP represent another unique environment that was not specifically modeled in EPA’s
analysis. EPA believes that the maximum exposure concentrations estimated in the hypothetical
river harbor can be used to inform effects determinations for species identified in the lake
environment. EPA determined that vessel populations operating on lakes and reservoirs are
significantly smaller than vessel populations operating in river harbors, and that the vessels also
operate (and discharge) for limited periods of time. Therefore, EPA concluded that the permitted
vessel equivalents in lake environments are very likely to be significantly lower than the vessel
equivalents estimated in the two river RAAs included in the effects analysis, which would result
in lower estimated pollutant concentrations in a lake environment than the maximum exposure
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