EPA's Vessel General Permit and Small Vessel General

traditionally requires applying data for the closest taxonomically related species with appropriate
data. EPA chose to analyze data for the most sensitive surrogate species rather than the closest
taxonomically related species. This decision provides an added measure of conservatism in the
risk analysis. EPA also assumed that listed species may be uniquely sensitive to pollutants
compared to surrogate species. For example, comparative analysis of the extreme chronic
sensitivity of the darter species, Etheostoma fonticola, and the generally-sensitive surrogate
species rainbow trout, to copper (Besser et al., 2005a) along with comparisons made in a number
of related studies by the same group of investigators, suggests that dividing water quality
guidelines by a factor of 2.0 may be a reasonable safety factor (e.g., Dwyer et al. 2005;
Gensemer et al. 2007; Mayer et al. 2008).
In addition to uncertainty associated with the use of effects data for standard lab species, there is
uncertainty associated with response to the net toxicity of vessel discharges as mixtures.
Organisms are simultaneously exposed to multiple pollutants in their natural environment, yet
risk assessment, as a science, still focuses on effects based on exposures to single pollutants.
Accordingly, the bulk of EPAs analysis evaluated toxicity on a single stressor basis. Mixture
toxicity is extremely complicated to assess due to the vast number of combinations of pollutants
that could be present in a mixture which differ in mechanism of action, biological availability,
and propensity to interact with coocurring toxicants or water quality characteristics which
influence toxicity and bioavailability. To address this uncertainty, EPA applied a concentration
addition approach, summing the RQs for pollutants with the same toxicological mechanism of
action: metals, which cause oxidative injury of biological tissues (Stohs and Bagchi 1995), and
type 1 narcotics, such as PAHs which induce stupor through altering nerve transmission
(USEPA, 2005c; McGrath and DiToro, 2009). In this analysis, EPA summed the RQs for watercolumn
only exposure based on a mixture of metals or PAHs to give a total RQ representative of
the risk of those mixtures to listed aquatic species.
EPA acknowledged issues with its use of NOECs (no observable effect concentration)/ LOECs
(lowest observable effect concentration)/MATCs (maximum acceptable toxicant concentration),
as sources of uncertainty that have the potential to influence the response analysis. EPA
described the shortcomings of NOECs/ LOECs/MATCs as follows: “Historically, most chronic
toxicity data were evaluated using hypothesis testing to derive a no observed effect concentration
(NOEC) and lowest observed effect concentration (LOEC) (and resultant maximum acceptable
toxicant concentration or MATC) from the test concentration series. The resulting values are
highly dependent on the test concentrations selected, and the number of replicates at each
treatment level. In many cases, the level of protection afforded by the NOEC-LOEC approach is
driven more by study design and data precision than ecological significance (see Mount et al.,
2003 for more discussion).” EPA did acknowledge that use of point estimates of chronic
toxicity, such as the EC10 or EC20, overcome many of the shortcomings of the NOEC-LOEC
approach, but the availability of point estimates associated with chronic effects is limited, and
debate continues with regards to the significance to the individual organism (and population) of
an estimated 10% reduction in chronic response, such as growth.
EPA also asserted that it was correct in not considering sublethal and nontraditional effects when
selecting its response thresholds. EPA stated that:
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