122
7. Developing Site-Specific Water Quality Criteria
Justification
What is an Appropriate
Modification of a Criterion?
The main objective of the procedure given in this chapter is to modify a
criterion (by default, the USEPA national criterion) by an appropriate
amount. Appropriate, as used here, is the extent necessary to prevent
bioaccumulation from reaching levels that induce toxic effects in fish and
aquatic birds. A criterion that is appropriate will be both effective and
reasonable. That is, it will provide the needed level of protection to biota
throughout the HU without being overly conservative or too liberal. This
translates to a criterion which does not require that reference conditions
be achieved in every instance but also does not permit toxic impacts to
biota. For example, in situations when a criterion is not exceeded, and
diagnostic residues are present, but no effects are evident, the decision
tree indicates that no modification is necessary (Fig. 7.1). Conversely,
when diagnostic residues are present without effects but the criterion is
exceeded, no change is indicated. This, in practice, sets 2 major conditions for triggering criterion modification: (1) diagnostic residues and toxic
effects must be coupled (both must be present) in order to lower a criterion, and (2) diagnostic residues and toxic effects must be coupled (both
must be absent) in order to raise a criterion. Coupling residues and effects
makes the procedure sensitive to the natural inter- and intraspecific variation in bioaccumulation and toxic responses exhibited by fish and wildlife
in aquatic ecosystems.
An additional precaution is necessary if the scenario under evaluation
is one in which the criterion is not exceeded, and diagnostic residues are
present, but there are no observed toxic effects. In this case, the recommended decision is to monitor reproduction but maintain the criterion at
the current level. However, it is essential that all of the fish and aquatic
bird species that are characteristic of the site-that is, those that are typical for the habitat type and are commonly found in nearby reference
locations-be screened for possible use in the assessment of reproductive
effects. Selenium-sensitive species can experience reproductive failure
and be virtually eliminated while at the same time, some of the most
tolerant species remain present and only exhibit elevated tissue residues
(Lemly 1985a). This scenario could lead to the false impression that there
are no effects and thus no need to lower the criterion, when in fact,
selenium-sensitive species have been severely affected and perhaps locally extirpated. This type of error is also possible when the scenario is
one in which the criterion is exceeded but no diagnostic residues or toxic
effects are evident among the species that are present. It is possible that
sensitive species have been eliminated, and the tolerant species that re-
7. Developing Site-Specific Water Quality Criteria
Justification
What is an Appropriate
Modification of a Criterion?
The main objective of the procedure given in this chapter is to modify a
criterion (by default, the USEPA national criterion) by an appropriate
amount. Appropriate, as used here, is the extent necessary to prevent
bioaccumulation from reaching levels that induce toxic effects in fish and
aquatic birds. A criterion that is appropriate will be both effective and
reasonable. That is, it will provide the needed level of protection to biota
throughout the HU without being overly conservative or too liberal. This
translates to a criterion which does not require that reference conditions
be achieved in every instance but also does not permit toxic impacts to
biota. For example, in situations when a criterion is not exceeded, and
diagnostic residues are present, but no effects are evident, the decision
tree indicates that no modification is necessary (Fig. 7.1). Conversely,
when diagnostic residues are present without effects but the criterion is
exceeded, no change is indicated. This, in practice, sets 2 major conditions for triggering criterion modification: (1) diagnostic residues and toxic
effects must be coupled (both must be present) in order to lower a criterion, and (2) diagnostic residues and toxic effects must be coupled (both
must be absent) in order to raise a criterion. Coupling residues and effects
makes the procedure sensitive to the natural inter- and intraspecific variation in bioaccumulation and toxic responses exhibited by fish and wildlife
in aquatic ecosystems.
An additional precaution is necessary if the scenario under evaluation
is one in which the criterion is not exceeded, and diagnostic residues are
present, but there are no observed toxic effects. In this case, the recommended decision is to monitor reproduction but maintain the criterion at
the current level. However, it is essential that all of the fish and aquatic
bird species that are characteristic of the site-that is, those that are typical for the habitat type and are commonly found in nearby reference
locations-be screened for possible use in the assessment of reproductive
effects. Selenium-sensitive species can experience reproductive failure
and be virtually eliminated while at the same time, some of the most
tolerant species remain present and only exhibit elevated tissue residues
(Lemly 1985a). This scenario could lead to the false impression that there
are no effects and thus no need to lower the criterion, when in fact,
selenium-sensitive species have been severely affected and perhaps locally extirpated. This type of error is also possible when the scenario is
one in which the criterion is exceeded but no diagnostic residues or toxic
effects are evident among the species that are present. It is possible that
sensitive species have been eliminated, and the tolerant species that re-