Justification
123
main exhibit very low bioaccumulation, which may partially explain their
ability to tolerate selenium. This could lead to the false assumption that
increasing the criterion value is appropriate. Thus, in certain situations
the biota that "should be present" must be considered, particularly if they
represent known selenium sensitive species, in addition to those that are
actually present (see previous section, Step 2, for additional guidance in
this process). Comparing the fauna and selenium residues of adjacent
HUs is one way to evaluate such a situation and gain additional insight
into the need to raise a criterion.
Why Use Percentages and Ranges?
Percentages are used because they represent the simplest practical way
of modifying criteria while also addressing the main objective from
both an ecological and a regulatory perspective. The decision tree specifies that selenium concentrations and effects should be monitored in
each case when criterion modification is indicated (Fig. 7.1). It will
quickly become apparent if a criterion derived using the percentage
method is too conservative or too liberal. In the decision tree, the
monitoring requirements function as feed-back loops that provide an
on-going field test of the appropriateness of the criterion. Monitoring
of residues and biological effects in the HU is an essential part of the
validation process for site-specific criteria.
There is a very narrow range covering typical reference waterborne
concentrations of selenium in aquatic systems (0.1-0.3 J..lg SelL), concentrations that can cause bioaccumulation and toxicity in fish and
wildlife (1-3 J..lg SelL), and the current national and state criteria for
selenium (2-5 J..lg SelL). Moreover, the values for site-specific criteria
that are likely to be developed by states and approved by USEPA probably fall within the range of 1 to 20 J..lg SelL. Within this narrow range,
the most straightforward way to address necessary changes in environmental selenium residues is by using percentages (25%, 50%, 75%)
that are linked to different degrees of biological effects. This technique
is appropriate because the magnitude of teratogenic deformity andlor
embryomortality, and resultant ecosystem-level impacts to fish and
aquatic birds are distinctly greater in cases when a 75 % reduction in a
criterion is indicated (all residues are greater than the upper end of
the toxicity threshold ranges) than when a 25% reduction is indicated
(some residues are below and some within the threshold ranges).
The use of ranges for threshold values, rather than a single number,
makes the criterion-modification process sensitive to natural variation in
bioaccumulation and toxic responses of biota. It prevents the process from
being overly conservative in cases when a lowered criterion is indicated.
Using reference values and reproductive assessment prevents the process
123
main exhibit very low bioaccumulation, which may partially explain their
ability to tolerate selenium. This could lead to the false assumption that
increasing the criterion value is appropriate. Thus, in certain situations
the biota that "should be present" must be considered, particularly if they
represent known selenium sensitive species, in addition to those that are
actually present (see previous section, Step 2, for additional guidance in
this process). Comparing the fauna and selenium residues of adjacent
HUs is one way to evaluate such a situation and gain additional insight
into the need to raise a criterion.
Why Use Percentages and Ranges?
Percentages are used because they represent the simplest practical way
of modifying criteria while also addressing the main objective from
both an ecological and a regulatory perspective. The decision tree specifies that selenium concentrations and effects should be monitored in
each case when criterion modification is indicated (Fig. 7.1). It will
quickly become apparent if a criterion derived using the percentage
method is too conservative or too liberal. In the decision tree, the
monitoring requirements function as feed-back loops that provide an
on-going field test of the appropriateness of the criterion. Monitoring
of residues and biological effects in the HU is an essential part of the
validation process for site-specific criteria.
There is a very narrow range covering typical reference waterborne
concentrations of selenium in aquatic systems (0.1-0.3 J..lg SelL), concentrations that can cause bioaccumulation and toxicity in fish and
wildlife (1-3 J..lg SelL), and the current national and state criteria for
selenium (2-5 J..lg SelL). Moreover, the values for site-specific criteria
that are likely to be developed by states and approved by USEPA probably fall within the range of 1 to 20 J..lg SelL. Within this narrow range,
the most straightforward way to address necessary changes in environmental selenium residues is by using percentages (25%, 50%, 75%)
that are linked to different degrees of biological effects. This technique
is appropriate because the magnitude of teratogenic deformity andlor
embryomortality, and resultant ecosystem-level impacts to fish and
aquatic birds are distinctly greater in cases when a 75 % reduction in a
criterion is indicated (all residues are greater than the upper end of
the toxicity threshold ranges) than when a 25% reduction is indicated
(some residues are below and some within the threshold ranges).
The use of ranges for threshold values, rather than a single number,
makes the criterion-modification process sensitive to natural variation in
bioaccumulation and toxic responses of biota. It prevents the process from
being overly conservative in cases when a lowered criterion is indicated.
Using reference values and reproductive assessment prevents the process