120
7. Developing Site-Specific Water Quality Criteria
gree of bioaccumulation present in the HU (measured selenium residues), the known threshold concentrations for reproductive effects in
fish and wildlife (diagnostic residues for teratogenesis and
embryomortality), and any observed reproductive impacts in the HU
(terata or embryomortality).
A more restrictive (lowered) criterion is indicated when the existing or proposed criterion value for waterborne selenium is not exceeded,
but residues in the HU equal or exceed diagnostic toxic thresholds, and
reproductive effects are observed. The criterion value should be lowered by an amount that corresponds to the magnitude of bioaccumulation present. Three sets of conditions are possible, each signifying a
different level of toxic threat which, in turn, indicates a different amount
of revision to the criterion: (1) if residue values fall partly below the
threshold toxic ranges (see Diagnostic Residues, Table 7.1) and partly
within the threshold toxic ranges, then the criterion should be lowered by 25%; (2) if residue values fall entirely within the threshold
ranges or span the ranges such that some are below, some are within,
and some are above, then the criterion should be lowered by 50%; (3)
if all of the residue values exceed the upper end of the threshold ranges,
then the criterion should be lowered by 75 %. If these guidelines are
followed, criteria are lowered in each instance by an amount that is
proportional to the degree of biological hazard present.
No change in the criterion is necessary if: (1) the criterion is not
exceeded and no selenium problem is suspected; (2) the criterion is
not exceeded, a selenium problem is suspected (diagnostic residues
mayor may not be present), but there are no observed reproductive
effects; (3) the criterion is exceeded, and residues exceed or fall within
the toxic threshold ranges, or are above the ranges for uncontaminated aquatic systems (Table 7.1); or (4) the criterion is exceeded,
residues are within the ranges for uncontaminated systems, but downgradient impacts are possible in the HU (Fig. 7.1).
If the existing or proposed criterion is exceeded, yet residues fall
within the range of values for reference conditions in uncontaminated
systems (eg, background concentrations for the local area), which
would indicate that little bioaccumulation is taking place, then it may
be reasonable to consider raising the criterion. However, the appropriateness of raising a criterion is contingent upon firmly establishing
that there are no reproductive impacts to any of the species in the HU,
especially selenium-sensitive species such as centrarchid and salmonid fish, ducks, and wading birds. It is not sufficient to simply make an
observational assessment and assert that if fish and birds are plentiful,
then there must not be a selenium problem. In open aquatic systems,
fish and birds can move about freely, both into and out of the HU. This
can lead to the erroneous assumption that all of the observed individu-
7. Developing Site-Specific Water Quality Criteria
gree of bioaccumulation present in the HU (measured selenium residues), the known threshold concentrations for reproductive effects in
fish and wildlife (diagnostic residues for teratogenesis and
embryomortality), and any observed reproductive impacts in the HU
(terata or embryomortality).
A more restrictive (lowered) criterion is indicated when the existing or proposed criterion value for waterborne selenium is not exceeded,
but residues in the HU equal or exceed diagnostic toxic thresholds, and
reproductive effects are observed. The criterion value should be lowered by an amount that corresponds to the magnitude of bioaccumulation present. Three sets of conditions are possible, each signifying a
different level of toxic threat which, in turn, indicates a different amount
of revision to the criterion: (1) if residue values fall partly below the
threshold toxic ranges (see Diagnostic Residues, Table 7.1) and partly
within the threshold toxic ranges, then the criterion should be lowered by 25%; (2) if residue values fall entirely within the threshold
ranges or span the ranges such that some are below, some are within,
and some are above, then the criterion should be lowered by 50%; (3)
if all of the residue values exceed the upper end of the threshold ranges,
then the criterion should be lowered by 75 %. If these guidelines are
followed, criteria are lowered in each instance by an amount that is
proportional to the degree of biological hazard present.
No change in the criterion is necessary if: (1) the criterion is not
exceeded and no selenium problem is suspected; (2) the criterion is
not exceeded, a selenium problem is suspected (diagnostic residues
mayor may not be present), but there are no observed reproductive
effects; (3) the criterion is exceeded, and residues exceed or fall within
the toxic threshold ranges, or are above the ranges for uncontaminated aquatic systems (Table 7.1); or (4) the criterion is exceeded,
residues are within the ranges for uncontaminated systems, but downgradient impacts are possible in the HU (Fig. 7.1).
If the existing or proposed criterion is exceeded, yet residues fall
within the range of values for reference conditions in uncontaminated
systems (eg, background concentrations for the local area), which
would indicate that little bioaccumulation is taking place, then it may
be reasonable to consider raising the criterion. However, the appropriateness of raising a criterion is contingent upon firmly establishing
that there are no reproductive impacts to any of the species in the HU,
especially selenium-sensitive species such as centrarchid and salmonid fish, ducks, and wading birds. It is not sufficient to simply make an
observational assessment and assert that if fish and birds are plentiful,
then there must not be a selenium problem. In open aquatic systems,
fish and birds can move about freely, both into and out of the HU. This
can lead to the erroneous assumption that all of the observed individu-