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7. Developing Site-Specific Water Quality Criteria
from being too liberal in cases when it is appropriate to raise a criterion.
Increases are contingent upon finding low levels of bioaccumulation and
normal reproduction in selenium-sensitive species. Thus, modification is
always linked to a "no effect" range. Allowing a 50% increase in a criterion is reasonable if there is no evidence of bioaccumulation or reproductive impacts in the HU. Moreover, the feed-back loop provided by
monitoring will indicate if further revisions (upward or downward) should
be undertaken once a criterion is raised by this amount. This is also true
for criteria that are lowered. It is possible that more than one revision will
be necessary before an appropriate criterion is achieved.
Implementing Criteria
The USEPA provides states the option of developing site-specific criteria, and regulations governing their implementation, if adequate justification is given. The goal of the derivation process at the state and
local levels should be to establish criteria that are sensitive to selenium's
local variations in aquatic cycling and toxic effects. The purpose of this
chapter is to provide a procedure for deriving criteria at those levels.
However, it is also important to consider how criteria will be implemented and enforced. Will there be provisions for averaging concentrations over several days in order to meet criterion levels? Will periodic
exceedance of criteria be allowed? Will mixing zones be used to dilute
selenium inputs and achieve criterion concentrations? In many cases,
states may choose to follow the example set by the USEPA (eg, Stephan
et al. 1985; EPA 1985, 1987), which allows averaging, exceedances,
and mixing zones to factor into the implementation of criteria. However, the guidance provided by USEPA is generic and can greatly reduce the effectiveness of site-specific selenium criteria. It is essential
that selenium's profile of environmental cycling and toxicity be used
to formulate implementation policies at state and local levels. There
are several important precautions that should be taken by water quality regulators if they intend to rely on USEPA guidance documents to
implement site-specific selenium criteria.
USEPA guidelines allow the national criterion for chronic exposure
to be exceeded periodically (once every 3 years, on average) as long
as the 4-day average concentration is 5 Ilg SelL or less (EPA 1987).
During exceedances, the permissible ambient (ecosystem-wide) waterborne concentration can be as high as 20 Ilg SelL (Fig. 7.2). Stephan et
al. (1985) gives the rationale for this approach: "the averaging period
of 4 days was selected by the USEPA on the basis of data concerning
how rapidly some aquatic species react to increases in the concentrations of some pollutants, and 3 years is the Agency's best scientific
judgement of the average amount of time aquatic ecosystems should
7. Developing Site-Specific Water Quality Criteria
from being too liberal in cases when it is appropriate to raise a criterion.
Increases are contingent upon finding low levels of bioaccumulation and
normal reproduction in selenium-sensitive species. Thus, modification is
always linked to a "no effect" range. Allowing a 50% increase in a criterion is reasonable if there is no evidence of bioaccumulation or reproductive impacts in the HU. Moreover, the feed-back loop provided by
monitoring will indicate if further revisions (upward or downward) should
be undertaken once a criterion is raised by this amount. This is also true
for criteria that are lowered. It is possible that more than one revision will
be necessary before an appropriate criterion is achieved.
Implementing Criteria
The USEPA provides states the option of developing site-specific criteria, and regulations governing their implementation, if adequate justification is given. The goal of the derivation process at the state and
local levels should be to establish criteria that are sensitive to selenium's
local variations in aquatic cycling and toxic effects. The purpose of this
chapter is to provide a procedure for deriving criteria at those levels.
However, it is also important to consider how criteria will be implemented and enforced. Will there be provisions for averaging concentrations over several days in order to meet criterion levels? Will periodic
exceedance of criteria be allowed? Will mixing zones be used to dilute
selenium inputs and achieve criterion concentrations? In many cases,
states may choose to follow the example set by the USEPA (eg, Stephan
et al. 1985; EPA 1985, 1987), which allows averaging, exceedances,
and mixing zones to factor into the implementation of criteria. However, the guidance provided by USEPA is generic and can greatly reduce the effectiveness of site-specific selenium criteria. It is essential
that selenium's profile of environmental cycling and toxicity be used
to formulate implementation policies at state and local levels. There
are several important precautions that should be taken by water quality regulators if they intend to rely on USEPA guidance documents to
implement site-specific selenium criteria.
USEPA guidelines allow the national criterion for chronic exposure
to be exceeded periodically (once every 3 years, on average) as long
as the 4-day average concentration is 5 Ilg SelL or less (EPA 1987).
During exceedances, the permissible ambient (ecosystem-wide) waterborne concentration can be as high as 20 Ilg SelL (Fig. 7.2). Stephan et
al. (1985) gives the rationale for this approach: "the averaging period
of 4 days was selected by the USEPA on the basis of data concerning
how rapidly some aquatic species react to increases in the concentrations of some pollutants, and 3 years is the Agency's best scientific
judgement of the average amount of time aquatic ecosystems should