126
7. Developing Site-Specific Water Quality Criteria
aquatic habitats are threats to lentic systems (reservoirs, wetlands,
and off-channel bays and impoundments) and are threats from
power-plant discharges, agricultural irrigation, and other sources,
not from POTWs (see Chapters I and 9). The environmental dynamics of selenium in lentic ecosystems is quite different than the
riverine conditions used for the USEPA model.
(2) The 4-day average is based on organism responses to waterborne exposure alone. However, food-chain bioaccumulation and
dietary intake are more important in causing chronic selenium
toxicity to aquatic life (Lemly 1985a, 1997). This component of
selenium cycling is not addressed in the USEPA model. Moreover, exposure-bioaccumulation-response times for selenium in
fish and aquatic birds (whether exposure is due to waterborne
or dietary selenium intake) are on the order of weeks or months
rather than 4 days (eg, Lemly 1982; Heinz et al. 1988; Coyle et
al. 1993; Heinz and Fitzgerald 1993). The USEPA model assumption of 4 days is not correct.
(4)
(1984), Lemly (1985a), Gillespie and Baumann (1986), and
Hamilton et al. (1996) show that concentrations of 10 to 20 Ilg
SelL can quickly cause dietary levels that are toxic to fish and
aquatic birds. Consider, for example, a scenario in which an
exceedance causes waterborne selenium in a reservoir or wetland to reach 15 Ilg SelL, an acceptable concentration in the
USEPA model (Fig. 7.2). By the time ambient locations attain
this level, the entire "bioaccumulation engine" of the ecosystem
will have been fueled by the influx of new selenium, which
substantially escalates the toxic threat to aquatic life (Lemly
1985b).
The 3-year period between excursions (exceedances), although
perhaps reflecting the best scientific judgement available for
some pollutants in the early 1980s, is not appropriate for selenium
given present knowledge of the environmental dynamics and cycling of this trace element. Once an aquatic ecosystem has captured the selenium dose delivered by an exceedance, the ecosystem
can continue to cycle the selenium tightly within the components
of the system for many years. For example, studies show that the
recovery period for reservoirs contaminated by 10 Ilg SelL could
7. Developing Site-Specific Water Quality Criteria
aquatic habitats are threats to lentic systems (reservoirs, wetlands,
and off-channel bays and impoundments) and are threats from
power-plant discharges, agricultural irrigation, and other sources,
not from POTWs (see Chapters I and 9). The environmental dynamics of selenium in lentic ecosystems is quite different than the
riverine conditions used for the USEPA model.
(2) The 4-day average is based on organism responses to waterborne exposure alone. However, food-chain bioaccumulation and
dietary intake are more important in causing chronic selenium
toxicity to aquatic life (Lemly 1985a, 1997). This component of
selenium cycling is not addressed in the USEPA model. Moreover, exposure-bioaccumulation-response times for selenium in
fish and aquatic birds (whether exposure is due to waterborne
or dietary selenium intake) are on the order of weeks or months
rather than 4 days (eg, Lemly 1982; Heinz et al. 1988; Coyle et
al. 1993; Heinz and Fitzgerald 1993). The USEPA model assumption of 4 days is not correct.
(4)
(1984), Lemly (1985a), Gillespie and Baumann (1986), and
Hamilton et al. (1996) show that concentrations of 10 to 20 Ilg
SelL can quickly cause dietary levels that are toxic to fish and
aquatic birds. Consider, for example, a scenario in which an
exceedance causes waterborne selenium in a reservoir or wetland to reach 15 Ilg SelL, an acceptable concentration in the
USEPA model (Fig. 7.2). By the time ambient locations attain
this level, the entire "bioaccumulation engine" of the ecosystem
will have been fueled by the influx of new selenium, which
substantially escalates the toxic threat to aquatic life (Lemly
1985b).
The 3-year period between excursions (exceedances), although
perhaps reflecting the best scientific judgement available for
some pollutants in the early 1980s, is not appropriate for selenium
given present knowledge of the environmental dynamics and cycling of this trace element. Once an aquatic ecosystem has captured the selenium dose delivered by an exceedance, the ecosystem
can continue to cycle the selenium tightly within the components
of the system for many years. For example, studies show that the
recovery period for reservoirs contaminated by 10 Ilg SelL could