Toxic Thresholds
21
by other selenium sources that a common number can be given as the
threshold for detrimental effects. I recommend that waterborne selenium concentrations of 2 flg SelL or greater (total recoverable basis in
0.45 flm filtered samples) be considered highly hazardous to the health
and long-term survival of sensitive fish and aquatic birds. This threat
is due to food-chain bioaccumulation and resultant dietary exposure
and reproductive effects, not direct waterborne toxicity. Some species
will be relatively unaffected at the 2 flg SelL level, but sensitive species, many of which are the most important in terms of ecological
integrity and public recreational value, can be seriously impacted. It
should also be recognized that, under certain environmental conditions, waterborne concentrations of 1 flg SelL or less have the potential
to bioaccumulate to levels in the food chain that are toxic to predatory
species.
Sediments
In order to identify an accurate and reliable threshold for hazardous
amounts of selenium in sediments, it is necessary to have long-term
measurements of selenium concentrations and associated detrital foodchain bioaccumulation across a gradient from little or no accumulation/effects to detrimental levels. One of the most useful data sets for
this purpose is from Belews Lake, NC. Various investigators monitored
environmental conditions and selenium concentrations in water, sediments, and associated biota from the precontamination stage (19701973) through the time of selenium input (1974-1986) and for a 10-year
period of natural recovery after selenium inputs stopped (1987-1996)
(Cumbie 1978; Cumbie and Van Horn 1978; Weiss and Anderson 1978;
Holland 1979; Lemly 1985a, 1993, 1997).
Selenium in sediments typically exhibits wide spatial and temporal
variability, and benthic organisms also undergo seasonal cycles of
growth, movement, emergence, and recolonization. These factors make
it difficult to sort out the variability and come up with a reliable toxic
threshold. The extensive data set from Belews Lake makes it possible
to reduce the variability and link a narrow range of selenium concentrations in sediments to a narrow range of concentrations in benthic
macroinvertebrates. The threshold value given here is the approximate selenium concentration in sediments that elevates residues in
benthos to levels that are toxic to fish and aquatic birds that eat them.
Benthic organisms can tolerate body burdens of selenium far greater
than the dietary toxic level for fish (3 J.1g Se/g dry weight) and aquatic
birds (7 flg Se/g dry weight) without suffering ill effects (see next section of this chapter). Thus, the most important aspect of selenium residues in sediments is not direct toxicity to benthic organisms themselves,
21
by other selenium sources that a common number can be given as the
threshold for detrimental effects. I recommend that waterborne selenium concentrations of 2 flg SelL or greater (total recoverable basis in
0.45 flm filtered samples) be considered highly hazardous to the health
and long-term survival of sensitive fish and aquatic birds. This threat
is due to food-chain bioaccumulation and resultant dietary exposure
and reproductive effects, not direct waterborne toxicity. Some species
will be relatively unaffected at the 2 flg SelL level, but sensitive species, many of which are the most important in terms of ecological
integrity and public recreational value, can be seriously impacted. It
should also be recognized that, under certain environmental conditions, waterborne concentrations of 1 flg SelL or less have the potential
to bioaccumulate to levels in the food chain that are toxic to predatory
species.
Sediments
In order to identify an accurate and reliable threshold for hazardous
amounts of selenium in sediments, it is necessary to have long-term
measurements of selenium concentrations and associated detrital foodchain bioaccumulation across a gradient from little or no accumulation/effects to detrimental levels. One of the most useful data sets for
this purpose is from Belews Lake, NC. Various investigators monitored
environmental conditions and selenium concentrations in water, sediments, and associated biota from the precontamination stage (19701973) through the time of selenium input (1974-1986) and for a 10-year
period of natural recovery after selenium inputs stopped (1987-1996)
(Cumbie 1978; Cumbie and Van Horn 1978; Weiss and Anderson 1978;
Holland 1979; Lemly 1985a, 1993, 1997).
Selenium in sediments typically exhibits wide spatial and temporal
variability, and benthic organisms also undergo seasonal cycles of
growth, movement, emergence, and recolonization. These factors make
it difficult to sort out the variability and come up with a reliable toxic
threshold. The extensive data set from Belews Lake makes it possible
to reduce the variability and link a narrow range of selenium concentrations in sediments to a narrow range of concentrations in benthic
macroinvertebrates. The threshold value given here is the approximate selenium concentration in sediments that elevates residues in
benthos to levels that are toxic to fish and aquatic birds that eat them.
Benthic organisms can tolerate body burdens of selenium far greater
than the dietary toxic level for fish (3 J.1g Se/g dry weight) and aquatic
birds (7 flg Se/g dry weight) without suffering ill effects (see next section of this chapter). Thus, the most important aspect of selenium residues in sediments is not direct toxicity to benthic organisms themselves,