Conclusions
151
acids can produce bioaccumulation factors in excess of 350000 (Besser
et a1. 1993). This magnitude of bioaccumulation means that a waterborne concentration of only 0.1
SelL organic selenium, in the right
chemical form, is sufficient to elevate residues in food-chain organisms to levels that are toxic in the diet of fish and aquatic birds (5-15
Se/g).
The threat of selenium contamination from intensive feedlot operations becomes apparent when one considers the size and number of
these operations in combination with the magnitude of pollution events
that have occurred due to drainage or spills from manure pits. For
example, in the Coastal Plain region of eastern North Carolina, modern swine rearing farms house 5000 to 20 000 animals each. Hog production nearly tripled during the 5-year period from 1990 to 1995,
increasing from 5 million to 14 million animals. The large volume of
liquid manure produced is stored in multimillion-gallon lagoons until
it is removed and sprayed onto fields. Spills or overflows from these
sewage lagoons or pollution from runoff of manure from excessively
sprayed fields is a common occurrence (Schildgen 1996). In 1995, a
25-million gallon spill entered North Carolina's New River and killed
fish for 20 miles downstream. Hog waste was also implicated in massive pollution of the state's Neuse River, which was quarantined for a
35-mile stretch in 1995 (Schildgen 1996). Hog wastes also likely contributed to outbreaks of a toxic dinoflagellate (Pfiesteria piscidia) in the
Pamlico River Estuary (Glasgow et a1. 1995).
Concerns for aquatic impacts of livestock wastes are currently focused on nutrient enrichment, bacterial contamination, and depression of dissolved oxygen. There is little or no recognition of selenium
issues. Nevertheless, intensive livestock feeding operations generate a
large volume of seleniferous waste that has the potential to severely
impact nearby aquatic ecosystems as well as downstream habitats such
as wetlands and estuaries. This is an escalating threat that could affect
many areas of the United States. Two types of monitoring are needed:
first, to evaluate the acute or residual selenium contamination of aquatic
life in pollution events associated with feedlot operations; and second,
to determine selenium movement from fields sprayed with large
amounts of liquid manure.
Conclusions
The risk of widespread impacts from new selenium threats is greater
than ever before. Human activities that increase waterborne concentrations of selenium and provide conditions favorable for bioaccumulation are on the rise. Overshadowed by other concerns, selenium is
the "unknown contaminant" for some of these activities, yet it may
151
acids can produce bioaccumulation factors in excess of 350000 (Besser
et a1. 1993). This magnitude of bioaccumulation means that a waterborne concentration of only 0.1
SelL organic selenium, in the right
chemical form, is sufficient to elevate residues in food-chain organisms to levels that are toxic in the diet of fish and aquatic birds (5-15
Se/g).
The threat of selenium contamination from intensive feedlot operations becomes apparent when one considers the size and number of
these operations in combination with the magnitude of pollution events
that have occurred due to drainage or spills from manure pits. For
example, in the Coastal Plain region of eastern North Carolina, modern swine rearing farms house 5000 to 20 000 animals each. Hog production nearly tripled during the 5-year period from 1990 to 1995,
increasing from 5 million to 14 million animals. The large volume of
liquid manure produced is stored in multimillion-gallon lagoons until
it is removed and sprayed onto fields. Spills or overflows from these
sewage lagoons or pollution from runoff of manure from excessively
sprayed fields is a common occurrence (Schildgen 1996). In 1995, a
25-million gallon spill entered North Carolina's New River and killed
fish for 20 miles downstream. Hog waste was also implicated in massive pollution of the state's Neuse River, which was quarantined for a
35-mile stretch in 1995 (Schildgen 1996). Hog wastes also likely contributed to outbreaks of a toxic dinoflagellate (Pfiesteria piscidia) in the
Pamlico River Estuary (Glasgow et a1. 1995).
Concerns for aquatic impacts of livestock wastes are currently focused on nutrient enrichment, bacterial contamination, and depression of dissolved oxygen. There is little or no recognition of selenium
issues. Nevertheless, intensive livestock feeding operations generate a
large volume of seleniferous waste that has the potential to severely
impact nearby aquatic ecosystems as well as downstream habitats such
as wetlands and estuaries. This is an escalating threat that could affect
many areas of the United States. Two types of monitoring are needed:
first, to evaluate the acute or residual selenium contamination of aquatic
life in pollution events associated with feedlot operations; and second,
to determine selenium movement from fields sprayed with large
amounts of liquid manure.
Conclusions
The risk of widespread impacts from new selenium threats is greater
than ever before. Human activities that increase waterborne concentrations of selenium and provide conditions favorable for bioaccumulation are on the rise. Overshadowed by other concerns, selenium is
the "unknown contaminant" for some of these activities, yet it may