148
9. Emerging Selenium Contamination Issues
conveyed to Kesterson National Wildlife Refuge, where it caused death
and deformities in thousands of migratory waterfowl and shorebirds
(Lemly et al. 1993). A variety of treatment options for removing selenium from subsurface irrigation drainage and reducing hazards to fish
and wildlife in downstream waters were examined. One method, tested
experimentally in the late 1980s and promoted during the 1990s, is the
use of constructed wetlands. In addition to treating irrigation drainage, this "phytoremediation" approach has also been advocated as a
means to remove selenium from oil refinery effluents (Terry and Zayed
1998). However, it is important to recognize that serious ecological
risks may accompany this treatment technology.
The major objective of treatment wetlands is to remove materials
that could threaten the health and biological integrity of down-gradient receiving waters. If that goal is achieved, ecological benefits result. However, if the wastewater being treated contains selenium, the
apparent benefits to downstream water quality can be more than offset by toxic hazards created within the wetlands because of bioaccumulation. Moreover, wetlands constitute attractive habitat for fish and
wildlife, which are then likely to be exposed to hazardous levels of
selenium. The end result can be a net loss of benefits and creation of
an ecological liability that did not previously exist. Treatment wetlands may thus create selenium problems rather than solve them. That
result did take place at the Chevron USA Oil Refinery in Richmond,
California in the mid -1990s. A 40 ha constructed wetland, intended to
provide "water enhancement" by removing conventional pollutants
(biological oxygen demand, total organic carbon, total suspended solids, ammonia, etc.), was also effective in removing selenium from the
waste stream. This was initially viewed as an unanticipated net benefit. The habitat feature the wetland provided attracted large numbers
of migratory waterfowl and shorebirds, which was also promoted as a
benefit of the wetland. However, bioaccumulation caused selenium
levels to exceed toxic thresholds for wildlife, and waterbirds were poisoned. In 1995, selenium concentrations in birds were "high enough
to reduce hatchability of eggs and may cause some reduction of posthatch survival among chicks" (Ohlendorf and Gala 2000). That finding
prompted the implementation of an alternative management plan designed to reduce wildlife exposure by manipulating vegetation and
water levels in areas of highest selenium concentrations so as to make
them less attractive to birds. After several years under this new management plan, selenium levels in bird eggs had decreased but still
exceeded thresholds for reproductive toxicity (Chevron 2000). This
example illustrates the difficulty of meeting water-quality treatment
objectives without creating toxic hazards to wildlife. In the United States,
there are also important legal issues associated with the creation of
9. Emerging Selenium Contamination Issues
conveyed to Kesterson National Wildlife Refuge, where it caused death
and deformities in thousands of migratory waterfowl and shorebirds
(Lemly et al. 1993). A variety of treatment options for removing selenium from subsurface irrigation drainage and reducing hazards to fish
and wildlife in downstream waters were examined. One method, tested
experimentally in the late 1980s and promoted during the 1990s, is the
use of constructed wetlands. In addition to treating irrigation drainage, this "phytoremediation" approach has also been advocated as a
means to remove selenium from oil refinery effluents (Terry and Zayed
1998). However, it is important to recognize that serious ecological
risks may accompany this treatment technology.
The major objective of treatment wetlands is to remove materials
that could threaten the health and biological integrity of down-gradient receiving waters. If that goal is achieved, ecological benefits result. However, if the wastewater being treated contains selenium, the
apparent benefits to downstream water quality can be more than offset by toxic hazards created within the wetlands because of bioaccumulation. Moreover, wetlands constitute attractive habitat for fish and
wildlife, which are then likely to be exposed to hazardous levels of
selenium. The end result can be a net loss of benefits and creation of
an ecological liability that did not previously exist. Treatment wetlands may thus create selenium problems rather than solve them. That
result did take place at the Chevron USA Oil Refinery in Richmond,
California in the mid -1990s. A 40 ha constructed wetland, intended to
provide "water enhancement" by removing conventional pollutants
(biological oxygen demand, total organic carbon, total suspended solids, ammonia, etc.), was also effective in removing selenium from the
waste stream. This was initially viewed as an unanticipated net benefit. The habitat feature the wetland provided attracted large numbers
of migratory waterfowl and shorebirds, which was also promoted as a
benefit of the wetland. However, bioaccumulation caused selenium
levels to exceed toxic thresholds for wildlife, and waterbirds were poisoned. In 1995, selenium concentrations in birds were "high enough
to reduce hatchability of eggs and may cause some reduction of posthatch survival among chicks" (Ohlendorf and Gala 2000). That finding
prompted the implementation of an alternative management plan designed to reduce wildlife exposure by manipulating vegetation and
water levels in areas of highest selenium concentrations so as to make
them less attractive to birds. After several years under this new management plan, selenium levels in bird eggs had decreased but still
exceeded thresholds for reproductive toxicity (Chevron 2000). This
example illustrates the difficulty of meeting water-quality treatment
objectives without creating toxic hazards to wildlife. In the United States,
there are also important legal issues associated with the creation of