104
6. Hydrological Units and Selenium Criteria
mechanism of toxicity also makes it imperative to closely scrutinize
the pattern of bioaccumulation that is taking place. For example, deposition of dietary selenium in eggs can cause reproductive failure in
fish even though there is little or no outward evidence of toxicity to
the spawning fish themselves (Lemly 1985a, 1985b). Therefore, factors contributing to food-chain transfer of selenium to adult fish need
to be identified and accounted for as water quality criteria are being
developed.
The degree of mobility (inter-habitat transport), transformation (from
inorganic selenium to organic selenium and vice versa), and
bioaccumulation of selenium all influence the toxic threat to biota
and, consequently, the need for site-specfic criteria. This chapter presents a brief overview of the selenium cycle and discusses why site-tosite differences in bioaccumulation and threats to down-gradient aquatic
habitats favor the use of a Hydrological Unit (HU) approach for deriving water quality criteria.
The Selenium Cycle
Three things can happen to dissolved selenium when it enters an aquatic
ecosystem: (1) it can be absorbed or ingested by organisms; (2) it can
bind or complex with particulate matter or surficial sediments; or (3) it
can remain free in solution. Over time, most of the selenium is either
taken up by organisms or bound to particulate matter (Fig. 6.1). Through
deposition of biologically incorporated selenium and settling of particulate matter (sedimentation), most of the selenium usually accumulates in the top layer of sediment and detritus. However, because
biological, chemical, and physical processes move selenium out of, as
well as into, sediments, this top layer is only a temporary repository
for selenium. Aquatic systems are dynamic, and selenium can be cycled
back into the biota and remain at elevated levels for years after waterborne inputs of selenium are stopped (Lemly 1997).
Immobilization Processes
Selenium can be removed from solution and sequestered in sediments
through the natural processes of chemical and microbial reduction of
the selenate form (Se VI) to the selenite form (Se IV), followed by
adsorption (binding and complexation) onto clay and the organic carbon phase of particulates, reaction with iron species, and coprecipitation
or settling (Fig. 6.2). Regardless of the route, once selenium is in the
sediments, further chemical and microbial reduction may occur resulting in insoluble organic, mineral, elemental, or adsorbed selenium.
Most selenium in animal and plant tissues is eventually deposited as
6. Hydrological Units and Selenium Criteria
mechanism of toxicity also makes it imperative to closely scrutinize
the pattern of bioaccumulation that is taking place. For example, deposition of dietary selenium in eggs can cause reproductive failure in
fish even though there is little or no outward evidence of toxicity to
the spawning fish themselves (Lemly 1985a, 1985b). Therefore, factors contributing to food-chain transfer of selenium to adult fish need
to be identified and accounted for as water quality criteria are being
developed.
The degree of mobility (inter-habitat transport), transformation (from
inorganic selenium to organic selenium and vice versa), and
bioaccumulation of selenium all influence the toxic threat to biota
and, consequently, the need for site-specfic criteria. This chapter presents a brief overview of the selenium cycle and discusses why site-tosite differences in bioaccumulation and threats to down-gradient aquatic
habitats favor the use of a Hydrological Unit (HU) approach for deriving water quality criteria.
The Selenium Cycle
Three things can happen to dissolved selenium when it enters an aquatic
ecosystem: (1) it can be absorbed or ingested by organisms; (2) it can
bind or complex with particulate matter or surficial sediments; or (3) it
can remain free in solution. Over time, most of the selenium is either
taken up by organisms or bound to particulate matter (Fig. 6.1). Through
deposition of biologically incorporated selenium and settling of particulate matter (sedimentation), most of the selenium usually accumulates in the top layer of sediment and detritus. However, because
biological, chemical, and physical processes move selenium out of, as
well as into, sediments, this top layer is only a temporary repository
for selenium. Aquatic systems are dynamic, and selenium can be cycled
back into the biota and remain at elevated levels for years after waterborne inputs of selenium are stopped (Lemly 1997).
Immobilization Processes
Selenium can be removed from solution and sequestered in sediments
through the natural processes of chemical and microbial reduction of
the selenate form (Se VI) to the selenite form (Se IV), followed by
adsorption (binding and complexation) onto clay and the organic carbon phase of particulates, reaction with iron species, and coprecipitation
or settling (Fig. 6.2). Regardless of the route, once selenium is in the
sediments, further chemical and microbial reduction may occur resulting in insoluble organic, mineral, elemental, or adsorbed selenium.
Most selenium in animal and plant tissues is eventually deposited as