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6. Hydrological Units and Selenium Criteria
Criteria Based on Hydrological Units
The physical area from which measurements are taken to evaluate
selenium residues and biological effects-that is, the database for setting site-specific criteria-must encompass more than a limited area
such as an isolated segment of river or a tributary stream. The
overarching principle for establishing environmentally sound waterquality criteria for selenium is that bioaccumulation needs to be kept
below levels that pose significant threats to biota. In order for this to be
accomplished, hazards to down-gradient aquatic habitats, most or all
of which may fall outside the immediate "site" under consideration,
must be assessed and taken into account.
Differences in selenium cycling among aquatic habitats is one of
the main reasons that USEPA national criteria are being questioned
more frequently. One size doesn't fit all. However, the reasoning that
individual sites need specific criteria can be taken too far. For example,
isolating a segment of a river and setting a criterion for it that will not
prevent harm to biota in the receiving waters downstream is not a
prudent approach. Because of hydrological connections between the
various aquatic habitats that may be present in a watershed basin (wetlands, rivers, streams, lakes, reservoirs, and other impoundments), the
risk of toxic impacts from selenium contamination is also connected.
The hydrologically connected parts of a basin down-gradient of a selenium discharge or other input (any natural or anthropogenic selenium
source), extending to the point at which new sources of low-selenium
water dominate the hydrology (eg, confluence with a larger tributary or
river, a spring, or groundwater inflow, resulting in concentrations that do
not exceed background levels for the area), should be the area evaluated
and given a specific criterion, not isolated components of that larger area
(Fig. 6.4). Thus, a Hydrological Unit (HU) should be identified and used as
the "site" for the purpose of setting criteria. A simple screening-level
assessment of waterborne selenium concentrations is all that is necessary
to delineate the boundaries of the HU.
A designated HU is a major departure from the traditional concept
of what constitutes a "site". Consequently, it may be useful to refer to
criteria derived with the HU approach as being watershed-specific or
basin-specific rather than site-specific. While still providing for focused
local refinement of USEPA national criteria, this method ensures an
integrated assessment of ecosystems within a watershed and allows
basinwide protection of aquatic life. Importantly, criteria derived in
such a manner will reflect the transport and cycling of selenium within
the entire HU rather than simply focusing on a small, artificially designated segment of the system-that is, a section of river, a tributary
stream, or a similar limited area.
6. Hydrological Units and Selenium Criteria
Criteria Based on Hydrological Units
The physical area from which measurements are taken to evaluate
selenium residues and biological effects-that is, the database for setting site-specific criteria-must encompass more than a limited area
such as an isolated segment of river or a tributary stream. The
overarching principle for establishing environmentally sound waterquality criteria for selenium is that bioaccumulation needs to be kept
below levels that pose significant threats to biota. In order for this to be
accomplished, hazards to down-gradient aquatic habitats, most or all
of which may fall outside the immediate "site" under consideration,
must be assessed and taken into account.
Differences in selenium cycling among aquatic habitats is one of
the main reasons that USEPA national criteria are being questioned
more frequently. One size doesn't fit all. However, the reasoning that
individual sites need specific criteria can be taken too far. For example,
isolating a segment of a river and setting a criterion for it that will not
prevent harm to biota in the receiving waters downstream is not a
prudent approach. Because of hydrological connections between the
various aquatic habitats that may be present in a watershed basin (wetlands, rivers, streams, lakes, reservoirs, and other impoundments), the
risk of toxic impacts from selenium contamination is also connected.
The hydrologically connected parts of a basin down-gradient of a selenium discharge or other input (any natural or anthropogenic selenium
source), extending to the point at which new sources of low-selenium
water dominate the hydrology (eg, confluence with a larger tributary or
river, a spring, or groundwater inflow, resulting in concentrations that do
not exceed background levels for the area), should be the area evaluated
and given a specific criterion, not isolated components of that larger area
(Fig. 6.4). Thus, a Hydrological Unit (HU) should be identified and used as
the "site" for the purpose of setting criteria. A simple screening-level
assessment of waterborne selenium concentrations is all that is necessary
to delineate the boundaries of the HU.
A designated HU is a major departure from the traditional concept
of what constitutes a "site". Consequently, it may be useful to refer to
criteria derived with the HU approach as being watershed-specific or
basin-specific rather than site-specific. While still providing for focused
local refinement of USEPA national criteria, this method ensures an
integrated assessment of ecosystems within a watershed and allows
basinwide protection of aquatic life. Importantly, criteria derived in
such a manner will reflect the transport and cycling of selenium within
the entire HU rather than simply focusing on a small, artificially designated segment of the system-that is, a section of river, a tributary
stream, or a similar limited area.