Procedure
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the RC include bioaccumulation, detrital retention, physical and chemical sequestration, and recycling within the RU. The more that selenium
is held within a RU-through incorporation into biota, deposition in sediments, etc.,-the higher the RC. It is necessary to know the RC in order
to develop an environmentally sound TMDL because the higher the RC,
the lower the TMDL has to be to prevent toxic threats to fish and wildlife.
To a large extent, the RC depends on the degree of bioaccumulation
and internal recycling in the RU, which is reflected in: (1) the primary
productivity, (2) the water flow regime, and (3) the sediment type.
From the characterization of RU done in Step 1, information on these
3 factors should be available for each type of aquatic habitat. Use the
matrix in Table 8.1 to assign each of these factors a separate RC rating;
low, medium, or high. A rating should be done for each distinct habitat in the RU, that is, for each main-stem river, off-channel bay, stream,
wetland, lake, reservoir, and other impoundment.
An overall RC rating for each habitat is determined by combining
the 3 factor ratings as follows:
3 low ratings =low RC
2 low and 1 medium =low RC
2 low and 1 high =medium RC
2 medium and 1 low =medium RC
2 medium and 1 high = medium RC
3 medium ratings =medium RC
2 high and 1 low =medium RC
2 high and 1 medium =high RC
3 high ratings = high RC
The final RC rating for the RU should be set equal to the highest individual habitat rating. For example, if there are two habitats with low RC
and one with medium RC, the final RC rating for the RU is medium.
Step 5: Calculate the Total
Allowable Selenium Load
Begin by plugging the hazard rating from Step 2, and the RC from Step
4, into the matrix in Table 8.2. The table indicates the appropriate
amount of load reduction needed: small, medium, or large. The total
existing selenium load should be reduced by 10% if the amount designated is small, 25% if it is medium, and 50% if it is large. Subtract the
indicated amount from the total existing selenium loading rate (kg!
139
the RC include bioaccumulation, detrital retention, physical and chemical sequestration, and recycling within the RU. The more that selenium
is held within a RU-through incorporation into biota, deposition in sediments, etc.,-the higher the RC. It is necessary to know the RC in order
to develop an environmentally sound TMDL because the higher the RC,
the lower the TMDL has to be to prevent toxic threats to fish and wildlife.
To a large extent, the RC depends on the degree of bioaccumulation
and internal recycling in the RU, which is reflected in: (1) the primary
productivity, (2) the water flow regime, and (3) the sediment type.
From the characterization of RU done in Step 1, information on these
3 factors should be available for each type of aquatic habitat. Use the
matrix in Table 8.1 to assign each of these factors a separate RC rating;
low, medium, or high. A rating should be done for each distinct habitat in the RU, that is, for each main-stem river, off-channel bay, stream,
wetland, lake, reservoir, and other impoundment.
An overall RC rating for each habitat is determined by combining
the 3 factor ratings as follows:
3 low ratings =low RC
2 low and 1 medium =low RC
2 low and 1 high =medium RC
2 medium and 1 low =medium RC
2 medium and 1 high = medium RC
3 medium ratings =medium RC
2 high and 1 low =medium RC
2 high and 1 medium =high RC
3 high ratings = high RC
The final RC rating for the RU should be set equal to the highest individual habitat rating. For example, if there are two habitats with low RC
and one with medium RC, the final RC rating for the RU is medium.
Step 5: Calculate the Total
Allowable Selenium Load
Begin by plugging the hazard rating from Step 2, and the RC from Step
4, into the matrix in Table 8.2. The table indicates the appropriate
amount of load reduction needed: small, medium, or large. The total
existing selenium load should be reduced by 10% if the amount designated is small, 25% if it is medium, and 50% if it is large. Subtract the
indicated amount from the total existing selenium loading rate (kg!