MIL-HDBK-1138
2.4.1.1
Waste Load Allocation. Most NPDES permits include
limits on oxygen demanding substances (such as CBOD and ammonia).
Development of these limits is typically based on a waste load
allocation for the receiving stream. Stream modeling is used to
assess the assimilative capacity of the stream based on the
applicable dissolved oxygen standard. This capacity is then
allocated among all the dischargers in the area. Generally, some
portion of the stream's capacity is reserved for future
dischargers.
Waste load allocation modeling typically consists of a
desk-top effort for small discharges and a calibrated and
verified model based on field measurements for larger discharges.
Modeling can be performed by the discharger or by the state
agency. Regardless of who performs the modeling, the results
receive a detailed review by both the state and the EPA.
Typically, these results are put out for public comment. In many
cases, the public comment period is concurrent with the public
notice for the NPDES permit.
2.4.1.2
Chemical-Specific Criteria. Water quality-based
effluent limits can be based on chemical-specific criteria from
the water quality standards (such as for metals or toxics) or on
general narrative criteria. Specific criteria are used in the
development of effluent limits, and in many cases an allowance
for dilution in the receiving stream is provided. Typically,
some portion of the 7Q10 low-flow for the receiving stream is
used for dilution purposes. (7Q10 is a hydrogeological
determination of the lowest average flow over 7 consecutive days
with an average recurrence frequency of once in 10 years.)
Background concentrations in the receiving stream must also be
considered in the dilution calculations. Where the 7Q10 low-flow
is zero, the criteria will apply at the point of discharge, prior
to any dilution.
Aquatic Life Criteria. For aquatic life criteria,
2.4.1.3
acute or chronic values apply. The application of acute versus
chronic criteria is dependent on a number of items, including the
use classification and the available dilution in the receiving
stream. (Generally, if the available dilution is greater than
100 to 1, then the acute criteria apply.)
2.4.1.4
General Narrative Criteria.
An example of a
general narrative criteria follows:
Toxic substances shall not be present in receiving waters,
after mixing, in such quantities as to be toxic to human,
animal, plant or aquatic life or to interfere with the
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