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General Policies
[Why
are WQS Important?] [WQS
Review and Revision] [Role
of the Public] [Designated
Uses] [Water
Quality Criteria] [Antidegradation]
[General Policies]
[Indian Tribe Participation]
[WQS Program History]
General Policies: States
and Tribes may adopt policies and provisions regarding water quality
standards implementation, such as mixing zone, variance, and low-flow
policies. Such policies are subject to EPA review and approval.
Mixing Zones: States and Tribes may, at their discretion,
allow mixing zones for point source discharges. A mixing
zone is a defined area surrounding or downstream of a point source
discharge where the effluent plume is progressively diluted by
the receiving water and numeric criteria otherwise applicable
to the segment may be exceeded. Mixing zone procedures describe
the methodology for determining the location, size, shape, and
in-zone quality of mixing zones.
Variances: As an alternative to removing a designated
use, a State or Tribe may wish to include a variance as part of
a water quality standard. Variances temporarily relax a
water quality standard. They are subject to public review
every three years, and may be extended upon expiration.
A variance may specify an interim water quality criterion which
is applicable for the duration of the variance. Variances
can help to assure that further progress toward improving water
quality is achieved.
Low Flows: State and Tribal water quality standards
may identify policies and procedures to be applied in determining
critical low flow conditions. Such procedures are applied,
for example, when calculating discharge permit requirements to
be included in National Pollutant Discharge Elimination System
(NPDES) permits.
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