BOARD01494

• Board Meeting Date: 9/21/1998
• Description: Federal "Clean Water Action Plan" and EPA's Advance Notice of Proposed Rule Making
• Board Meetings - Doc Type: Memo

· &EPA

Fact Sheet

Mixing Zones - 40 CFR 131.13

A mixing zone is an area where an effluent discharge undergoes initial dilution and is extended
to cover the secondary mixing in the ambient water body. A mixing zone is an allocated impact
zone where water quality criteria can be exceeded as long as acutely toxic conditions are
prevented (USEPA Water Quality Standards Handbook, 1994). EPA regulations at 40 CFR
131.13 provide that, .. States may, at their discretion, include in their State standards, policies
generally affecting their application and implementation, such as mixing zones, low flows and
variances. Such policies are subject to EPA reviewand approval."

.

Other definitions of mixing zones have been offered, differing primarily by perspective (i.e.,
engineering, hydrological, ecological, regulatoty) and their application. From a
hydrological/engineering perspective, mixing zones can be defined based upon the recognition of
incomplete mixing of an effluent with its receiving water (e.g., "that area or volume of dilution
water necessary to reduce contaminant concentrations to some acceptable level or to a totally
mixed condition"). Biologically, mixing zones can be defined based on the premise that surface
water quality criteria can be exceeded under limited circumstances without causing unacceptable
toxicity or, more broadly, impairment of the designated beneficial uses (e.g., "the area contiguous
to a discharge where receiving water quality is not required to meet water quality criteria nor
other requirements applicable to the receiving water").

EPA's current policy addresses mixing zones as allocated impact zones (AIZs) where certain
numeric water quality criteria may be exceeded as long as:

. there is no lethality to organisms passing through the mixing zone,
. there are no significant risks to human health, and
. the designated and existing uses of the water body are not impaired as a result.

These AIZs or mixing zones, if disproportionately large, could unacceptably impact the integrity
of the aquatic ecosystem and have unanticipated ecological consequences on the water body as a
whole resulting in impairment of the designated or existing uses. Therefore, EP A's policy has
emphasized a holistic approach to mixing zone regulation which considers location, size, shape,
outfall design and in-zone quality. Mixing zone guidance produced by EP A since 1972 has
consistently emphasized the need to protect both nonmotile benthic and sessile organisms in the
mixing zone as well as swimming and drifting organisms 0N ater Quality Criteria 1972). As
originally designed, EP A's mixing zone policy provided for the prevention of lethality to
. swimming and drifting organisms by limiting the size of the mixing zone and to nonmotile