Mississippi Administrative Code › Title 11 - Mississippi Department of Environmental Quality › Part 6 - Wastewater Pollution Control Regulations › Chapter 1 - Mississippi Commission on Environmental Quality National Pollutant Discharge Elimination System (NPDES) Permits, Underground Injection Control (UIC) Permits, State Permits, Water Quality Based Effluent Limitations and Water Quality Certification › Subchapter 2 - Water Quality Based Effluent Limitations › Exhibit 11-6-1-2-E to Chapter 1, Subchapters 1 and 2 - ANTIDEGRADATION IMPLEMENTATION METHODOLOGY

Current through June 25, 2024
I. Introduction
The Mississippi Department of Environmental Quality (MDEQ)
					 State of Mississippi Water Quality Criteria for Intrastate, Interstate, and
					 Coastal Waters (WPC-2) provides:
"Antidegradation: The policy inherent in the standards
					 shall be to protect water quality existing at the time these water quality
					 standards were adopted and to upgrade or enhance water quality within the State
					 of Mississippi. Waters whose existing quality is better than the established
					 standards will be maintained at high quality unless the Commission finds, after
					 full satisfaction of the intergovernmental coordination and public
					 participation provisions of the State's continuing planning process, that
					 allowing lower water quality is necessary to accommodate important economic or
					 social development in the area in which the waters are located. In no event,
					 however, may degradation of water quality interfere with or become injurious to
					 existing instream water uses. Further, in no case will water quality be
					 degraded below (or above) the base levels set forth in these standards for the
					 protection of the beneficial uses described herein. In addition, the State will
					 assure that there shall be achieved the highest statutory and regulatory
					 requirements for all new and existing point sources and all cost-effective and
					 reasonable best management practices for nonpoint source control. Where the
					 Commission determines that high quality waters constitute an Outstanding
					 National Resource, such as waters of National and State Parks and Wildlife
					 Refuges and waters of exceptional recreational or ecological significance, that
					 water quality shall be maintained and protected. For the purposes of this rule,
					 existing uses are defined as those uses actually attained in the water body on
					 or after November 28, 1975, whether or not they are included in the Water
					 Quality Criteria."
Further, the federal antidegradation policy ( 40 CFR
					 Section 131.12(a)(2) ) provides:
"Where the quality of the waters exceed levels necessary to
					 support propagation of fish, shellfish, and wildlife, and recreation in and on
					 the water, that quality shall be maintained and protected, unless the State
					 finds, after full satisfaction of the intergovernmental coordination and public
					 participation provisions of the State's continuing planning process, that
					 allowing lower water quality is necessary to accommodate important economic or
					 social development in the area in which the waters are located."
Each state or tribe is required to identify methods for
					 implementation of its antidegradation policy ( 40 CFR Section 131.12(a) ):
"The State shall develop and adopt a statewide
					 antidegradation policy and identify the methods for implementing such policy
					 pursuant to this subpart."
On June 25, 2008, the State of Mississippi's
					 antidegradation policy was determined by the United States Environmental
					 Protection Agency, Region 4, to be consistent with the requirements of federal
					 regulations.
II. Water Body
				  Tiers
Antidegradation is generally considered in a tiered
					 approach. Tier 1 waters are those waters in which the existing water quality
					 does not support designated uses. Tier 2 waters are those waters in which the
					 water quality meets or exceeds the mandatory minimum levels to support the
					 Clean Water Act (CWA) goal of propagation of fish, shellfish, and wildlife, and
					 recreation in and on such waters. Tier 3 waters are those high quality waters
					 that constitute Outstanding National Resource Waters (ONRWs). MDEQ will decide
					 which tier applies for State waters based on a review of applicable information
					 as described below. All existing uses must be maintained and protected in all
					 waters of the State regardless of whether they are considered a Tier 1, 2, or 3
					 water.
All waters in Mississippi are considered to be Tier 2
					 waters unless one of the following conditions is met:
(1) the water is designated as an Outstanding
				  National Resource Water (ONRW), in which case, it is a Tier 3 water;
(2) the water is identified on Mississippi's
				  current §303(d) list, in which case it is defined as a Tier 1 water with
				  respect to the pollutant(s) causing impairment; or
(3) the water has been subject to an
				  established final Total Maximum Daily Load (TMDL), in which case it is defined
				  as a Tier 1 water with respect to the pollutant(s) addressed by the TMDL.
Tier 1 Waters
Tier 1 waters are those water body segments that are known
					 to be impaired by a pollutant. Such waters have an established as final TMDL or
					 are listed in the State's most recently adopted §303(d) list. For Tier 1
					 waters, the antidegradation policy is implemented through the State's NPDES
					 Permit Issuance Process. New or expanding discharges are not allowed in Section
					 303(d) listed waters if there is an increase in pollutants proposed for which
					 the water is listed, unless there is a TMDL developed that can be attained due
					 to available assimilative capacity within the wasteload allocation as part of
					 the TMDL. Tier 1 water bodies are pollutant specific, and this designation does
					 not relieve a permit applicant from the requirements of an antidegradation
					 report for this or other "non-listed" pollutants as required proposed to be
					 discharged.
Tier 2 Waters
Tier 2 waters are those water bodies that:
(1) have been determined to have assimilative
				  capacity based on assessment of water quality data and/or water quality
				  modeling tools; or
(2) are assumed
				  to have assimilative capacity because there are insufficient data or
				  information to conclude that there is no assimilative capacity.
An antidegradation report is required for all proposed new
					 or expanding discharges into Tier 2 waters. The level of detail involved in an
					 antidegradation review will generally be dependent upon the State's judgment of
					 the potential impact on water quality from a proposed activity considering
					 factors such as the type of activity (e.g., covered by a general or individual
					 permit) and magnitude of the discharge (e.g., major or minor).
Tier 3 Waters
When the Commission determines that high quality waters
					 constitute an Outstanding National Resource, such as waters of National and
					 State Parks and Wildlife Refuges and waters of exceptional recreational or
					 ecological significance, the water quality of such waters shall be maintained
					 and protected by adopting Tier 3 designation. Tier 3 waters are considered
					 Outstanding National Resource Waters (ONRW). In order to achieve this
					 designation, the stream must be nominated as a Tier 3 ONRW and adopted by the
					 Mississippi Commission on Environmental Quality (MCEQ). Any person may submit a
					 nomination to the Commission which will include the rationale and documentation
					 citing the historical, recreational or ecological significance of the water
					 body. The submission must include documentation as shown in Exhibit G. Upon
					 receipt of the nomination, the Commission staff will review the water body,
					 assess any available data or information to determine the impairment status,
					 identify the current NPDES permits in the watershed, and make a recommendation
					 to the Commission for consideration. If adopted as a Tier 3 water, the water
					 body will not be allowed to experience any further permanent
					 degradation.