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MEMORANDUM

SUBJECT: Tier 2 Antidegradation Reviews and Significance Thresholds 

FROM: Ephraim S. King, Director 

Office of Science and Technology 

TO: Water Management Division Directors, Regions 1-10 

I would like to share with you OST=s current recommendation regarding significance 

thresholds and lowering of water quality in high quality waters in the context of tier 2 

antidegradation reviews. This memorandum is intended to provide the Regions with technical 

recommendations for your consideration as you work with states and authorized tribes and as 

you review antidegradation implementation methods that adopt significance thresholds. Within 

this context, EPA will make decisions on a state=s or tribe=s antidegradation policy on a case-bycase basis, guided by the applicable requirements of the Clean Water Act and implementing 

regulations, and taking into account best available information. 

Antidegradation is an integral part of a state=s or tribe=s water quality standards, as it 

provides important protections that are critical to the fulfillment of the Clean Water Act 

objective Ato restore and maintain the chemical, physical, and biological integrity of the Nation=s 

waters.@ Of the three tiers of antidegradation protection, perhaps the most detailed in terms of 

implementation is tier 2, or high quality water protection. The intent of tier 2 protection is to 

maintain and protect high quality waters and not to allow for any degradation beyond a de 

minimis level without having made a demonstration, with opportunity for public input, that such 

a lowering is necessary and important. The available assimilative capacity of a waterbody - the 

difference between the applicable water quality criterion for a pollutant parameter and the 

ambient water quality for that pollutant parameter where it is better than the criterion - is a 

valuable natural resource. EPA=s regulations provide for public participation in decisions 

regarding whether a lowering of water quality is necessary (i.e., there are no alternatives to 

allowing a new or increased discharge that will lower water quality) to accommodate important 

development (i.e., the activity causing the lowering will provide for important economic or 

social development in the area in which the waters are located). See 40 CFR 131.12(a)(2). 
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We recognize that some states and tribes have chosen to target their antidegradation 

efforts by defining a significance threshold above which the effects on water quality require tier 

2 antidegradation findings of necessity and social and economic importance. Applying 

antidegradation review requirements only to those activities that may result in significant 

degradation of water quality is a useful approach that allows states and tribes to focus their 

resources where they may result in the greatest environmental protection. However, it is 

important that states and tribes set their significance thresholds at a level that can be 

demonstrated to be consistent with the purpose of tier 2 antidegradation requirements. 

Otherwise, a new or increased discharge may result in significant degradation that will not be 

subject to antidegradation review, and decisions about the lowering of water quality in high 

quality waters may be made without public consideration of necessity and importance, resulting 

in the loss or diminishment of a valuable natural resource. 

EPA has afforded the states and tribes some discretion in determining what constitutes a 

significant lowering of water quality. EPA has accepted a range of approaches to defining a 

Asignificance threshold@ over which a full antidegradation review is required. This issue was 

considered at length in the process of developing the Water Quality Guidance for the Great 

Lakes. Relying upon input offered during a four-year open public process involving 

environmental groups, industry representatives, and other experts, with numerous opportunities 

for public input, the directors of the eight Great Lakes states and EPA technical experts reached 

a consensus on a significance threshold value of ten percent (10%) of the available assimilative 

capacity, coupled with a cumulative cap. They determined that this threshold represented a 

reasonable balance between the need of the regulatory agencies to limit the number of actions 

involving non-BCCs (bioaccumulative chemicals of concern) that are subjected to the detailed 

antidegradation demonstration requirements, and the need to protect and maintain water quality. 

 They believed that any individual decision to lower water quality for non-BCCs that is limited 

to 10% of the available assimilative capacity represents minimal risk to the receiving water and 

is fully consistent with the objectives and goals of the Clean Water Act. A ten percent (10%) 

value is within the range of values for significance thresholds that EPA has approved in other 

states as well. EPA considers this approach to be workable and protective in identifying those 

significant lowerings of water quality that should receive a full tier 2 antidegradation review, 

including public participation. 

Given the different approaches states and tribes have taken recently to define 

significance, it is important to clarify that the most appropriate way to define a significance 

threshold is in terms of assimilative capacity. Other approaches for defining significance, such 

as considering only increases in pollutant loading, may not take into account the resulting 

changes in water quality, and in some cases may allow most or all of the remaining assimilative 

capacity of a waterbody to be used without an antidegradation review. Evaluations of 

significance based solely on the magnitude of the proposed increase without reference to the 

amount of change in the ambient condition of the waterbody need to be very carefully evaluated 

to determine how they translate to reduction in assimilative capacity in order to understand 

whether a significant decrease in assimilative capacity will occur. This analysis can be 

technically difficult when applied to all possible waterbody types and flow situations, thus 
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making justifications of these expressions by states and tribes and approvals by EPA more 

challenging. Further, given the importance of public participation and transparency, it is clear 

that a definition of significance that directly links to the resource to be protected (assimilative 

capacity) is more likely to be understood by the public. Therefore, OST strongly recommends 

that new or revised submissions of antidegradation implementation procedures to EPA that 

define Asignificant@ lowering of water quality define significance in terms of assimilative 

capacity, unless the state or tribe demonstrates that another approach is equally or more 

protective of the state=s high quality water resources. Increased loadings of BCCs to surface 

waters of the Great Lakes System must be consistent with the Great Lakes Water Quality 

Initiative Antidegradation Policy (40 CFR Part 132, Appendix E, II.A. ASignificant Lowering of 

Water Quality@). States and tribes that are concerned that new or increased discharges would not 

trigger antidegradation review on large waterbodies where the assimilative capacity is great 

should consider other approaches to defining significance, such as a combination of use of 

assimilative capacity and increase in pollutant loading.

To address situations where there are multiple or repeated increases in discharges, OST 

recommends that states and tribes incorporate a cumulative cap on the use of total assimilative 

capacity (i.e., the baseline assimilative capacity of a waterbody established at a specified point in 

time). This approach creates a backstop so that multiple or repeated discharges to a waterbody 

over time do not result in the majority of the total assimilative capacity being used without a 

single antidegradation review. For instance, the state or tribe may choose to subject any

lowering of water quality to antidegradation review after a certain percentage of the total 

assimilative capacity has been used. This ensures that where the ambient water quality is 

lowered closer to the criteria levels, the state or tribe will conduct an antidegradation review 

after a certain point to evaluate the necessity and importance of each lowering, regardless of the 

amount of assimilative capacity that would be used. 

OST recommends that, where states and tribes desire to establish a significance 

threshold, you work with them as they develop or revise their antidegradation implementation 

methods to ensure that any significance thresholds are consistent with the approaches described 

in this memorandum. 

If you have any questions or concerns, please do not hesitate to call me, or Denise 

Keehner, Director of the Standards and Health Protection Division, at (202) 566-1566. 

cc: Robbi Savage, ASIWPCA 

Water Quality Standards Branch Chiefs, Regions 1-10