Antidegradation and permits

The highest level of water quality protection applied to certain waters that constitute an outstanding state or national resource. The division maintains a GIS map of outstanding waters that shall be maintained and protected at their existing water quality.

Short-term degradation of existing quality is allowed for activities that result in long-term ecological or water quality benefit, or clear public interest. As a result, typically only discharges of pollutants from short-term construction activities are allowable in outstanding waters.

An intermediate level of water quality protection. Permits for discharge of pollutants into reviewable waters must prevent significant degradation of those waters since the baseline date when the receiving segment(s) first became reviewable.

A small increase in discharge of pollutants can be significant to low or zero flow segment(s) or when there are multiple dischargers in close proximity. Conversely, a large increase may not be significant if there is a lot of available assimilative capacity for the discharge of pollutants.

There is an exception for dissolved iron, dissolved manganese, and sulfate. These concentrations may reach the applicable water supply standard without an antidegradation review provided degradation for Aquatic Life based standards is not significant.

Determinations of insignificant degradation

• Temporary dischargers (less than 2 years) are generally considered not to result in significant degradation of existing water quality.
• Facilities with small discharge rates relative to segment(s) with large available assimilative capacity such that all pollutants will be diluted 100:1 or more at low flow are considered not to result in significant degradation.

Examples of antidegradation in permits

Facilities that start discharging after the baseline date (new dischargers) are required to prevent significant degradation and likely receive fairly stringent antidegradation-based effluent limitations. Typically the limits are 15% or less of the numeric water quality standards, not accounting for any available assimilative capacity (which is not available in zero low-flow segment(s) or when there existing dischargers).

Existing dischargers may be allowed to continue discharging the same amount of pollutants as before prior to the baseline date. These antidegradation-based limits are non-impact limits (NILs) and implicit NILs.

• NILs are discussed on page 6 of the antidegradation guidance. 
• The division developed an implicit NILs memo to clarify how they are determined.
• An increase in the existing discharge rate requires a reduction in the NILs to maintain the same discharge amount.

Exceptions to antidegradation protections — antidegradation alternatives analysis

The Water Quality Control Division can determine that degradation of existing water quality is necessary to accommodate important economic or social development in reviewable waters.

• This is referred to as the “necessity of degradation determination” in Regulation 31.8(3)(d) and is discussed starting on page 38 of the antidegradation guidance.
• It includes an antidegradation alternatives analysis required by 31.8(3)(d)(iii) to determine whether “there are no water quality control alternatives available that (A) would result in no degradation or less degradation of the state waters and (B) are determined to be economically, environmentally, and technologically reasonable.”
• The division has provided guidance and examples of antidegradation alternative analyses.
• All antidegradation alternative limits are subject to a public comment process.