1. Antideg and Municipal Stormwater Discussion Sept. 23, 2009

2. Guiding principles Tier 2 Protection: Where water quality exceeds the applicable standard, water quality may not be lowered unless there is a demonstrated need for important economic or social development in the area where the waters are located. Antidegradation requirements are implemented and enforced through CWA control documents (e.g. NPDES permits). Antidegradation requirements that are specified in general permit conditions must maintain the quality of receiving waters. In future MS4 general permits, a determination will need to be made whether the MS4 is an expanded discharge.

3. Antidegradation Review Process 1. Application to discharge into waters of the state 2. Will water quality be lowered? 3. Are there reasonable non-degrading alternatives? 4. If not, are there reasonable minimally- degrading alternatives? 5. Where water quality will be lowered, it must be justified based on the need for important economic or social development

4. Determination an expanded discharge Step 1. Determine the “raw” loading Question: For stormwater, can surrogate measures be used to estimate pollutant loading? Surrogate measures may include: land use, impervious surfaces, population Step 2. Is there an increase in “raw” loading? Step 3. Will control measures mitigate increased loading? (Will water quality be lowered?)

5. Baselines “lowering of water quality” = “new or expanded discharges” Implies a starting point or baseline from which actions are measured. Baselines may be defined in at least one of two ways: 1. Water quality conditions of the receiving water at a specific point in time. Based on monitoring and or modeling, includes loading contributions from non- regulated sources 2. Permitted loading at a specific point in time (For wastewater discharges this is based on flow and numeric effluent limits)

6. Adjustment of baselines If water quality is allowed to be lowered as a result of a demonstrated need for important economic or social development, then a new baseline is set (is adjusted downward). If water quality improves as a result of decreased loading then the baseline is adjusted upward.

7. Maintaining water quality Question: Can we develop antidegradation requirements (control measures) in the permit that will maintain water quality? –Control measures may be: i. Performance objectives, e.g. infiltration requirements, percent TSS removal ii. Evidence-based mandatory control measures, e.g. use of a particular treatment system under certain conditions What are our best options? Level of confidence? Adaptive management – assessment of control measure effectiveness –Ambient monitoring of receiving water –Discharge monitoring –Validation that control measures are designed, constructed and maintained to meet objectives