Over recent years, much attention has been placed on State policies and procedures regarding impaired waters, 303d listing, and TMDLs. However, State Antidegradation Policies can be as significant, and in many cases can affect significantly larger number of dischargers.
In essence, the two regulatory programs address two subsets of "waters of the State" 1) those that currently do not meet their water quality standards, and 2) those that do currently meet standards. Impaired Waters Programs address waters that do not meet their respective water quality standards. Studies and implementation plans ("TMDLs) are required to move these impaired waters back into compliance.
In contrast, Antidegradation policies or programs address waters that current meet their respective standards. In this case, policies or rules are in place to assure that NEW or EXPANDED discharges to these waters do not result in an unacceptable degradation in water quality (even if still below water quality standards). Antidegradation policies will generally set thresholds for new or expanded discharges above which Antidegradation Reviews may need to be conducted before the discharge is permitted.
One of the complicating factors in antidegradation policies is the application to stormwater discharges which require an NPDES permit. Typically the antidegradation policy thresholds are not expressed in units that are easily applied to stormwater discharges. For example, an existing industrial facility which has a permit to discharge stormwater expands its truck parking area, which technically increases flow. Depending on the specific requirements of the State's antidegradation policy, this increase may require an antidegradation review.
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