According to information assembled by the Environmental Protection Agency (EPA) from state Clean Water Act (CWA) Section 303(d) lists, a large fraction of impaired waters are located within or near urbanized areas (EPA, 2000). CWA Section 402(p) and its implementing regulations currently
require municipal separate storm sewer system (MS4) operators serving populations greater than 100,000 people to obtain authorization to discharge storm water via National Pollutant Discharge Elimination System (NPDES) permits. In March 2003 MS4 operators located in urbanized areas will also
be required to obtain NPDES MS4 permits under pending Phase II storm water regulations. Also, under Phase II rules, certain MS4 operators can be required to obtain a permit even if they are located outside of urbanized areas, based upon their location and their receiving waters (NARA, 2001).
Compliance with NPDES MS4 permit conditions is necessary to avoid civil penalties through enforcement action by regulators and exposure to third-party lawsuits. Most existing MS4 permits contain narrative conditions that stipulate pollutant discharges in storm water must be reduced to the
maximum extent practicable (known as the “MEP standard”). Many MS4 general permits promulgated under the pending Phase II program are anticipated to contain similar programmatic provisions. However, some recently proposed TMDL implementation plans, particularly in California, have
called for the addition of enforceable effluent limits in MS4 permits to achieve TMDL load reductions. Of particular note is the TMDL for trash proposed for the Los Angles River in early 2001 (CRWQCB, 2001). This TMDL explicitly requires MS4 permits in the Los Angeles area to be revised to
include monitoring requirements and also requires the imposition of diminishing waste-load allocations over a defined compliance period. The final trash allocation after this compliance period is zero. Since many impaired waters are located in areas subject to storm water permitting regulations
and discharge to impaired waters is a likely designation trigger for MS4 operators located outside of urbanized areas, it is likely that as TMDLs are developed and load reduction targets established, more and more MS4 permit holders will be required to achieve TMDL derived load reductions
and more and more MS4 permits will be used as a TMDL implementation tool. Enforceable NPDES MS4 permits will be renewed or reopened to include specific load reduction targets. This paper will present a national review of known proposed or issued TMDLs or TMDL implementation plans that call
for the imposition of numeric effluent limitations in MS4 permits as a means to achieve TMDL-derived load reductions. The paper will also highlight some of the policy implications of this emerging trend.
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