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A. National Pollutant Discharge Elimination System (NPDES) permitting, 

 limits derivation 



Industries and sewage treatment plants that discharge effluent into 

 US waters are required to have permits that establish limits on the quantity 

 of pollutants they can release. Today, those limits are derived in order to 

 protect water quality -- i.e., the chemical content in the water column. 

 Permit writers use. state standards, plus information on effluent 

 concentration, flow (the "dilution" of the waste stream that will occur once 

 it hits the water) and patterns of mixing to calculate the level of a pollutant 

 that is permissible in the effluent. 1 



However, it is known that pollutants present in low concentrations in 

 the water settle onto sediment and over time accumulate in high 

 concentrations. Sediment quality criteria should insure that clean sediments 

 stay clean. 



Once sediment quality criteria are available, they can be used like 

 water quality criteria to back-calculate the level of pollutant discharges that 

 can safely be made without exceeding sediment quality criteria. 2 Permit 

 limits then can be modified to protect both water and sediment quality. 



For many waters, multiple dischargers of a toxic contaminant of 

 concern pollute waterways. In such cases, single-facility discharges cannot 

 be analyzed in isolation. A 'Total Maximum Daily (sediment) Load" 

 (TMDL) -- the maximum daily amount of a certain pollutant that the 

 sediment bottom can safely receive - must be calculated. Once the TMDL 

 for sediment is determined, that load must be allocated among all 

 dischargers and pollutant sources from both point sources and runoff 

 sources. 



Because TMDL and load allocation calculations can be time 

 consuming, interim protection should be provided for areas whose 

 sediments already exceed standards. In these cases, a staged cutback or 

 freeze at current levels or discharges should be incorporated into NPDES 

 permits until the more precise TMDL and load allocations are available. 



U.S. Environmental Protection Agency (1991), 'Technical Support Document for 

 Water Quality-based Toxics Control", Office of Water, EPA 505/2-90-001. 



U.S. Environmental Protection Agency (1991), "Pre-Draft Guidance on the 

 Application of Sediment Quality Criteria for the Protection of Aquatic Life", 

 Office of Science and Technology, Draft Document, Washington, D.C. 



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