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Water Quality


Mass transportation projects can impact water quality by increasing runoff or altering surface or sub-surface drainage patterns. If a project results in discharge of wastewater into the stormwater system, a permit may be required under the National Pollutant Discharge Elimination System (NPDES). 

Process For Protecting Water Quality

Wastewater generated by mass transit system maintenance and storage facilities can contain a number of pollutants that, through improper handling or treatment, can be released into stormwater systems. Environmental documents for a proposed project should discuss any activities that could generate wastewater (e.g., steam cleaning, vehicle washing, and floor wash-downs) and the provisions for containing possible pollutants. Typical bus garage effluent contains concentrations of oil and grease, detergents, chemicals and metals, and solid materials that pass into the sewer system.

Stormwater runoff from transit parking areas may contain harmful pollutants such as lead, zinc, and cadmium. Procedures should be developed to prevent these pollutants from entering the storm drainage systems of bus and commuter rail parking and maintenance facilities. The project's potential for increasing runoff, and measures that will be used to reduce runoff or prevent pollutants from entering stormwater systems, should be described in the environmental document.

In addition, if a transit project changes the existing runoff pattern it could contribute to local flooding. In this case, an analysis should evaluate the magnitude of the impact and identify measures that will be used to reduce the potential for flooding. The analysis also should include the findings of the appropriate local or state agencies (e.g., public works agencies or flood control districts). The results of the analysis should be presented in the environmental document. Examples of transit projects that may contribute to flooding include paving of an area where previously a permeable surface existed, modifying a drainage system, or channeling existing runoff.

A National Pollutant Discharge Elimination System (NPDES) permit issued under Section 402 of the Clean Water Act (33 U.S.C. 1342) may be required if wastewater is discharged into the stormwater system. The US Environmental Protection Agency (US EPA) sets pretreatment effluent limits for NPDES permits and issues the permits and localities may have their own limits and identified specific disposal sites. The environmental document should state whether an NPDES permit is required and whether there are local or state pretreatment requirements.

Mitigation measures should be used to control chemical and detergent use and to remove oil, grease, and solid materials at the source. As a mitigation measure, dry detention facilities may be preferable to wet detention ponds because the transformation of particulate metals (from diesel exhaust emissions) to soluble forms that are susceptible to washout can be minimized. The soluble form is much more toxic to aquatic organisms than the particulate form. Periodically, pipes should be flushed, exterior pavement washed, and sediments removed from detention ponds in dry weather.

Federal Statute

Section 402 of the Clean Water Act (33 U.S.C. 1342) - National Pollution Discharge Elimination System (NPDES)

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Updated: Wednesday, March 16, 2016
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