Expert Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

The rapid conversion of land to urban and suburban areas has profoundly altered how water flows during and following storm events, putting higher volumes of water and more pollutants into the nation's rivers, lakes, and estuaries. These changes have degraded water quality and habitat in virtually every urban stream system. To help address this problem, Congress in 1987 amended the Clean Water Act, expanding the U.S. Environmental Protection Agency's (EPA) oversight from about 100,000 point source permittees (wastewater discharged from industries and sewage treatment plants) to more than 500,000 permittees in order to encompass stormwater discharges from municipal areas, industry, and construction sites. Adding to the challenge of more permittees, it is much more difficult to collect and treat stormwater than wastewater. In light of these challenges, EPA asked the National Research Council to review its stormwater program. The report finds that the program will require significant changes if it is to improve the quality of the nation's waters. This report calls for an entirely new permitting structure that would put authority and accountability for stormwater discharges at the municipal watershed level. A number of additional actions, such as conserving natural areas, reducing hard surface cover (e.g., roads and parking lots), and retrofitting urban areas with features that hold and treat stormwater, are recommended.

Key Messages

  • Because of a 10-year effort to collect and analyze monitoring data from MS4s nationwide, the quality of stormwater from urbanized areas is well characterized.
  • Continuous, flow-weighted sampling methods should replace the traditional collection of stormwater data using grab samples.
  • EPA�s current approach to regulating stormwater is unlikely to produce an accurate or complete picture of the extent of the problem, nor is it likely to adequately control stormwater�s contribution to waterbody impairment.
  • Individual controls on stormwater discharges are inadequate as the sole solution to stormwater in urban watersheds.
  • Industry should monitor the quality of stormwater discharges from certain critical industrial sectors in a more sophisticated manner, so that permitting authorities can better establish benchmarks and technology-based effluent guidelines.
  • Nonstructural SCMs such as product substitution, better site design, downspout disconnection, conservation of natural areas, and watershed and land-use planning can dramatically reduce the volume of runoff and pollutant load from a new development.
  • Roads and parking lots can be the most significant type of land cover with respect to stormwater.
  • SCMs that harvest, infiltrate, and evapotranspirate stormwater are critical to reducing the volume and pollutant loading of small storms.
  • The full distribution and sequence of flows (i.e., the flow regime) should be taken into consideration when assessing the impacts of stormwater on streams.
  • The protection of aquatic life in urban streams requires an approach that incorporates all stressors.
  • There is a direct relationship between land cover and the biological condition of downstream receiving waters.
  • Watershed models are useful tools for predicting downstream impacts from urbanization and designing mitigation to reduce those impacts, but they are incomplete in scope and do not offer definitive causal links between polluted discharges and downstream degradation.