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Stormwater managers are increasingly faced with the need to address many potentially-conflicting issues as part of their activities, ranging from flood prevention to protection of downstream habitat. New urban development philosophies, such as low impact development, no adverse impact, water-sensitive urban drainage systems, etc., establish goals similar to other professions – Do No Harm. This is addressed often through criteria such as maintaining pre-development hydrology to the maximum extent practicable after development with treatment provided to reduce pollutants to their pre-development level. These new management plans incorporate both infiltration and surface treatment/discharge into the development.

A series of flowcharts were developed to aid stormwater managers in stepping through the process of selecting an appropriate management/treatment technology that meets both the low-impact goals and the regulatory requirements. These charts refer to the specific document sections that address the issues of concern that should be considered when selecting appropriate stormwater practices. Appendices were developed to provide supporting information for users not familiar with the terminology (Glossary in Appendix A) or the methods and supporting literature.  

This work has been based on the literature and guidance documents developed around the world. Surface-treatment technologies such as detention and filtration have a long history with much literature published about them. They are better understood than the infiltration practices currently gaining favor. Therefore, several chapters are devoted to the concerns associated with selecting infiltration. The desire to incorporate infiltration raises two potential concerns: 1) acceptance of small-scale infiltration devices, especially by the regulatory authorities, and 2) the potential for groundwater contamination. For the first concern, Appendix B summarizes several references on the issues raised by current stormwater management authorities regarding low impact development and infiltration devices dispersed throughout a watershed.

The second concern is that the potential for groundwater contamination is not well-known or documented. Pollutant removal in the subsurface has not been addressed regularly by authors who have either advocated infiltration and/or researched the effectiveness of infiltration at reducing surface water discharge volumes and pollutant loadings, although more recent reports are providing data showing both pollutant removals in the surface discharge, plus subsurface water quality.

The third activity under this literature review/guidance document development activity was to identify questions/data gaps in the selection and use of stormwater treatment devices, with a focus on infiltration. The following questions were raised.

  1. How effective are current and proposed stormwater management techniques at protecting surface water quality and habitat, as well as protecting groundwater? Groundwater and vadose zone water chemistry information is limited in the literature; it comes primarily from laboratory investigations or basins where the soil has been engineered. Few studies have looked at transport of stormwater-borne pollutants in the vadose zone of non-engineered soils. What soil and water chemistry data should be collected to improve the understanding of pollutant transport?
  2. Can water and wastewater treatment theory provide a better foundation in the models that predict BMP performance? Would this reduce the uncertainty that has to be associated with every stormwater pollutant removal calculation? Water and wastewater theory has been applied successfully to the design of sedimentation devices, but it has not been as successful at predicting pollutant removal in filtration or infiltration devices.
  3. How do the chemicals secreted by microorganisms affect pollutant mitigation in the surface and subsurface environment? What role does the local microbiological flora and fauna play in the trapping of pollutants and in the release of previously-trapped pollutants?

 

If infiltration is selected as a stormwater management technique, what issues of concern must be addressed outside of the pollutant transport? Issues to be addressed during treatment device selection include, but are not limited to, the following: groundwater mounding (and localized basement flooding), transport of pollutants through the subsurface to the groundwater, increased maintenance of the infiltration basins, interference with septic tanks and leach fields, soil stability problems due to longer periods where the soil is ‘wet’, legal issues of increased flooding and/or reduced slope stability. 

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