BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

4.4 STORMWATER TREATMENTThe primary purpose of stormwater treatment BMPs is to remove pollutantsfrom stormwater prior to discharge from a developed site or a second BMP in a“treatment train”. Stormwater treatment BMPs rely on settling, filtration, anduptake from vegetation, or filtration through an engineered media to removepollutants. Included in this category are BMPs which remove large particlesand trash and BMPs which are capable of removing dissolved pollutants. TheseBMPs may provide some hydrologic source control in the form of detentionfor flood control, but generally are not designed to significantly reduce thetotal volume of water released from the site. Stormwater treatment BMPs areoften required on sites that have high pollutant loading or have the potential tocontaminate ground or surface water.Factors that should be considered prior to choosing a stormwater treatmentBMPs include the type of contaminates likely to be present on the site, theamount of space available for the BMP footprint, and the proximity of groundand surface water. The level of treatment necessary for a BMP which will be astand-alone system will be higher than the level of treatment required if thetreatment BMP is part of a series of BMPs (treatment train). The effectivenessof stormwater treatment BMPs is determined by the volume and flow rates onthe site therefore, it is essential to evaluate drainage rates and patternscorrectly. Failure to accurately size filtration BMPs will result in system bypassand will not result in stormwater treatment.Maintenance is critical to the continued effectiveness of a treatment BMP.Failure to frequently remove sediment and other pollutants from a BMP thatrelies on settling or contact with vegetation will result in the re-suspension andpossible release of these collected pollutants. Failure to replace cloggedfiltration media will result in system bypass. In addition, because filtrationBMPs are typically used in areas which are very sensitive to pollutants, failureto maintain these BMPs will have a direct effect on water quality.4.4.1 BIOFILTRATION AND DETENTIONBiofiltration and retention BMPs rely on adequate stormwater residence timewithin the system to provide a water quality benefit. Because this type of BMPmainly relies on settling to remove pollutants, the level of treatment achievedwill be affected by the amount of stormwater which enters the system and howlong it stays in the BMP. A basin or a bioswale will have reduced effectivenessfor removal of pollutants if the basin is overwhelmed. For this reason, thesetypes of BMPs should be designed with bypass mechanisms that will divertrun-off around the BMP once the design storm is captured. Failure to do thesemay result in a release of pollutants captured from previous events.Basins and bioswales will remove sediment particles which are large enough tosettle, and the pollutants which have an affinity for these particles (metals, oilTRPA BMP HandbookChapter 4: BMP Toolkit2012 Page 1