Development of an Improved Bioretention Soil Media for Use in Washington State

摘要

The current Washington State, Phase I and Phase 2 municipal National Pollutant Discharge Elimination System stormwater permits (NPDES stormwater permits), effective January 1, 2017, require the use of low impact development (LID) practices where feasible as the first option for managing stormwater. Bioretention is the most widely applicable and flexible LID practice for flow control and runoff treatment. Bioretention systems may include under-drains, especially in areas with soils that are less suitable for infiltration. In these cases, a portion of the treated runoff is discharged back into the stormwater conveyance system and into local receiving water bodies. The current Washington State Department of Ecology (Ecology) specification for bioretention soil media (BSM) in western Washington (Ecology 2014) is a blend of 60 percent sand and 40 percent compost (60sand/40comp). While the 60sand/40comp blend can provide reliable water quality treatment for some contaminants (e.g., solids removal, zinc [Zn], hydrocarbons, and possibly bacteria), regional and national research indicate that nitrogen (N), phosphorus (P), and copper (Cu) are often exported from BSMs containing compost (Herrera 2014, Herrera 2015a). The use of bioretention with underdrains will increase dramatically with the new NPDES stormwater permit requirement for on-site stormwater management. As a result, the export of contaminants to receiving waters from the currently specified BSM will be an increasing concern.