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Perhaps the most recognized Low Impact Development technique, bioretention, incorporates landscaped features to slowly percolate stormwater runoff through plants and engineered soil prior to infiltrating that water into native soils. Where feasible, these systems can be a low cost, high efficiency stormwater management practice that can also provide other benefits including a reduction in heat island effect, improved site aesthetics, and water supply augmentation.  Although bioretention is extremely versatile, two common site constraints may limit applicability. In some cases, infiltration is not feasible due to lack of adequate infiltration capacity or other geotechnical issues.  Also, conventional bioretention can be prohibitively large, particularly in dense urban environments.

Thankfully there are solutions.  Where infiltration is prohibitively slow or creates other hazards, a bioretention cell can be converted easily to a tree box filter (biofilter) by adding underdrains to discharge the treated stormwater.  To shrink the footprint of bioretention systems, engineered soils with higher infiltration rates can be used.  Many jurisdictions have adopted variations of a non-proprietary sand/compost blend that can be designed at rates up to 10 inches per hour. Beyond that rate, specialized media are commercially available with infiltration rates up to 100 inches per hour.  In order to provide these rapid infiltration rates while providing adequate organic content, cation exchange capacity and moisture holding capabilities, media suppliers must have robust quality control processes to ensure consistent properties.  Amended soil provides the greatest footprint advantage when paired with an underdrain or with additional subsurface detention volume where water can slowly infiltrate to native soils over a period of days. 

Even where site constraints abound, tree box filters with these optimized features can usually be integrated into the landscape to provide most of the low impact development benefits associated with conventional bioretention.

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