Improved VDOT Bioretention Media Specification

About the project

DOT uses bioretention as a stormwater control measure (SCM); however, the most recent special provision for VDOT bioretention soil media requires that testing the media’s infiltration rate use a unique mesocosm test method, outlined in VTM-134 (VDOT, 2025), which presents five challenges.  These are (1) inconsistency in how the media is placed and ultimately compacted in the test apparatus and therefore potential variability in the test results; (2) lack of labs willing to run the mesocosm test (only one in Virginia does this); (3) large amount of media required (40 5-gallon buckets of media and related materials); (4) lack of information proving this test is needed to procure successful bioretention soil media; and (5) a cost of $6,000 to run one test.  Due to these challenges, few media providers both try to meet the requirements and then succeed in doing so, which ultimately increases the overall project costs when bioretention is selected as the SCM.  A unique aspect of testing a media’s infiltration rate using the mesocosm test is consideration of how both de-icing salts and wet and dry cycles (to mimic rainfall patterns) impact a media’s infiltration rate. These unique aspects of the mesocosm test appear to be why it’s included in the current special provision. This study will recommend a specification for VDOT bioretention soil media that addresses the five challenges of the mesocosm test method to determine the media’s infiltration rate based on laboratory testing.

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