Bioretention Soil Media Export and Impacts on Underlying Native Soils

Bioretention systems are commonly designed to provide full or partial infiltration of the design storm. An initial flushing period is expected for bioretention soil media (BSM) where mobile media components are released, however, research has indicated that BSM mass export can vary substantially among suppliers. BSM characteristics depend on many factors including variability in source materials, blending and storage practices, and installation methods. To address concerns that significant mass export from prolonged BSM flushing could occlude underlying native soils, thereby limiting infiltration rates, volumes, and pollutant reductions, a bench-scale study was conducted to evaluate BSM export via effluent mass evaluation. Test columns representing high-rate and low-flow media from numerous suppliers received multiple, simulated storms following a prescribed hydrograph for each hydraulic loading rate (HLR) with stair-stepped flow pacing. Repetitive simulations coupled with suspended sediment concentration sample collection and analysis allowed for observation of change in BSM export. Testing ceased when effluent quality throughout a simulation was consistently below a low threshold turbidity and/or SSC value, indicating that flushing was complete. BSM mass export results for various suppliers and design HLRs will be presented. This presentation will provide suggested test designs and methods for evaluating BSM mass export. Recommendations will be provided to practitioners on how use this data when considering installation and long-term performance of both low-flow and high-rate BSM over infiltrating native soils. Designers must consider the role of BSM export and hydraulic capacity sustainability of the underlying, native soils to achieve long-term infiltration, pollutant reduction, and permit compliance.