356-6 Exploring Impacts of Groundwater Interactions on Urban Stormwater Control Measure Performance

Abstract:

Stormwater control measures (SCMs) are designed to ameliorate flooding, limit stream erosion, and improve water quality from urban developments. Infiltrating SCMs, such as bioretention and sand filters, are often designed for groundwater recharge to supply baseflow to nearby streams. Two field monitoring campaigns, one at a large sand filter draining an industrial site near Columbus and another with four bioretention cells in a parking lot at Cleveland Metroparks zoo, were recently completed with notable stormwater-groundwater interactions. The sand filter was located on poorly draining soils with a known high-water table; water level loggers within the sand layers and significant increases in hardness showed extended dewatering times (>1 week after rainfall) and a unique connection with the groundwater table. For 20 sampled storm events, pollutant concentrations were significantly reduced for heavy metals (>80%), total suspended solids (72%), and nutrients in runoff, inclusive of dissolved forms like nitrate (48% reduction). Ongoing dissolved oxygen and dissolved organic carbon monitoring may support denitrification. At Cleveland Metroparks zoo, bioretention cells with varying plant palettes and loading ratios (i.e., ratio of catchment area to SCM surface area) were monitored over one year. While runoff volumes were substantially reduced by three cells (71-89%), effluent volumes exceeded influent runoff at the low-lying Tree cell. Suspected groundwater interactions were confirmed by soil borings, which revealed redoximorphic features below the underdrain. Similarly, though mean concentrations of TSS were effectively reduced (by 50-83%), nutrients (i.e., N and P) were discharged at higher concentrations in the bioretention effluent, indicative of leaching of organics from compost in the bioretention media blends. Collectively, these two field studies highlight the need to consider groundwater interactions in SCM design, particularly since labile pollutants might be transferred to groundwater.

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