Nutrient Leaching and Greenhouse Gas Emissions in Grassed Detention and Bioretention Stormwater Basins

Nutrient cycling was compared in a grassed detention basin and a bioretention basin was amended with compost, mulch, and diverse plantings. The authors monitored dissolved nutrients in basin inflows and outflows and emissions of greenhouse gases (GHGs), methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) from basin soils. Few previous studies have evaluated GHGs in stormwater control measures, and none to the authors' knowledge have quantified GHGs along with nutrient leaching. Although these basins are intended to improve runoff quality, the bioretention basin was a source of nitrate, soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC). SRP and DOC leaching was driven by high phosphorous (P) and carbon (C) content of the bioretention soil media, and nitrate leaching was driven by the low C:N of added compost. Emissions of N2O and CH4 were low from both basins, although there were periodically high N2O emission rates at both sites. CO2 emissions were greater from the bioretention basin, where soil C content was greater. Looking at bioretention design standards, organic matter (OM) recommendations are quite varied. Based on the results from this study, bioretention basin design should minimize OM additions to soil media and choose OM with high C:N (>20) and low P content to minimize availability of excess nutrients for leaching or GHG production.