TRACE METALS IN BIOSWALES OF NEW YORK CITY: IMPLICATIONS ON METAL FLUXES FROM AIR AND OTHER SOURCES

Thousands of bioswales have been built on the sidewalk of streets in New York City in the past 15 years. They effectively capture runoff from streets, reducing the volume of combined sewer overflow – the leading cause of water quality problem for the waterways around the City. These bioswales also serve as a sink for trace metals scavenged from the atmosphere by rain, and those from the street surfaces. The majority of trace metals are bound to the particles and remain in the topsoil in the bioswale. Therefore, trace metal levels in the topsoil of a bioswale can serve as an indicator of metal fluxes in a specific watershed or neighborhood. We selected 12 different neighborhoods in 4 boroughs in NYC, varying in land use and traffic density. Within each neighborhood, we randomly selected 3-7 different bioswales. These bioswales, usually 20 ft by 5 ft in size, vary in watershed size and age. Seven topsoil samples were collected within each bioswale: three from the center line (lower elevation) near the inlet, middle and outlet area; four from the two flanks of the bioswale (higher in elevation). A portable X-ray Fluorescence instrument was used to scan directly the soil sample contained in a ziplock bag, after the soil sample was dried and sieved to <2 mm in size. Our preliminary results show that trace metal concentrations vary, sometimes by several orders of magnitude, among different bioswales and at different locations within the same bioswale. This suggests that trace metal fluxes can vary a lot among different neighborhoods, and that metals have uneven distribution within a bioswale (due to flow patterns and sediment deposition). Annual fluxes for different land uses can be estimated based on the topsoil metal concentrations, and can be used as a proxy for air quality for different urban neighborhoods. Further analysis will also reveal the relationship between annual fluxes and traffic density. This is the first time that bioswale systems are used as indicators or monitors of air quality.