SEDIMENT DYNAMICS IN AN URBAN WATERSHED USING SHORT-LIVED RADIONUCLIDES, DEAD RUN, BALTIMORE, MARYLAND, USA

Urban watersheds are geomorphically complex areas where sediment flux is related to the age and extent of urban development and infrastructure. In an urban-suburban watershed in Baltimore, Maryland (Dead Run; 1.6 km²), developed prior to the 1970s, sediment flux was studied using short-lived radionuclides (⁷Be, ²¹⁰Pb_ex, and ¹³⁷Cs). Pavement dust (street sweepings from roads and parking lots), fluvial sediment (streambed and suspended sediment), surface soil, and rainfall samples were collected to quantify sediment yield and radionuclide activities. The ⁷Be and ²¹⁰Pb_ex activities in Dead Run are some of the highest measured in the U.S., possibly because this is one of the first studies to include radionuclide sampling on and near urban impervious surfaces. Based on these data, a conceptual model of particle and radionuclide dynamics in urban settings is proposed, where: (1) areas of exposed soils are the primary source of pavement-dust particles; (2) ambient pavement dust has higher ⁷Be and ²¹⁰Pb_ex activities relative to soils because of dosing by previous rain events (however, rainfall dosing of ¹³⁷Cs, a bomb-fallout radionuclide, is no longer occurring); (3) during rainfall, additional dosing of ⁷Be and ²¹⁰Pb_ex to particles mobilized by runoff leads to the most elevated radionuclide activities; (4) upon entry into streams, dilution of particles in runoff by particles from soil and bank erosion reduces fallout radionuclide activities. Mass-balance calculations indicate that pavement dust contributes a relatively small fraction of stream sediment, yet it might contribute substantially to loading of some urban contaminants. This work has implications for understanding sources and loading of sediment and particle-associated contaminants to streams from urban impervious surfaces and soils.