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Dirt, sand, and other particulate material on parking lots and streets is washing off into urban streams, reports a new study by the U.S. Geological Survey Regional Stream Quality Assessment. The study used novel particle tracers to determine that particles washed from pavement made up almost half of the streambed sediment in parts of Dead Run, an urban stream in the Baltimore area.
Sources of urban sediment are critical from a water-quality standpoint, because sediment from pavement can carry contaminants such as metals and polycyclic hydrocarbons, whereas eroded streambank material has low contaminant levels. Contaminants in suspended and bed sediment can degrade aquatic communities—the fish, algae, and aquatic invertebrates such as miniature crustaceans and damselfly larvae that live in the stream.
Pavement sediment has a unique radionuclide “fingerprint”, which enabled the investigators to quantify its contribution to stream sediment. The study measured three radionuclides that come from the atmosphere and adhere to sediment: beryllium-7, cesium-137, and excess lead-210 (the part of lead-210 not related to radioactive decay in rock and sediment). The contribution of particulate material from pavement—almost one-half of total stream sediment—was greatest just downstream from where parts of the urban stream were buried and routed through pipes and culverts. The remaining stream sediment consisted of topsoil and eroded streambanks.