Tracking Atmsopherically-Derived Contaminants through An Urban Watershed

Airborne contaminants are deposited on the landscape and are transported in aqueous or sediment phases to stream channels. The purpose of this study is to evaluate suspended sediment sources and associated trace metal contaminants in the NE branch of the Anacostia River near Washington, D.C. Four main sub-watersheds occupy most of the land area; 3 have impervious surfaces areas of 18-20%, the fourth watershed has 11%. Smaller, heavily-urbanized tributaries (with ~40% impervious surfaces and concrete-lined channels) enter the NE branch above the mouth. We sampled sites (upland soils, stream banks, floodplain surfaces, point bars, and road surfaces) to determine the amount of contamination as a function of topographic position. Suspended sediment samples were collected during storm events and were concentrated via centrifuge. Elemental analysis was conducted on the fine fractions of prepared samples by ICP-MS. These data indicate that site contamination for most elements is in the following order: streets >point bars >stream banks >floodplains >uplands. We calculated enrichment factors for metals in each sample by normalizing by crustal abundances. Air samples show the following enrichment order: Se> Pb> V> Cd> Zn> As> Cu> Ni. Bank samples had a wide range of Pb contamination, from 0 to values above modern atmospheric levels, which record pre-1972 atmospheric contamination of suspended sediment deposits. Street samples are heavily enriched in Cd. These contributions are reflected in the suspended sediment enrichment order: Cd>Pb>Se>Zn>Cu>Mn. Sediment budgets and sediment fingerprinting analyses were used to determine that suspended sediment is largely derived from bank erosion (70-80%) and from streets (10-15%. Suspended sediment samples had higher concentrations of Cd, Cu, Co, Zn, Ni and Mn than source samples. This suggests that high pH values (7-9)generated by concrete channels in the lower watersheds causes contaminants transported initially in the aqueous phase to become attached to sediment during transport events.