Hydrogeochemistry and transport of organic contaminants in an urban watershed of Chesapeake Bay (USA)

Stream water samples were collected in the two main free-flowing branches of the Anacostia River watershed above the head of tide over a one year time period. Both the Northeast and Northwest Branches drain large suburban and urban land areas that flow into the more urbanized tidal portion of the Anacostia River within Washington, DC. Large volume (40–75 l) water samples were filtered, and the suspended particulate matter and filtrate were analyzed for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and organochlorine pesticides (OCPs) at sub-nanogram per liter concentrations using ultra-trace analytical methods. Higher amounts of PCBs, PAH, and OCPs in the tidal Anacostia River occurred primarily in the particulate phase during high flow events. Polycyclic aromatic hydrocarbons in the particulate phase within fluvial transport consisted primarily of pyrogenic homologues characteristic of weathered or combusted petroleum products. Fluxes were exceptionally high for PAHs which showed annual fluxes to the tidal Anacostia River comparable to those determined for the much larger mainstem Potomac River. Aromatic hydrocarbons in runoff from urban regions may serve as an important source of PAH fluxes to the tidal waters of Chesapeake Bay.