TIDAL RIVER SEDIMENTS IN THE WASHINGTON, DC AREA .1. DISTRIBUTION ANDSOURCES OF TRACE-METALS

Thirty-three bottom sediments were collected from the Potomac and Anac ostia rivers, Tidal Basin, and Washington Ship Channel in June 1991 to define the extent of trace metal contamination and to elucidate sourc e areas of sediment contaminants. In addition, twenty-three sediment s amples were collected directly in front of and within major storm and combined sewers that discharge directly to these areas. Trace metals ( e.g., Cu, Cr, Cd, Hg, Pb, and Zn) exhibited a wide range in values thr oughout the study area. Sediment concentrations of Pb ranged from 32.0 mu g Ph g(-1) to 3,630 mu g Pb g(-1), Cd from 0.24 mu g Cd g(-1) to 4 .1 mu g Cd g(-1), and Hg from 0.13 mu g Hg g(-1) to 9.2 mu g Hg g(-1), with generally higher concentrations in either outfall or sewer sedim ents compared to river bottom-sediments. In the Anacostia River, conce ntration differences among sewer, outfall, and river sediments, along with downriver spatial trends in trace metals suggest that numerous st orm and combined sewers are major sources of trace metals. Similar res ults were observed in both the Tidal Basin and Washington Ship Channel . Cadmium and Pb concentrations are higher in specific sewers and outf alls, whereas the distribution of other metals suggests a more diffuse source to the rivers and basins of the area. Cadmium and Pb also exhi bited the greatest enrichment throughout the study area, with peak val ues located in the Anacostia River, near the Washington Navy Yard. Enr ichment factors decrease in the order: Cd>Pb>Zn>Hg>Cu>Cr. Between 70% and 96% of sediment-bound Pb and Cd was released from N-2-purged 1N HC l leach. On average, less than or equal to 40% of total sedimentary Cu was liberated, possibly due to the partial attack of organic componen ts of the sediment. Sediments of the tidal freshwater portion of the P otomac estuary reflect a moderate to highly contaminated area with sub stantial enrichments of sedimentary Ph, Cd, and Zn. The sediment phase that contains these metals indicates the potential mobility of the se diment-bound metals if they are reworked during either storm events or dredging.