TRACE METAL DISTRIBUTION IN THE OLIGHALINE REACHES OF THE POTOMAC ESTUARY AS DETERMINED FROM SURFACE SEDIMENT CORES

Anthropogenic input of trace metals to estuaries is a well-documented environmental concern, and these metals often accumulate preferentially in the oligohaline reaches of estuaries due to scavenging and flocculation processes. The Potomac River is known to have elevated trace metals concentrations in the Washington D.C. region, and here we test the hypothesis that elevated trace metals concentrations will be found further down river in the oligohaline reaches of the estuary. Six surface sediment cores (lengths varying from 24-64cm) were recovered along the distance between the Quantico Military Base and Westmoreland State Park, VA (distance of ~68km). Subsamples taken at discreet depths in each core were digested using a partial digestion technique (nitric acid, hydrochloric acid, hydrogen peroxide) and analyzed for trace metals (Pb, Cu, Cr, Ni) using flame atomic absorbance spectrometry. All metal concentrations were relatively low, and below estuarine sediment quality guidelines. Surface concentrations in each core are within previously published ranges for the upper reaches of the Potomac River as well as the Potomac Estuary. An insignificant decrease in concentration with distance down-estuary was noted for each of the four metals, suggesting a common up-estuary source combined with local influences. Down-core data reveal historic deposition at each location. Two of the locations (Quantico and Westmoreland) have Pb concentrations that transition from background values to higher values at ~35cm, likely a result of anthropogenic influences. The common depth at these sites, and lack of the lead increase feature at other sites, suggests similar sedimentation rates for the two locations, but slower rates than the other four locations. A sediment core taken off the Fair View Marina contains subsurface maxima peak in Pb and Cr concentrations at 20cm, and a Cu peak at 18cm. These data suggest a local and short-lived source of these metals at some point within the past 50 years. Overall, this stretch of the Potomac Estuary has relatively low concentrations of trace metals, and local changes in sources and sedimentation rates are highlighted by the records.