GEOSPATIAL-GEOCHEMICAL ANALYSIS OF HEAVY METALS IN THE NEPONSET RIVER: FATE, TRANSPORT AND GEOCHEMICAL FINGERPRINT OF SEDIMENT IN A HISTORIC NEW ENGLAND WATERSHED

This study presents a geospatial-geochemical analysis of Pb, Cu, Cr, and Zn in the Neponset River Watershed, a historic New England watershed industrialized in the 17^th century. Two hundred surface and core river sediment samples were collected from a 21 km reach of the river spanning from the suburban headwaters to the densely populated mouth. Sediment was analyzed for elemental abundances by x-ray fluorescence. A unique distribution of metals was observed that can be portrayed as a series of point sources superimposed over a regional geochemical fingerprint. Metal concentrations, often exceeding 1,000 mg/g, are found in historically industrialized stretches of the watershed. Contrary to other studies, our results indicate that surface sediment quality is not directly related to current land use or urbanization. Research on the industrial history of the watershed, combined with the spatial distribution of metals, suggests that legacy sources of metals coupled with flow control structures are controlling the current distribution of sediment metal loads.

Principal Component Analysis (PCA), an exploratory statistical tool, was used to combine the geospatial dataset with geochemical analysis in order to evaluate linkages between hydrological conditions in the watershed and geochemically unique legacy point sources. After normalizing for sedimentological controls on metal abundances, such as grain size and mineralogy, the modeling results suggest that the watershed can be broadly characterized into distinct geochemical zones. In each zone it is possible to assess the relative contributions of three dominant endmembers associated with sediment metal concentrations including: legacy /regional sources, nutrient loading, and watershed specific “background”. This is of major interest to local watershed associations and state environmental agencies charged with evaluating metal mobility potentials under changing hydrological conditions and with setting remediation benchmarks. This unified multi-variable watershed-wide approach that combines historical analysis and geospatial mapping with geochemical fingerprinting, represents a new direction for defining regional “urban metal baselines” in river sediments and has implications for the management and remediation of urban watersheds.