A GEOCHEMICAL, GEOSPATIAL, AND GEOSTATISTICAL FRAMEWORK FOR TRACE ELEMENT FATE AND TRANSPORT IN URBAN WATERSHEDS

In the Neponset River Watershed, a New England Watershed industrialized in the 17^th century, hotspots reflecting historical industry record Cr, Cu, Zn, and Cl concentrations exceeding 1,000 μg/g in surface sediments. Sediment contamination is linked with numerous sources, and may be remobilized downstream during high flow events. The highly engineered and developed watershed is characterized by eutrophic headwater sites, numerous dammed millponds, and a main branch that is mixture of natural state riparian zones to a fully channelized river. Traditional, watershed-level analyses alone – often reliant on techniques like Principal Component Analysis (PCA) and GIS-- cannot define specific contaminant sources given the geochemical and hydrological complexity of the watershed superimposed on a diverse history of human uses. Diffuse metal contamination from atmospheric deposition and urban runoff, for example, can create a baseline of pollution that obscures point sources and transport. In response, this study outlines a novel framework for defining elements and places of interest in the fate and transport chemistry of industrialized urban watersheds, using a suite of geochemical, geospatial, and statistical tools. Over 190 surface samples, representing 17 ponds and tributaries in the watershed, were analyzed for elemental abundance by x-ray fluorescence. Elements and places of interest were defined by geohistorical knowledge coupled with PCA and geospatial analysis by GIS. Variograms were then used to account for spatial and hydrological factors controlling metal distributions and identified spatially correlated elements of interest. A statistical test of dissimilarity defined five geochemically distinct sub-regions in the watershed based on previous observations. In each region it was possible to assess the relative contributions of diverse contaminant sources, segregate regional and point sources, and define the role dams play in containing contaminated sediment. The above process allows for the differentiation of watershed and sub-watershed level factors controlling metal concentrations, is useful for guiding local remediation efforts, and is intended to guide similar fate and transport studies in other urban watersheds.