Processes Controlling the Export of Trace Metals from Contrasting Watersheds

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9630277 Armstrong Watersheds have a major influence on water quality. Concern over the impacts of metals on receiving waters emphasizes the need for information on the factors controlling the forms and amounts of metals exported from watersheds and the influences of extreme hydrologic events and changes in land use. Our premise is that the forms, concentrations, and loadings of metals in streams can be used to discern the watershed processes controlling metal export. We also postulate that the nature and amounts of dissolved organic carbon (DOC) and suspended particulate matter (SPM) transported from watersheds are the main geochemical factors controlling metal export. Our goal is to identify and quantify key geochemical and hydrologic processes controlling the forms and amounts of trace metals exported from watersheds representing diverse geology, hydrology, soil type, and land use/cover. Specific objectives are to determine (1) The importance of hydrologic events in controlling metal export from watersheds of contrasting geology and land use; (2) Factors controlling within-watershed differences between base flow and event periods in metal levels, partition, and yields; (3) Relationships controlling metal speciation in dissolved and particulate phases and (4) Factors and processes controlling delivery of key metal transport phases from watersheds. A matrix of six relatively homogenous watersheds will be compared. (iradients in stream DOC, SPM, and ionic strength will be represented. Export will be determined by concurrent measurements of discharge and metal concentrations. "Ultra clean" sampling and analysis methods will be employed to quantify metals. Speciation will include particulate, colloidal, and "labile" fractions, and free and DOC-associated "dissolved" forms. We will model geochemical processes regulating dissolved metal concentrations and loadings. Speciation techniques include ultrafiltration, selective resin partitioning, electrochemical/voltammetric measurements, solid -phase partitioning experiments, chemical extractions, and oxidation state measurements. The properties of a suite of metals (Ag, Al, As, Be, Cd, Cr, Co, Cu, Hg, Mo, Ni, Pb, Sb, Tl, U, V, Zn) will be used to probe the watershed characteristics and geochemical processes controlling metal export. We expect to provide a basis for assessment and prediction of both metal export. Speciation will be quantified and modeled as a function of geochemical factors. Predictive capability for metal export will be based on watershed characteristics and discharge-geochemistry relationships. This proposal was submitted to the Environmental Geochemistry and Biogeochemistry solicitation, and the project is being jointly supported by the Divisions of Earth Sciences (Geosciences) and Chemical and Transport Systems (Engineering).