Biogeochemical cycling processes of metals (copper and lead) in Boston Harbor, Massachusetts Bay and Cape Cod Bay

Distributions of dissolved Cu (Cudiss) and Pb (Pbdiss ) in the waters of Boston Harbor (BH), Massachusetts Bay (MB) and Cape Cod Bay (CCB) were simulated by integrating dissolved metals into an existing 3-D hydrodynamic model. Comparison with observed data (2003-2004) showed that, in the Bays, Cudiss could be successfully modeled as a conservative constituent and Pbdiss as a non-conservative one. The scavenging process of Pb was simulated using settling particulate organic carbon (POC) as a scavenging phase. In BH, the concentrations of Cudiss and Pbdiss have remained relatively constant over the past 15 years despite significant reduction in metal loading in the effluents. With all contemporary known metal sources input into the 2002 Harbor models, only 10% and 20% of observed Cudiss and Pbdiss distributions respectively could be accounted for. Incorporation of reasonable benthic fluxes of these elements provided in the model simulations resulted in satisfactory comparison between field observations. The model based analysis suggests that previously contaminated Harbor sediments are now a major source of these metals to the overlying water column. The models were also used to explore the sensitivity of the Harbor-Bays system to the changes on Cu and Pb sources quantitatively.;The developed trace metal model was then employed to assist the application of a sediment-water dynamic equilibrium model (SDEM) for Cu in BH and MB to describe the interdependence of copper concentrations between surface sediments and the overlying water column and to provide insights into regional and temporal changes in water column Cu speciation. Significant correlations between Cu and organic carbon concentrations were observed in surface sediments from surveys conducted in 2005 in BH and MB, and from earlier studies in BH in 1987 and in MB in 1995. Estimation of the temporal and spatial free cupric ion activities in BH and MB using the SDEM were done in conjunction with an organic carbon-based sediment conditional binding constant (beta*SED = 108.9) recently determined by Pala (2009). Competing strengths (alpha') of the natural ligands in the water column were also estimated based on the model-simulated Cudiss distributions in bottom water in the study region.