U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

Processes Affecting the Benthic Flux of Trace Metals into the Water Column of San Francisco Bay

By James S. Kuwabara, Brent R. Topping, Kenneth H. Coale, and William M. Berelson

This paper is available in pdf format: Kuwabara.pdf

ABSTRACT

Locally regulated point sources represent a significant input of trace metals to San Francisco Bay, especially in the southern component (South Bay) during the dry seasons and periods of drought. However, with recent recognition of the importance of non-point sources, a variety of approaches are being used to evaluate sediment remobilization and subsequent benthic flux of trace contaminants. The extent to which chemical processes couple with diffusive and advective physical processes in regulating benthic flux is also being examined. In terms of metal-speciation controls on solute remobilization and benthic flux, we are investigating the importance of metal-sulfide complexation relative to complexation with dissolved organic substances by determining both metal and ligand fluxes simultaneously. Consistent with previously reported water-column measurements, dissolved pore-water sulfide concentrations, measured at two South Bay sites in July, 1997 and September, 1998, increased with sediment depth from around 10 nM near the sediment-water interface to as high as 980 nM within the top 10 cm, suggesting a source to the water column. Diffusive flux for sulfides estimated from pore-water profiles ranged from 11 to 45 nmoles-m^-2-h^-1. As a complement to the pore-water approach, core-incubation experiments were used to directly measure sulfide benthic flux which ranged from 92 to 480 nmoles-m^-2-h^-1 over the same period. The significant difference between these two approaches was attributable to biologically enhanced advection (bioturbation/irrigation). Although sulfide benthic flux has been found to be consistently positive (that is, out of the sediment as much as 948 nmoles-m^-2-h^-1), benthic fluxes for dissolved organic carbon (0.2 µm filtered, DOC), copper and cadmium are temporally variable in direction across the sediment-water interface, suggesting the importance of DOC complexation in regulating metal benthic flux. Quantifying and understanding processes that affect the variability of these fluxes would enhance water-quality modeling for this estuary.