Field and laboratory studies were conducted in April and November 2003 to provide the first direct measurements of the benthic flux of dissolved mercury species (total and methylated forms) between the bottom sediment and water column at two sampling locations within the southern component of San Francisco Bay, California (hereafter referred to as South Bay): one within the main channel and the other in the western shoal area. Because of interest in the effects of historic mercury mining within watersheds that drain into South Bay, the solutes of primary interest were dissolved-mercury species and the predominant ligands that often control mercury speciation (dissolved sulfide and dissolved organic carbon). Benthic flux, sometimes referred to as internal recycling, is the transport of dissolved chemical species between the water column and the underlying sediment. Because of the affinity of mercury to adsorb onto particle surfaces and to form insoluble precipitates (particularly with sulfides), the mass transport of mercury in mining-affected watersheds is typically dominated by particles. As these enriched particles accumulate at depositional sites such as estuaries and reservoirs, benthic processes facilitate the repartitioning, transformation, and transport of mercury in dissolved, biologically reactive forms (dissolved methyl-mercury being the most bioavailable for trophic transfer). These are the forms of mercury examined in this study.
During two sampling events, three replicate sediment cores from each of two South Bay locations were used in incubation experiments to provide flux estimates and benthic biological characterizations. Incubation of these cores provided “snapshots” of solute flux across the sediment-water interface in this component of the estuary, under environmental conditions representative of the time and place of collection. Ancillary data, including nutrient and ligand fluxes, were gathered to provide a water-quality framework from which to compare the results for mercury. The following major observations from interdependent physical, biological, and chemical data were made:
Note: The dissolved-mercury concentrations discussed in this section refer to samples filtered with 0.7-micrometer quartz-fiber filters pre-combusted at 500 oC for 12 hours.
Because the benthic flux of mercury appears to represent a dominant transport process for dissolved, more bioavailable forms, an important management implication is suggested. Remediation efforts and Total Maximum Daily Load (TMDL) allocations along the Guadalupe River have dual objectives of decreasing concentrations and loads to down-gradient systems in an effort to reduce bioaccumulation of mercury in fish consumed by humans and wildlife. Using preliminary mercury-flux estimates into the estuary, our results indicate that a significant (and possibly predominant) percentage of dissolved mercury in the water column presently comes from the bay sediment (Mercury flux discussion). If upstream sources are controlled, which is desirable even apart from estuary effects, the change in inflow loads is likely to be compensated in part by increases in benthic flux.
In contrast to typical scientific manuscripts, this report is formatted in a pyramid-like structure to serve the needs of diverse groups who may be interested in reviewing or acquiring information at various levels of technical detail. The report enables quick transitions between the initial summary information (figuratively at the top of the pyramid) and the later details of methods or results (figuratively towards the base of the pyramid) using hyperlinks to supporting figures and tables, and an electronically linked Table of Contents.
Physical and Biological Characterizations
Results and Discussion
Study Design and Methods
Appendix 1: Comments on the Report Structure
Appendix 2: List of Figures
Appendix 3: List of Tables
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