Sediment Remobilization of Mercury in South San Francisco
Bay, California
By Brent R. Topping, James S. Kuwabara, Mark C. Marvin-DiPasquale,
Jennifer L. Agee, Le H. Kieu, John R. Flanders, Francis Parchaso, Stephen W.
Hager, Cary B. Lopez, and David P. Krabbenhoft
ONLINE ONLY
This report is also available as a pdf.
September 2004
Scientific Investigations Report 2004-5196
U.S. Department of the Interior
U.S. Geological Survey
EXECUTIVE SUMMARY
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:
Physical and Biological Characterizations
- Porosity: The surficial sediment at Station 29A was generally of higher
porosity than that at Station 25 on both sampling dates. Station 29A porosities
ranged from 0.77 to 0.92 (0.84 + 0.05), while those of Station 25 ranged from
0.73 to 0.82 (0.77 + 0.04). This difference, however, is not statistically
significant at the 95% confidence interval.
- Benthic Biota: Macroinvertebrate densities varied temporally and spatially,
in a manner consistent with previous studies (Topping and others, 2001; see
macroinvertebrate discussion). Given that the first sampling event coincided
with the annual spring phytoplankton bloom in the South Bay, Chlorophyll and
phaeophytin concentrations also exhibited temporal and spatial differences.
See the results section for details.
Chemical Characterizations
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.
- Dissolved mercury in the water column: Dissolved methyl-mercury
concentrations were below detection limits at all sites and dates. Total dissolved-mercury
concentrations ranged from ~4 to ~10 picomolar, so the undetectable methyl-mercury
values were not surprising because total mercury concentrations are typically
two or more orders of magnitude greater than methylmercury concentrations.
Total dissolved-mercury concentrations were appreciably elevated at the western-shoal
Station 25 relative to the main-channel Station 29A, and in spring relative
to autumn.
- Benthic flux of dissolved forms of mercury: When the three
cores for each site were averaged, benthic flux estimates for total dissolved-mercury
showed no temporal change for Station 25, but showed higher flux in spring
relative to fall for Station 29A. Still, the averages for each site and date
were within the same order of magnitude. When the average for all sites and
dates (135 ± 94 g/day; is extrapolated over the greater South Bay,
the magnitude of the values is consistently comparable to or greater in magnitude
than estimates of major riverine sources. Notably, benthic flux of dissolved
mercury is of the same magnitude as particulate mercury inputs from the Guadalupe
River watershed (318 ± 88 g/day in 2003; McKee 2004, draft copy). Transport
of dissolved-mercury species between the estuary bed and water column may
therefore be a potentially critical process regulating the fate of mercury
species in the water column (Mercury flux discussion).
All twelve individual core incubations in the study resulted in positive total-mercury
flux values. In other words, at all sites and dates, each core indicated that
dissolved total mercury was transported out of the sediment into the overlying
water column.
Dissolved methyl-mercury fluxes could not be directly calculated due to the
undetectable values for all sites and dates (<0.5 picomolar in May, <0.2
picomolar in November). These undetectable values are common and have been
observed in other studies (Conaway and others, 2003). However, when comparing
loads of dissolved methyl-mercury into South Bay, benthic flux cannot be summarily
discounted as a possible source due to analytical limitations (Load comparison
discussion).
Ancillary measurements characterizing the sediment were compared with water-column
and benthic flux values. Four correlations were observed including sediment
mercury’s correlation with benthic flux of dissolved mercury (Ancillary
sediment characterization discussion).
- Benthic flux of mercury-binding ligands:
Dissolved-sulfide benthic fluxes were similar at all dates and sites, and
consistently positive. Sulfides are associated with reducing conditions, so
the similarities at all stations and dates might indicate reducing-oxidizing
(redox) conditions were similar throughout the study, despite significantly
higher dissolved oxygen (DO) consumption in May (DO discussion). Given that
the South Bay water column is consistently oxic (that is, dissolved-sulfide
species are only metastable), the relative consistency of the mercury fluxes
may be associated with the flux of sulfides, which have a strong affinity
to complex with mercury. Dissolved organic carbon (DOC) fluxes were markedly
different between sites and significantly so between dates. Autumn fluxes
were much higher than spring fluxes at both sites, while Station 29A exhibited
higher fluxes than Station 25 for both dates. Unlike sulfides, negative DOC
fluxes (solute transport from the water column to the bottom sediment) were
observed in some cores.
- Benthic flux of other metals:
Comparisons between this study and a previous one by the authors (Topping
and others, 2001) indicated that 2003 conditions may be less conducive to
metal remobilization and release than in previous years (Dissolved nickel
and copper discussion). This indicates that estimates of longterm mercury
flux could be incorrectly low if based only on 2003 sampling.
Upgradient Remedial Implications
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.
Comment on the Report Structure
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.
CONTENTS
Executive Summary
Physical and Biological Characterizations
Chemical Characterizations
Remedial Implications
Background
Results and Discussion
Physical Data
Biological Data
Chemical Data
Study Design and Methods
Coring Operation
Core Incubations
Physical Data
Biological Data
Chemical Parameters
References Cited
Acknowledgments
Appendix 1: Comments on the Report Structure
Appendix 2: List of Figures
Appendix 3: List of Tables
For additional information write to:
Brent R. Topping
U.S. Geological Survey
345 Middlefield Road, MS 439
Menlo Park, CA 94025
Copies of this report may be obtained from the authors or
U.S. Geological Survey
Information Center
Box 25286, MS 517
Denver Federal Center
Denver, CO 80225
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