Mercury in the San Francisco Estuary - Reviews of Environmental Contamination and Toxicology - page 39

Table 3 Estimate of recent or present-day fluxes of mercury (kg yr −1 ) in San Francisco Estuary

Mercury (kg yr −1 ) Reference

Sources

Atmospheric deposition

Direct wet deposition

4.4-4.8 Steding and Flegal 2002; Tsai and

Direct dry deposition 22.0 Hoenicke 2001

Atmospheric emissions

Stationary sources

244 CARB 2005

Areawide sources

1074

On-road mobile

29

Other mobile

83

Gasoline combustion

0.7-13 Conaway et al. 2005

Watershed

Central Valley

440-800 Domagalski 2001; McKee et al.

Guadalupe River 4-116 2005; SFRWQCB 2006;

Thomas et al. 2002

Wastewater 12 Ellgas 2001

Erosion of contaminated 460 SFRWQCB 2006

benthic sediments

Stormwater runoff

Urban

160 SFRWQCB 2006

Nonurban

25

Sinks

Ocean export

513 MacLeod et al. 2005

Burial

732

(Conaway et al. 2004; Hornberger et al. 1999). The physical distribution and
chemical speciation of this mercury in part determine the relationship between the
sources and present-day human health and ecological effects described above. The
distribution of MMHg and total mercury has been generally described by Choe and
associates (Choe and Gill 2003; Choe et al. 2003), Heim et al. (2007), and Conaway
et al. (2003). Focusing on the northern reach, the studies by Choe are distinguished
by their detailed treatment of the surface water, including colloidal fractions, and
are complemented by the study of Heim et al. which provides data on mercury spe-
ciation in sediment over an annual period. The study by Conaway et al. details total
mercury and MMHg distribution in both water and sediment with a multiannual,
multiseasonal statistical approach.
A general trend discernible from the data available is that water column concen-
trations of total mercury are higher in the rivers draining into the estuary than in the
estuary itself. The northern reach, with large riverine inputs, has higher dissolved
and total mercury concentrations than the southern reach, where so-called freshwa-
ter inputs are dominated by wastewater discharges. Total mercury concentrations in
surficial sediments, averaging approximately 1 nmol g −1 (Choe et al. 2004; Conaway
et al. 2003), are similar throughout the estuary as a result of mixing and resuspen-
sion, but decrease moving east into the delta (Heim et al. 2007). Water and sedi-
ment MMHg levels vary substantially both spatially and temporally, with highest