Mercury in the San Francisco Estuary - Reviews of Environmental Contamination and Toxicology - page 41

community, and respiration rate in controlling the rates of microbially mediated
MMHg production and degradation in sediments of different environments in the
estuary and delta.
Although there has been an apparent decrease in total concentrations in surface
sediment through time in some parts of the estuary, there has been no observed
corresponding decrease of mercury concentrations in sportfish (Conaway et al.
2007). This discrepancy is primarily attributed to MMHg being the dominant form
in fish (Kuwabara et al. 2007), and that mercury methylation and uptake is control-
led by a complex interaction of various biogeochemical factors. Nevertheless,
because of active erosion and redistribution of sediment in the estuary (Cappiella et
al. 1999; Foxgrover et al. 2004; Jaffe and Foxgrover 2006), buried sediments with
relatively higher mercury concentrations (Conaway et al. 2004; Hornberger et al.
1999; Marvin-DiPasquale and Agee 2003) may be remobilized and increase levels
of bioavailable mercury.

B Water: The Importance of Flux and Complexation

The majority of studies on water column mercury have so far focused on the distri-
bution and transport of contaminant mercury to and within the estuary, making
regional model and mass balance calculations possible (MacLeod et al. 2005).
Water column measurements used to support flux calculations in the estuary have
established the importance of investigating concentrations in suspended matter
entering the estuary and also the dominance of sediment resuspension on water
column mercury concentrations (Conaway et al. 2003; Domagalski 2001; McKee
et al. 2006). Evasion of dissolved gas mercury from surface waters appears to be a
small flux out of the estuary (MacLeod et al. 2005); however, high-temporal resolu-
tion studies with better spatial resolution are still required (Conaway 2005). In
addition to flux calculations, studies on the relationship between salinity and fresh-
water flow on dissolved concentrations in the water column (Choe and Gill 2003;
Choe et al. 2003; Conaway et al. 2003) are important in understanding the uptake
of mercury by organisms, and water column measurements of mercury and MMHg
have been used to show that sediments are an important source to overlying water
(Choe et al. 2004; Topping et al. 2004).
Despite the many studies of the biogeochemical cycling of mercury in the
estuary, there are few studies to date describing the element's complexation and
speciation, which are relevant to its bioavailability. Detailed investigations of this
type in the surface waters have been limited to studies of surface water in tributar-
ies and wastewater effluent that showed the presence of strong, or nonlabile,
Hg(II)-complexing ligands with conditional stability constants similar to those of
reduced sulfur-containing ligands (Black et al. 2007; Hsu and Sedlak 2003).
However, further studies of the complexation and chemical speciation of Hg(II)
and MMHg in surface waters or pore waters of the estuary, as well as riverine
inputs to the estuary, are needed.