Monomethyl mercury (MeHg) constitutes only a few percent of total Hg in natural waters. However, MeHg is a more potent toxin than inorganic Hg, and the major form of Hg that bioaccumulates in the soft tissue of fish and shellfish. Biomagnification of MeHg through the food web makes a potential health risk to humans through the consumption of MeHg-enriched fish.

Recently, colloids have been postulated as important carriers of trace metals, radionuclides and hydrophobic trace organic contaminants in marine systems. The scavenging processes of trace metals in the water column, combined with adsorption onto macromolecules, coagulation into particulate phases, and removal processes, suggests that colloids play an important role in the transportation, distribution and bioavailability of trace metals in estuarine and coastal environments

An accurate cross-flow ultrafiltration technique was developed to isolate colloidal material for the determination of the phase speciation of MeHg in natural waters. This technique was applied to the one of the most Hg-contaminated estuaries, San Francisco Bay (SFB), to elucidate the importance of colloidal material in the biogeochemical cycling of MeHg in estuaries.

Surface water samples were collected from the SFB estuary in September - October 2000 and March 2001. Mercury (Hg) (inorganic mercury + MeHg) and MeHg concentrations were measured in unfiltered, filter-passing (< 0.45 micro-m), colloidal (1 kDa – 0.45 micro-m), and (truly) dissolved (< 1 kDa) fractions. Particulate Hg was the dominant phase species (88 +/- 7 %, n = 29) in unfiltered water. Particulate MeHg averaged 52 +/- 18 % (n = 29) of the total MeHg in unfiltered water. A significant portion of the filter-passing Hg (49 +/- 17 %, n = 21) and MeHg (46 +/- 18 %, n = 21) were associated with colloidal material. Strong correlations were observed between colloidal Hg and colloidal organic carbon, and between colloidal MeHg and colloidal organic carbon, in the filter-passing fraction. These relationships suggest that organic material is an important transport medium of the filter-passing Hg and MeHg in the SFB estuary.

Benthic flux chamber studies demonstrated that sediment-water exchange is an important source of MeHg in the SFB estuary and Delta regions. Fluxes for both Hg and MeHg were generally higher in May than in September or February. The Cosumnes River, Franks Tract, Franks Tract marsh, and Prospect Slough had higher MeHg fluxes (90 +/- 18 pmol/m2/day, n = 6) compared with Little Holland and Sherman Island (5 +/- 37 pmol/m2/day, n = 3) in May 2000 and May 2001, suggesting a MeHg source in the upper Delta region.

Evidence of colloidally associated MeHg was observed in interstitial porewater of SFB Delta sediments. Both absolute and relative colloidal MeHg concentrations were significantly higher in upper (0 – 2 cm) sediments (32 +/- 9 %, n = 8) compared to deeper sediments (18 +/- 16 %, n = 16).

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