Distribution of particulate, colloidal, and dissolved mercury in San Francisco Bay estuary. 2. Monomethyl mercury

The phase speciation and estuarine behavior of monomethyl mercury (MeHg) were determined in surface waters of the San Francisco Bay estuary in September-October 2000 (low flow)and March 2001 (high flow). Colloidally associated MeHg was isolated using a cross-flow ultrafiltration technique with a nominal molecular weight cutoff of I kDa. Filter-passing MeHg was 57 +/- 18% of the MeHg in unfiltered water in the fall and 39 +/- 12% in the spring. Colloidal MeHg averaged 34 +/- 11% of the filter passing MeHg in the fall and 56 +/- 15% in the spring. Significantly higher particle-water partition coefficients were observed between colloidal and dissolved MeHg (log K-C = 5.6 +/- 0.3, n = 21) compared with those between particulate and dissolved MeHg (log K-P = 4.9 +/- 0.5, n = 21), which suggests that MeHg is preferentially associated with colloidal material. Strong correlations of MeHg with organic carbon content in the filter-passing, colloidal, and dissolved fractions confirmed the importance of organic matter in the MeHg cycle. Both absolute and relative (as a percentage of Hg) MeHg concentrations were highest in the river water end-member under both flow-regimes, which suggests that riverborne MeHg is the major source of MeHg introduced to the estuary. A nonconservative estuarine mixing model suggests that significant amounts of colloidal and dissolved MeHg are removed in the estuary under both flow regimes, standing in marked contrast to Hg, which had a source within the estuary under the low flow condition.