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

Choe, Key-Young, Gary A. Gill

Limnol. Oceanogr., 48(4), 2003, 1547-1556 | DOI: 10.4319/lo.2003.48.4.1547

ABSTRACT: 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 1 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 Kc = 5.6 ± 0.3, n = 21) compared with those between particulate and dissolved MeHg (log KP = 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.

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