Distribution of particulate , colloidal , and dissolved mercury in San Francisco Bay estuary . 1 . Total mercury

Surface water samples were collected from the San Francisco Bay estuary in September–October 2000 (low flow) and March 2001 (high flow). Total mercury (Hg) concentrations were measured in unfiltered, filter-passing (,0.45 mm), colloidal (1 kDa–0.45 mm), and dissolved (,1 kDa) fractions. Particulate Hg was the dominant phase (88 6 7%, n 5 29) in unfiltered water. Suspended particulate matter (SPM) explained most particulate Hg concentrations in the northern reach. A significant portion of filter-passing Hg was associated with colloidal Hg, accounting for 38 6 18% (n 5 9) in the fall and 57 6 10% (n 5 12) in the spring. Seasonal variability of the filter-passing Hg concentration observed in the upper estuary was attributed to the temporal change in the riverine colloidal Hg. The strong correlation observed between Hg and organic carbon in the filter-passing fraction indicates that organic material is an important transport medium of Hg in San Francisco Bay. Assessment of various forms of the particle– water distribution coefficient revealed that Hg was preferentially associated with SPM during the low flow period but that colloidal material played as important a role in Hg phase speciation as SPM during the high flow condition. A steady-state nonconservative estuarine mixing model suggested that the northern reach had an internal source of colloidal Hg in September–October, possibly from resuspended sediments, and a sink of colloidal Hg within the estuary in March 2001. Phase speciation (or size distribution) of trace metals is important to their bioavailability, toxicity, and fate in aquatic environments. For instance, many investigators have shown that only the solution-phase or free ion species are bioavailable to phytoplankton (Tessier and Turner 1995 and references therein). Colloids are operationally defined as inorganic or organic material in the size range of ;1 nm to ;1 mm. Because of their short residence time (Baskaran et al. 1992; Moran and Buesseler 1992) and strong reactivity with trace metals (Honeyman and Santschi 1989), colloidal materials have been receiving considerable attention recently with respect to the biogeochemistry of trace metals in natural waters (Benoit et al. 1994; Dai and Martin 1995; Powell et al. 1996; Wen et al. 1999). In contrast to traditional approaches that have used conventional filters with pore sizes between 0.1 and 1.0 mm, where colloid-sized macromolecules are included in the solution phase, cross-flow ultrafiltration techniques provide a great capability to isolate colloids and associated trace elements from the filter-passing fraction. Numerous investigations have been conducted on total mercury (Hg) (inorganic Hg 1 monomethyl Hg) biogeo1 Corresponding author. Present address: McGill University, Earth and Planetary Sciences, Montreal, Quebec H3A 2A7 (choe@eps. mcgill.ca).