Abstract
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 ( n = 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 ± 18% ( n = 9) in the fall and 57 ± 10% ( n = 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.