Five field trips were conducted in the San Francisco Bay-Delta between May 2000 and October 2001 to investigate the sediment-water exchange of total mercury (Hg) and monomethyl mercury (MMHg). Solid-phase Hg averaged ϳ1 nmol g Ϫ1 and did not show any variability with depth or time or among sites. In contrast, solid-phase MMHg showed considerable vertical, temporal, and spatial variability (0.4-66 pmol g Ϫ1 ), with the highest values occurring at a peat-rich environment in May 2001, suggesting that MMHg production was largely controlled by temporal factors and habitat type. In pore water, both Hg and MMHg concentrations were generally elevated near the sediment-water interface during warm months. Sediment-water exchange flux of MMHg, determined with benthic chamber deployments, ranged from Ϫ92 to 850 pmol m Ϫ2 d Ϫ1 , with higher values occurring in May. In most cases, diffusional fluxes of Hg and MMHg, estimated with the use of interfacial concentration gradients, constituted only a minor portion of the measured fluxes, suggesting the importance of advective processes on sediment-water exchange. Surface-water transect and time series studies conducted in Franks Tract support the commonly held belief that wetland and marsh regions are major sources for MMHg within the Delta. The integrated sediment-water fluxes of Hg and MMHg in the study area were estimated to be 130 and 6 mmol d Ϫ1 , respectively, and the benthic input was as important a source of Hg and MMHg as the riverine input within the Delta during low-flow months.

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 (Ͻ0.45 m), colloidal (1 kDa-0.45 m), and dissolved (Ͻ1 kDa) fractions. Particulate Hg was the dominant phase (88 Ϯ 7%, n ϭ 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 Ϯ 18% (n ϭ 9) in the fall and 57 Ϯ 10% (n ϭ 12) in the spring.

Recent studies indicate significant amounts of mercury (Hg) are annually transported into the San Francisco Bay-Delta (Bay-Delta) as a result of historic gold and Hg mining activities. We examined temporal and spatial variation in concentrations of total Hg (Hg(T)) and monomethylmercury (MMHg) in surficial sediments of various ecosystem types in the Bay-Delta. We sampled surficial sediments across the Bay-Delta system and found Hg(T) sediment concentrations in the central Delta were generally 100-200 ng g(-1) and increased westward through Suisun Bay to 250-350 ng g(-1). MMHg concentrations in the central Delta were between 1 and 3 ng g(-1), while those in sediments in the perimeter waterways and adjacent bays were less than 1 ng g(-1). Six sites were monitored monthly for over a year to identify seasonal changes in Hg sediment concentrations. Hgtau sediment concentrations ranged from 48 to 382 ng g(-1) and varied as a function of location not season. However, MMHg concentrations varied seasonally, increasing from 1 ng g(-1) during winter months to 6 ng g(-1) during spring and summer. Transects conducted at three marshes in the central Delta revealed MMHg sediment concentrations of 4-8 ng g(-1) at the interior and 2 ng g(-1) at the exterior of the marshes. Habitat type was a major factor controlling MMHg concentration and the MMHg to Hg(T) ratio in sediments of the Bay-Delta. MMHg was significantly correlated to Hgt (r2 = 0.49) in marsh sediments.

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