Microbial cycling of mercury in contaminated pelagic and wetland sediments of San Pablo Bay, California

Abstract

San Pablo Bay is an estuary, within northern San Francisco Bay, containing elevated sediment mercury (Hg) levels because of historic loading of hydraulic mining debris during the California gold-rush of the late 1800s. A preliminary investigation of benthic microbial Hg cycling was conducted in surface sediment (0-4 cm) collected from one salt-marsh and three open-water sites. A deeper profile (0-26 cm) was evaluated at one of the open-water locations. Radiolabeled model Hg-compounds were used to measure rates of both methylmercury (MeHg) production and degradation by bacteria. While all sites and depths had similar totalHg concentrations (0.3-0.6 ppm), and geochemical signatures of mining debris (as Nd, range: -3.08 to -4.37), in-situ MeHg was highest in the marsh (5.4±3.5 ppb) and £ 0.7 ppb in all open-water sites. Microbial MeHg production (potential rate) in 0-4 surface sediments was also highest in the marsh (3.1 ng g -1 wet sediment day -1 ) and below detection (<0.06 ng g -1 wet sediment day -1 ) in open-water locations. The marsh exhibited a methylation/ demethylation (M/D) ratio more than 25· that of all open-water locations. Only below the surface 0-4-cm horizon was significant MeHg production potential evident in the open-water sediment profile (0.2-1.1 ng g -1 wet sediment day -1 ). In-situ Hg methylation rates, calculated from radiotracer rate constants, and in-situ inorganic Hg(II) concentrations compared well with potential rates. However, similarly calculated in-situ rates of MeHg degrada tion were much lower than potential rates. These preliminary data indicate that wetlands surrounding San Pablo Bay represent important zones of MeHg production, more so than similarly Hg-contaminated adjacent open-water areas. This has significant implications for this and other Hg-impacted systems, where wetland expansion is currently planned.

Pre-Construction Biogeochemical Analysis of Mercury in Wetlands Bordering the Hamilton Army Airfield Wetlands Restoration Site

“…The latter export would be 2.5 times higher than projected using the values generated by the study. Reasons for the differences in methylation, demethylation rates, and methylation:demethylation ratios in this study and in the Marvin-DiPasquale et al (2003) study may be the following:…”

“…The nonvegetated bay sediments assayed in the current study originate from the marsh currently bordering the HAAF that are richer in organic matter and have a higher LOI than the bay sediments assayed by Marvin-DiPasquale et al (2003), possibly causing higher methylation rates. This comparison demonstrates the sensitivity of this value to MeHg export projections.…”

“…Aside from the values provided herein, little other data are available. However, one recent study reports values on methylation, demethylation rates, and ratios measured in surficial bay sediments and in one marsh site, obtained using the less sensitive 14 C-method (Marvin-DiPasquale et al 2003). Using the latter values, the estimates of annual potential MeHg export would change significantly, from 2.5 g MeHg for a nonvegetated HAAF to 405 g MeHg for a vegetated HAAF (Table 3-13).…”

“…A more sensitive stable isotope approach has been used in the current study than the radioactive isotope approach used by Marvin-DiPasquale et al (2003), therefore methylation rates are above the method-detection level, whereas the methylation rates measured by Marvin-DiPasquale et al (2003) in bay sediments are below their higher detection level.…”

Sediment-adsorbed total mercury flux through Yolo Bypass, the primary floodway and wetland in the Sacramento Valley, California

“…However, there is still great uncertainty in the mass balance of total mercury (THg) delivered to and stored in lowland floodplain environments from different source areas. Such estimates are particularly important given that lowland floodplains integrate basinwide delivery of chemical constituents and engender conditions favorable for mercury methylation (Compeau and Bartha, 1985;Gilmour et al, 1992;Marvin-DiPasquale et al, 2003). In this paper, we develop empirical relationships from hydrology, sediment, and mercury data collected by various regional and state programs to evaluate the delivery of THg to a large, engineered floodplain in the Lower Sacramento River basin and its storage over a decade.…”