Selenium distribution and fluxes in intertidal wetlands, San Francisco Bay, California

Selenium (Se) concentrations exceeding ecological guidelines for sediments and suspended particulate matter (SPM) have been observed in the northern r each of the San Francisco Bay estuary. Long term availability of elevated S e in wetland sediments depends in part on the fluxes controlling Se distrib ution. The relative contribution of sedimentary vs. post-depositional Se fl uxes in two San Francisco Bay intertidal wetlands was estimated. Selenium c oncentrations on surface wetland sediments were compared with levels on SPM , and with previously established background levels in San Francisco Bay se diments. Sediment Se fluxes to the wetlands were measured directly using se diment traps. Although dissolved Se concentrations are higher than particul ate Se concentrations in San Francisco Bay water, sediment input into the s ystem provides the major flux of Se. Strong correlation between Se and C on SPM (r(2) = 0.81) indicates the importance of organic particulate depositi on. Dependence on sediment texture was qualitatively established by measuri ng Se on particle-size separates. Normalization to Al showed that 65% of Se spatial variability is related to sediment texture. Selenium is further en riched in the marsh via post-depositional inputs, probably due to in situ a dsorption from overlying water and chemical reduction. According to sedimen t flux measurements, enrichment in the marsh is equivalent to 20 to 25% of the particulate Se flux, thereby defining the marsh as a Se sink. These fin dings highlight the need for more intensive monitoring of SPM as the major source of Se to intertidal wetlands.