Benthic lead fluxes in San Francisco Bay, California, USA

Abstract

Porewater concentration gradients indicate relatively large benthic fluxes of Pb from sediments in the San Francisco Bay estuary. Gradients in total dissolved (<0.45 μm) Pb concentrations in sediment porewaters, which range from 0.07–19.2 nM, parallel gradients in ammonia and dissolved Fe in sediment cores from the bay. Corresponding Fickian diffusive fluxes range from 2.6 × 10 −9 moles m −2 d −1 to 3.1 × 10 −8 moles m −2 d −2 in anoxic surface (<2 cm) sediments along the periphery of the estuary. These indicate the net diffusive benthic flux of Pb from sediments in San Francisco Bay (3–31 moles d −1) is at least an order of magnitude greater than the fluvial input of dissolved Pb to the estuary (0.2 moles d −1) during low flow periods. Moreover, estimates of the total benthic Pb flux, which were based on Hammond et al. (1985) irrigation benthic flux model, are two- to six-fold greater (6–186 moles d −1) than the estimates of diffusive fluxes. Therefore, the total benthic flux of Pb from the bay's sediments may be within an order of magnitude of the total anthropogenic flux of Pb to the San Francisco Bay estuary (965–8,410 moles d −1).