Chromium in San Francisco Bay: superposition of geochemical processes causes complex spatial distributions of redox species

Processes controlling the geochemical cycle of Cr in San Francisco Bay were characterized with analyses of water samples taken from 25 stations distributed throughout the estuary. Mixing of water masses, localized inputs, in-situ reduction, and sediment resuspension contributed to complex spatial distributions of dissolved (< 0.45 μm) Cr(III) and Cr(VI), as well as suspended particulate Cr. Total dissolved Cr concentrations in the bay ranged from 1.9 to 8.3 nM, with the maximum concentration observed at the head of the estuary (the Delta) that was attributed to a relatively large input (5.2 nM) of Cr(III) within the San Joaquin River. The calculated Cr(III) scavenging residence time in that region was relatively long (> 3 days) for a particle reactive trace metal, suggesting either localized inputs or complexation by colloidal/organic matter. There was also a relative excess (2.6 nM) of Cr(VI) in another section of the Delta. Mass balance calculations showed that excess was the same order of magnitude as reported point source loadings of Cr in that area. The subsequent depletion of Cr(VI), along with a concurrent increase in Cr(III) concentration in a shallow region of the Delta, was tentatively attributed to in-situ reduction.