MERCURY CONCENTRATIONS AND LOADS IN A LARGE RIVER SYSTEM TRIBUTARY TO SAN FRANCISCO BAY, CALIFORNIA, USA

In order to estimate total mercury (Hg-T) loads entering San Francisco Bay, USA, via the Sacramento-San Joaquin River system, unfiltered water samples were collected between January 2002 and January 2006 during high flow events and analyzed for Hg-T. Unfiltered Hg-T concentrations ranged from 3.2 to 75 ng/L and showed a strong correlation (r(2) = 0.8, p < 0.001, n = 78) to suspended sediment concentrations (SSC). During infrequent large floods, Hg-T concentrations relative to SSC were approximately twice as high as observed during smaller floods. This difference indicates the transport of more Hg-contaminated particles during high discharge events. Daily Hg-T loads in the Sacramento-San Joaquin River at Mallard Island ranged from below the limit of detection to 35 kg. Annual Hg-T loads varied from 61 +/- 22 kg (n = 5) in water year (WY) 2002 to 470 +/- 170 kg (n = 25) in WY 2006. The data collected will assist in understanding the long-term recovery of San Francisco Bay from Hg contamination and in implementing the Hg total maximum daily load, the long-term cleanup plan for Hg in the Bay.