Geochemistry of fine sediment from San Francisco Bay shoals (2012) and tributaries (2010, 2012, 2013)

SEDIMENT COLLECTION. Single hand-grab samples were collected from local streams and inland rivers in March and April 2010. Surface sediment from the shoals and ebb tide delta of San Francisco Bay was collected in January 2012 using a benthic grab sampler deployed from the USGS research vessel Parke Snavely. Single hand-grab samples were collected from Petaluma River in March 2012 and inland rivers in February 2013.

SEDIMENT GRAIN SIZE ANALYSIS. Sediment grain size distributions were determined at the USGS Pacific Coastal and Marine Science Center Sediment Laboratory. To remove organic material, 10 milliliters of 35 percent hydrogen peroxide were added to 20 grams of sediment. The sample was heated to 250 degrees Celsius to remove any remaining hydrogen peroxide and was placed in an ultrasonic bath to liberate the less than 0.063 millimeter-diameter fine fraction. The sample was centrifuged for 1.5 hours in deionized water to remove soluble salts. Samples were funneled through 2 millimeter- and 0.063 millimeter-diameter mesh sieves to separate gravel-sized (greater than 2 millimeter), sand-sized (0.063 to 2 millimeter), and mud-sized (less than 0.063 millimeter) particles. The sand and gravel fractions were dried at 80 degrees Celsius and weighed. Five milliliters of sodium hexametaphosphate were added to the mud fraction and a 20-milliliter aliquot was taken from a 1-liter graduated cylinder to determine the weight of the mud. The particle-size distribution of the mud and sand fraction was analyzed separately using a Beckman Coulter LS 13 320, and the gravel, sand, and mud results were integrated into a single grain-size distribution for each sample.

ELEMENTAL CHEMISTRY. Bulk sediment was oven-dried at 60 degrees Celsius and disaggregated gently with a mortar and pestle to avoid changing grain sizes. Disaggregated sediment was dry-sieved through stainless steel sieves to obtain 2 grams of the less than 0.063 millimeter fine fraction. Fine sediments were sent to the Analytical Chemistry Project of the USGS Geology, Geophysics, and Geochemistry Science Center (GGGSC), which contracted SGS Inc. Laboratory for analyses of major, minor, and trace element contents using two methods: a near-total four-acid digestion (labeled multi-acid in the table of data) and a total digestion with a sodium peroxide fusion (labeled fusion in the table of data) for elements hosted in acid-resistant minerals. Quantification was performed by inductively coupled plasma atomic emission spectroscopy (ICP-OES) for major and some minor elements and by inductively coupled plasma mass spectroscopy (ICP-MS) for some minor and all trace elements.