Geochemistry of sediment and organic matter in drainages burned by the Altas and Nuns wildfires in October 2017 and of nearshore seabed sediment in north San Francisco Bay from March to April 2018

SAMPLE COLLECTION AND GROUND CONDITIONS. Terrestrial soil and sediment were collected five months post-fire and after a week of rainstorms. Nearshore sediment was collected six months post-fire and when daytime high tides allowed access to San Francisco Bay tidal flats.

STABLE CARBON ISOTOPE RATIOS (d13C) and CARBON TO NITROGEN RATIOS (C:N) in sedimentary organic matter. Dried and homogenized decarbonated sediments were analyzed for ratios of total carbon to total nitrogen and stable carbon-13/carbon-12 isotopes in sedimentary organic matter using a CE Instruments NC2500 elemental analyzer interfaced to a ThermoFinningan Delta Plus XP isotope ratio mass spectrometer.

SHORT-LIVED RADIONUCLIDE ACTIVITIES. Prior to quantification by gamma spectrometry, bulk sediment was oven-dried at 60 degrees Celsius, homogenzied, and packed into 10 ml graduated polystyrene vials with air-tight caps. Activities of short-lived radionuclides were counted for 24 hours in a high-purity low-background germanium well detector in the Radiochemistry Lab of the USGS Pacific Coastal and Marine Science Center. Beryllium-7 activities were counted the 477.6 kiloelectron volt (keV) peak, Cesium-137 activities were counted the 661.6 keV peak, and unsupported Lead-210 activities were quantified by subtracting Lead-210 activities counted at the 46.52 keV peak from the average of supported Radium-226 activities counted indirectly at the Bismuth-214 351.9 keV and Lead-214 609.3 keV peaks.

ELEMENTAL COMPOSITIONS. Prior to total elemental analyses, sediment was oven-dried at 60 degrees Celsius, disaggregated gently to avoid changing grain sizes, and dry-sieved through stainless steel sieves to obtain the less than 0.063 mm size fraction for geochemical analyses. Sieved sediment was sent to the Analytical Chemistry Project of the USGS Central Mineral and Environmental Resources Science Center (CMERSC), which contracted with SGS Laboratories, a nationally accredited laboratory, for analyses. Sediment was decomposed using two methods: a near-total four-acid digestion for major, minor, and trace elements, and a total digestion with a sodium peroxide fusion for major, minor, rare earth elements (REE), and other elements hosted in resistant minerals. Quantification was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) for major and some minor elements and by inductively coupled plasma mass spectroscopy (ICP-MS) for some minor and all trace elements.

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