Geochemistry of fine-grained sediment in Bellingham Bay, Nooksack River, and small creeks from June 2017 to September 2019

SEDIMENT COLLECTION: Bellingham Bay surface sediment was collected using a benthic grab sampler deployed from a boat. Nooksack River, tributary, and small creek sediment was collected by hand. The surface 0-2 cm layer of nearshore, river, and creek sediment was sampled into plastic wire-top bags and stored frozen.

SEDIMENT PARTICLE SIZE, TOTAL ORGANIC CARBON, AND CARBONATE CONTENT ANALSES: Sediment particle 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 was 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. Total carbon (TC), total inorganic carbon (TIC), and total organic carbon contents of bulk sediment were determined coulometrically (CM 5015 coulometer; UIC, Inc.) at the USGS Pacific Coastal and Marine Science Center sediment lab. Approximately 1 gram of sediment was dried overnight at 80 degrees Celsius and pulverized by hand in a mortar and pestle. TC was determined by incineration at 950 degrees Celsius (UIC CM 5200 furnace) of 20 milligrams of dry sediment loaded into in foil capsules. TIC was determined by perchloric acid digestion (UIC CM 5130 acid digester) of 20 milligrams of dry sediment. TOC was calculated as the difference between TC and TIC. The calcium carbonate (CaCO3) content of sediment in weight percent was determined by multiplying TIC by 8.333, the mass ratio of CaCO3 to carbon.

RATIOS OF STABLE ISOTOPES OF CARBON 13/12 (d13C) AND NITROGEN 15/14 (d15N), and TOTAL CARBON TO TOTAL NITROGEN (C:N): Sediment d13C, d15N, and C:N ratios were determined at the University of California at Santa Cruz Stable Isotope Lab using a CE Instruments NC2500 elemental analyzer interfaced to a ThermoFinningan Delta Plus XP isotope ratio mass spectrometer. Except for Bellingham Bay sediment collected in 2017, samples were decarbonated by fumigation with sulfurous acid prior to d13C analysis to remove inorganic carbonates. Sediment d15N was determined on unacidified samples. Sediment d13C ratios were referenced to the Vienna PeeDeeBelemnit and d15N ratios were referenced to air.

SHORT-LIVED COSMOGENIC RADIONUCLIDE ACTIVITIES: Prior to quantification of short-lived radionuclide activities, bulk sediment was oven-dried at 60 degrees Celsius, ground by hand in a mortar and pestle to a homogeneous grain size and packed into 10 milliliter graduated polystyrene vials. Vials were capped and sealed to prevent gas exchange. Samples and reference materials with the same geometry were counted for 24 hours in a high purity, low background germanium well detector at the USGS Pacific Coastal and Marine Science Center. Beryllium-7 activities were counted at 476 kiloelectron volts (keV), Cesium-137 at 661 keV, Lead-210 at 46.5 keV, and Radium-226 as the average of indirect peaks at 351.87 keV (Lead-214) and 609.31 keV (Bismuth-214). Unsupported, or excess, Lead-210 was calculated by subtracting supported Lead-210 (assumed to equal average Radium-226 activity) from total Lead-210.

MAJOR OXIDES PERCENTAGES AND MAJOR MINOR, AND TRACE ELEMENT CONTENTS: 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 size fraction. Sieved 2-gram samples were sent to the Analytical Chemistry Project of the USGS Central Mineral and Environmental Resources Science Center (CMERSC). Major oxides percentages were determined at CMERSC by wavelength dispersive x-ray fluorescence spectrometry (WDXRF). Sediment was fused with lithium metaborate/lithium tetraborate flux and the resultant glass disk was irradiated by x-rays tube. For major, minor, and trace element contents, CMERSC contracted AGAT Laboratories, a nationally accredited laboratory, for analyses by two methods: a near-total four-acid digestion and a total digestion with a sodium peroxide fusion for elements hosted in acid-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.