Sediment Radiochemical Data from Georgia, Massachusetts and Virginia Coastal Marshes

Scientists from VIMS and USGS PWRC collected 8 sediment push cores and 25 surficial sediment grabs from various marsh environments in Georgia, Virginia and Massachusetts between 2015 and 2019. The serrated marsh push cores collected were ES.A, ES.B (Mockhorn Island, VA), GW.B, GWLD.A (Goodwin Island, VA), PI.A, PI.B (Laws Point, Plum Island Estuary, MA), SA.A, and SA.B (South Altamaha, GA). The two sets of surficial sediment grabs were SA S1 through SA S10 (South Altamaha, GA) and GSS1 through GSS15 (Goodwin Island, VA). These samples were sent to the USGS SPCMSC for radiochemical and loss on ignition analysis of the sediments. The USGS Field Activity Numbers (FANs) 2019-302-CNT and 2019-306-CNT were assigned to these contract samples (ES.A, ES.B, PI.B, and GW.B) per the USGS SPCMSC data management requirements. The other cores and surficial sediment grabs were submitted for analysis before USGS SPCMSC data management required FANs to be assigned to contract samples.

The organic matter content for marsh push core GW.B was determined by loss on ignition (LOI), a mass loss technique. The sediment depth intervals submitted by VIMS scientists for analysis were pre-dried and ground. Approximately 1-5 grams (g) of sediment for each sample depth interval was added to a pre-weighed porcelain crucible. The mass of the sediment and crucible was recorded. The samples were then placed inside a laboratory muffle furnace with stabilizing temperature control. To remove hygroscopic water absorbed to sediment particles, the furnace was heated to 110 degrees Celsius (°C) for a minimum of 6 hours. Then the furnace temperature was lowered to 60 °C and the sediments were reweighed. Sediments were placed in the muffle furnace again and heated to 550 °C and held at that temperature for 6 hours. The furnace was again lowered to 110 °C and held at this temperature until the sediments could be reweighed. This step prevents absorption of additional moisture, which can impact the measurement. The mass lost during the 6-hour heating time in comparison to the 110 °C dried mass is used as a metric for organic matter content. The loss on ignition data is reported as a ratio of organic matter mass (g) to post-110 °C dry sediment mass (g). The limited amount of material for core GW.B did not allow for replicate analyses of the core intervals for quality assurance. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Total Lead-210 (Pb-210) activity was measured by alpha spectrometry for 25 surficial sediment grab samples (SA S1-SA S10 and GSS1-GSS15) and 8 marsh push cores (ES.A, ES.B, GW.B, PI.A, PI.B, SA.A, SA.B, and GWDL.A). Polonium-210 (Po-210, half-life = 138 days) is assumed to be in secular equilibrium with its parent Pb-210 (half-life = 22.3 years), allowing for the determination of the total Pb-210 activity in environmental sediments by directly measuring the activity of Po-210 through alpha particle decay. The USGS SPCMSC radioisotope laboratory uses a method that was originally developed by Martin and Rice (1981) to chemically separate Po-210 from sediments. This method utilizes polonium’s affinity to autoplate to silver planchets in order to isolate Po-210 for alpha counting (Flynn, 1968). To separate the Po-210, between 1.99 and 5 grams of dried, ground sediment sample was leached with 10 milliliters (mL) 16 Normality (N) nitric acid and a Po-209 tracer was added. The solution digested overnight and was then dried on a hotplate. The dried solution was washed with 5 mL of 8 N hydrochloric acid, followed by three washings with 30% hydrogen peroxide to break down organics. After two additional washings with 5 mL of 8 N hydrochloric acid, 5 mL of 8 N hydrochloric acid was added and once the sample dissolved completely the solution was brought to 50 mL by adding deionized water. Three reagents are added to buffer the pH of the solution during the deposition of Po-210 to the silver planchets. Both 20% hydroxylamine hydrochloride and 25% sodium citrate were added to reduce the inference of oxidants, such as Fe+3 and Cr+6 (Martin and Rice, 1981). A 2.2% weight by volume solution of bismuth nitrate was added to help prevent the deposition of Bismuth-212 (Martin and Rice, 1981). A calibrated handheld pH meter was used to monitor the pH of the solution. Ammonium hydroxide was added dropwise while mixing to achieve a pH of 1.85 to 1.95. The Po-210 was autoplated onto the 1.9-centimeter diameter sterling silver planchets as the solution was heated and stirred on a hotplate for 2 hours. All planchets had one side covered with tape to ensure Po-210 plating only occurred on one side. The planchets were removed from solution to be rinsed with deionized water and dried. The planchets were counted in low-level alpha spectrometers coupled to a pulse-height analyzer for 24 hours. Samples with low Po-210 activity were counted for 48 hours. The total Pb-210 activity and counting error are reported in the Excel spreadsheet. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Six dried, ground sediment push cores (ES.A, ES.B, GW.B, PI.B, SA.A, and GWLD.A) submitted by VIMS were analyzed for the detection of radionuclides by standard gamma-ray spectrometry (Cutshall and Larsen, 1986) at the USGS SPCMSC radioisotope lab. Selected depth intervals from each core were analyzed based on the results of alpha spectrometry. A total of 56 depth intervals were analyzed for radioisotopic activities. Analyzed sediments (5-50 g) from cores ES.A, ES.B, PI.B, SA.A, and GWLD.A were sealed in Teflon-taped, airtight polypropylene containers. Analyzed sediments (4.6 mL) from core GW.B were packed and sealed with epoxy in polypropylene graduated test tubes. The chosen sediment sample weights and volumes, for GW.B, and counting container geometries were matched to pre-determined calibration standards. Once sealed, the sediment samples remained for a minimum of 3 weeks before being analyzed to allow for Radium-226 (Ra-226) to come into secular equilibrium with its daughter isotopes Pb-214 and Bismuth-214 (Bi-214). Each sample sealed in a Teflon-taped, airtight polypropylene container was then counted for 24-72 hours on a 50-millimeter (mm) diameter planar-style, low energy, high-purity germanium, gamma-ray spectrometer. The GW.B samples sealed with epoxy in a test tube were counted for 24-72 hours on a 16x44-mm well-style, low energy, high-purity germanium, gamma-ray spectrometer. The USGS SPCMSC radioisotope lab gamma detectors measure the following anthropogenic and naturally-occurring radioisotopes, including their corresponding photopeak energies in kiloelectron volts (keV): Pb-210 (46.5 keV), Thorium-234 (Th-234, 63.3 keV), Pb-214 (295.7 and 352.5 keV; proxies for Ra-226), Bi-214 (609.3 keV; proxy for Ra-226), Cesium-137 (Cs-137, 661.6 keV), and Potassium-40 (K-40, 1460.8 keV). The sample counts were corrected for photopeak intensity, self-absorption and detector efficiency. Gamma detector efficiency was determined using International Atomic Energy Agency RGU-1 reference material. Sample specific self-absorption correction was determined using a Uranium-238 sealed source (Cutshall and others, 1983). The activities for Pb-210, Cs-137, and K-40 were decay corrected to the field collection date. The activities of Pb-210, Th-234, Cs-137, and K-40 were corrected for nuclide decay during the counting period. Radioisotope activities and counting error for all samples are reported in the Excel spreadsheet with the results for each core on separate tabs. The critical level is reported for each core. The critical level for Ra-226 is reported as the highest critical level amongst the 3 proxies (295.7 keV, 352.5 keV and 609.3 keV); therefore, the most conservative critical level for Ra-226 is provided. A comma-separated values data file containing the tabular data in plain text is included in the download file.

Added keywords section with USGS persistent identifier as theme keyword.