Cesium-137 isotope activity measured in sediment cores collected from Cargill Marsh, South San Francisco Bay, California during field activities 2022-643-FA and 2023-681-FA

Sediment cores were collected from the marsh environment on Cargill Marsh in Eden Landing Environmental Reserve, Alameda County, California using a Russian corer (a hand-held, side-filling peat auger). At each core site, two adjacent cores holes were used to sample sediment in 50 cm intervals with a 10 cm overlapping section resulting in undisturbed sediment cores of 90 cm length. Cores were transported back to the USGS Pacific Coastal and Marine Science Center for imaging, and subsequently sectioned in two cm samples for Cesium-137 analyses. Not all segments of the cores were analyzed.

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 and packed into 10 milliliter graduated polystyrene vials. Vials were capped and sealed to prevent gas exchange, then left to equilibrate for at least 21 days. Samples and reference materials with the same geometry were counted for 24 hours in a high purity, low background germanium (HPGe) well detector at the USGS PCMSC Radiochemistry Lab. Cesium-137 activities were counted at 661 kiloelectron volts (keV).