Dataset: Dissolved long-lived radium isotopes for submarine groundwater samples collected from November 2022 and March 2024 along the West Florida Shelf

U.S. Geological Survey small boat operations along the West Florida Shelf were conducted quarterly from November 2022 to March 2024, for a total of six seasonal campaigns. Surface water (~20 L), bottom water (~20 L), and submarine groundwaters (1.4 – 2.2 L) were collected via peristaltic pump along three coastal transects after characterization using a calibrated handheld water quality sonde (YSI ProDSS; Chestang et al., 2025). Dissolved radium samples were filtered at <1L/min onto a MnO₂-coated acrylic fiber to adsorb Ra. Short-lived radium isotopes and water physicochemical parameters for the U.S. Geological Survey small boat operations were separately characterized (Chestang et al., 2025) and are not included in this report. The Mn-coated acrylic fibers were combusted at 850°C for 16 hours, homogenized, sealed with epoxy resin, and incubated for a minimum of three weeks until ²²²Rn was in secular equilibrium with its parent ²²⁶Ra. Long-lived Ra isotopes were measured via gamma spectroscopy in a Ge well-type detector for ²²⁶Ra (352 keV photopeak) and ²²⁸Ra (average of 338 and 911 keV photopeaks), with efficiencies calibrated using standards prepared in the same fashion as the estuarine samples in the related dataset (Charette et al., 2001).