Short-Lived Radium-Isotope (Radium-223 and -224) Specific Activity for Samples Collected Between November 2022 and March 2024 Along the West Florida Shelf (Indian Rocks Beach, Nature Coast, and Venice Headland)

Six separate sampling activities in November 2022, January-February 2023, May-June 2023, September 2023, December 2023, and February-March 2024 from USGS FAN 2022-340-FA (altFANs 22WFS05, 23WFS01, 23WFS02, 23WFS03, 23WFS04, and 24WFS01) were made at Indian Rocks Beach, Hudson, and Venice (Florida). Throughout all field activities, samples from water column (surface water, bottom water) and groundwater wells were collected for the determination of radioisotopes of Radium-223 and Radium-224. Surface water is operationally defined as the upper one meter of the water column and bottom water is operationally defined as one to two meters above the seafloor.

SPCMSC researchers extracted radium isotopes from the water column samples using a known volume of water (Moore and Reid, 1973). When a large volume of water is exposed to Mn-fibers at flow rates less than two liters per minute (L/min), naturally occurring radium isotopes are quantitatively adsorbed onto the high surface area of hydrated manganese-oxides bound to the fibers producing a representative sample of the radium in surficial seawater (Moore and Reid, 1973; Moore and Arnold, 1996; ASTM International, 2017). Direct sampling involves the Mn-fiber encountering a known volume of collected water. During all sampling activities samples were collected using a direct method with a quantitative volume of water passed across the fiber. For groundwater sampling, 2 L of water was gravity-fed across 20 grams (g) Mn-fiber housed in a flow-through column. For 22WFS05 and 23WFS01 only, water column samples were collected and extracted using a flow through cell system in the field; surface samples for the remainder of the sampling activities were collected in one or more, field rinsed 20-L cube containers and extracted by gravity after returning to shore. Volumes were recorded to the nearest 0.1 L for groundwater samples and 1 L for water column samples; uncertainties were 0.1-0.2 L and 1-2 L, respectively. The date and time of collection and extraction were recorded for each sample.

Mn-fibers were analyzed for Radium-223 and Radium-224 using the Radium Delayed Coincidence Counter (RaDeCC) and methods described in Moore and Arnold (1996). The RaDeCC's ability to measure Radium-223 and Radium-224 isotopes relies on the short-lived radon daughters, Radon-219 and Radon-220, respectively. Mn-fibers were connected to the RaDeCC in a sample cartridge which was contained within a closed helium-circulated loop. Helium gas strips the short-lived Radon daughters from the fiber and is used as a carrier gas to transport these particles to a Zinc-Sulfide coated alpha scintillation vessel or cell (Ralph and Arnold, 1996). Flow of helium was regulated to allow Radon-219 (half-life of 3.96 seconds) and Radon-220 (half-life of 55.6 seconds) to undergo alpha decay into Polonium-215 and Polonium-216 within the cell. The base of the scintillation cell was coupled to a photomultiplier tube, pre-amplifier, and a high voltage supply sensitive enough to detect photons generated from the interaction of the alpha particles from polonium decay and the scintillator. These analog events and their timing are passed through a delay coincidence circuit that has been tuned to differentiate the mean statistical occurrence (time) of each polonium isotope. The analog counts for each polonium isotope passing for through the circuit is converted to a digital signal that is then passed to RaDeCC Software (Scientific Computer Instruments, 2016) computer program via a universal serial bus (USB).

Prior to the analysis of Radium-223 and Radium-224, each system's background radiation level was measured by circulating ambient air through the system. Mn-fiber samples were rinsed with radium-free, Milli-Q© deionized water to remove sea salts and the fibers masses were adjusted to roughly 50-100% total moisture content to ensure maximum emanation of radon from the Mn-fibers (Sun and Torgersen, 1998). The weight was recorded, and fibers were gently pulled apart to increase surface area and placed into sample cartridges. The cartridges were snapped into holders and the top flow tubing attached to the cartridge. Ultra-High Purity Helium ('helium' hence forth) was introduced to the system at the lower port of the column purging the system until all ambient air had been displaced (purged) as indicated by a stable flow rate of approximately 0.4 L per minute. Helium is superior to ambient air as a carrier gas in the counting chamber because more alpha particles from radon decay can reach the walls of the chamber and interact with the zinc-sulfide, thus dramatically increasing the systems efficiency (Moore, 2008). Once the system is saturated with helium and a stable flow rate reached, the helium hose was removed and quickly replaced with the system return flow hose to create a closed-loop system with the sample and the instrument. The pump was turned on and the analysis was started on the RaDeCC Software (Scientific Computer Instruments, 2016). The radium-radon-polonium isotopes within the closed system were allowed to reach secular equilibrium, which provided radon and polonium progeny half-lives takes 5 minutes. After this period, the RaDeCC Software (Scientific Computer Instruments, 2016) test was reset and left to count until Radium-220 counts reached a minimum of 300 counts or until the test reached 240 minutes, which ever came first (Diego-Feliu and others, 2020).

Raw Radium-223 and Radium-224 data were exported off the RaDeCC software (Scientific Computer Instruments, 2016) into a text file and included: background count rates, count time, Radon-219 and Radon-220 count rates, Radon-219 and Radon-220 total counts, and combined total counts. Determination of Radium-223 and Radium-224 via the delayed coincidence counter follows the procedure from Diego-Feliu and others (2020) and previously outlined by Moore and Arnold (1996). The count rates for the Radon-219 and Radon-220 circuits were corrected for chance coincidence events using the formula found in Giffin and others (1963). Giffin and others (1963) based this formula on total count rates and the time each circuit is open. Corrected Radon-219 and Radon-220 count rates were then determined by subtracting the correction factor from the raw count rates. Another adjustment to the Radon-220 data was required, due to Radon-219 and its daughter, Polonium-215. It is possible that two Radon-219 molecules decay while the Radon-220 window is open. This leaves the second Radon-219 and the Polonium-215 decays to be recorded in the Radon-220 channel. A second order correction must be made to produce a final Radon-220 count rate (Moore and Arnold, 1996). This second order correction is not needed for the Radon-219 channel (Moore and Arnold, 1996). Radium-223 and Radium-224 count rates were calculated using the final Radon-219 and Radon-220 count rates, respectively, and adjusting for decay, system efficiency, sample volume, and background measurements using the formula: Cx = (CRFinal x / [ DF * Ex]) * (100/V). Cx is the concentration of the desired isotope, CRFinal x is the final count rate of the respective circuit, DF is the decay factor, Ex is the system efficiency, and V is the sample volume (Moore and Arnold, 1996; Diego-Feliu and others, 2020). Data treatment follows that outlined in Diego-Feliu and others (2020) and contains sample sensitivity and uncertainty statements as well as metrics quantifying detection and determination limits (McCurdy and others, 2008) as defined in the accompanying data dictionary (Data_Dictionary_Radium_Measurements.docx) included with this data release.