Award: OCE-1028990

This project developed a state-of-the-art system capable of measuring dissolved radon-222 activities under the water. Radon-222 is a naturally-occurring radioactive gas that serves as a useful tracer of groundwater that has recently seeped into ocean waters. Such seepage has been shown to deliver substantial amounts of nutrients, trace metals, and other contaminants into ocean waters and therefore may exert a strong control over nearshore ocean ecosystem health. Prior to this project, our understanding of where such groundwater inputs to ocean waters occur has been limited due to our inability to measure small-scale discharge signals (e.g., radon-222) near the source in open waters. This project aimed to fill this analytical need by developing a system capable of measuring radon-222 activities continuously from a submersible vehicle, which allows the analytical system to measure very near the seabed where the discharges occur. This project was led by researchers at Coastal Carolina University (CCU) and Woods Hole Oceanographic Institution (WHOI). The CCU efforts of the project developed advancements in methods to transfer a dissolved gas like radon into an air phase from which its concentrations could be measured. The WHOI side of the project integrated this degassing system with electrical and physical engineering advancements to construct the entire submersible radon system (named the æRADXÆ). A number of ocean engineering design challenges were overcome in order to achieve a fully submersible radon detection system. Through several iterations, we miniaturized both the electronics and the high voltage detector domes to achieve a system compact enough for submersible deployment. As the radon system is open to the sea and exposed to full ocean pressure, we developed a carrier gas passive compensation system that adds carrier gas to the system as needed during descent and allows it to escape during ascent. As the carrier-gas is directly sparged through seawater we developed a compact sparging chamber that mounts laterally and directly to the underside of the instrument housing, deflects seawater spray and bubble films from the return air-flow to the instrument, and contains a water detector that shuts down the carrier gas flow if water enters the detector system. We optimized this system through repeated testing in the laboratory under a variety of pressures, detector voltages, and signal gains. We deployed this system in Waquoit Bay for operational submerged testing. The final version is 1.2 m long, 20 cm in diameter, a size that allows for a range of deployment options including on moorings, by diver, and by remote and autonomous ocean vehicles. With the introduction of the RADX, researchers will be able to better understand how groundwater seepage locations and associated delivery of nutrients may impact oceanic ecosystems (for example, around coral reef settings). Therefore, this tool will be useful to researchers across a broad spectrum of oceanographic disciplines who are interested in delivery of nutrients, trace metals, and chemical contaminants from this unseen pathway. Additionally, a number of undergraduate and graduate students at Coastal Carolina University, Smith College, and Woods Hole Oceanographic Institution received valuable research experience by participating in this project to develop a new analytical system. We have presented the need for such an analytical system and the progress of our instrument design at a variety of regional, national, and international conferences. Last Modified: 12/02/2015 Submitted by: John A Breier