Radon as a Tracer for Stratification Controls on Gas Fluxes in the Water Column

Understanding physical gas flux is essential to modeling biogeochemical processes, but it can be difficult to isolate. Radon-222 can serve as a tracer of physical transport as it is inert and yet easily detectable due to its radioactivity. The Martens’ Lab previously developed air-based radon detectors for use in forest canopies. This study aims to adapt them for use in aquatic environments using a custom-built water-air equilibration chamber, Nafion tubing, and a continuous air loop. This system was constructed and then deployed for testing at Jordan Lake, North Carolina. A profile of radon counts and stratification controls were collected at depths of 1, 3, and 3.5 meters. The adapted system successfully detected radon levels that aligned with expected gas transport patterns, with activities ranging from 0.1 in the surface water to 2.9 becquerels nearest the sediment. Short-term, wind-driven mixing events were detected, and radon levels responded as expected. When the lake was stratified, radon built up in the bottom water and was mixed out during the period where the lake was well-mixed. The response to these events was comparable to that seen by the optode sensors, indicating viability for monitoring short-lived mixing events. Over multiple weeks of deployment, the system delivered consistent results and showed no sign of sensor drift, showing the Nafion tubing is an effective standalone drying agent. Future work will focus on extending the length of data collection, improving vertical resolution, and integrating biologically active chemicals to develop flux models.