Development of a sparging chamber for field radon analysis

Radon-222 has become a widely used tracer of submarine groundwater discharge. However, remote field studies are often limited by the need to pump water to a spray chamber which degasses dissolved radon for subsequent analysis in the gaseous phase. We develop here a new method of degassing dissolved 222Rn, utilizing a stream of bubbles driven by the internal air pump of a commercial radon analyzer to achieve air:water partitioning equilibrium, eliminating the need to pump water. This system utilizes a sparging chamber, comprised of a slotted vertically-oriented pipe with bubbles produced in the bottom. A non-slotted section of the pipe at the top of the chamber forms a sealed headspace, allowing air to be circulated in a closed loop between the sparging chamber and a radon-in-air monitor. We found that such a sparging chamber needs to allow bubbles to rise through at least 45 cm of water column to function at equal efficiency as the standard protocol of the spray chamber. Under our optimized configuration, the sparging chamber operates as efficiently as the standard protocol at measuring dissolved 222Rn activities when encountering increasing 222Rn activities, and offers even greater gas exchange efficiency when dissolved 222Rn activities decrease. The sparging chamber offers a more field-friendly alternative to measuring 222Rn activities, as it eliminates the need to maintain a submersible pump throughout the measurement and it offers increased temporal resolution when variable 222Rn activities are expected. (author)