Kinetics of the Water/Air Phase Transition of Radon and Its Implication on Detection of Radon-in-Water Concentrations: Practical Assessment of Different On-Site Radon Extraction Methods

Abstract

The on-site measurement of radon-in-water concentrations relies on extraction of radon from the water followed by its detection by means of a mobile radon-in-air monitor. Many applications of radon as a naturally occurring aquatic tracer require the collection of continuous radon concentration time series, thus necessitating the continuous extraction of radon either from a permanent water stream supplied by a water pump or directly from a water body or a groundwater monitoring well. Essentially, three different types of extraction units are available for this purpose: (i) a flow-through spray chamber, (ii) a flow-through membrane extraction module, and (iii) a submersible (usually coiled) membrane tube. In this paper we discuss the advantages and disadvantages of these three methodical approaches with particular focus on their individual response to instantaneously changing radon-in-water concentrations. After a concise introduction into theoretical aspects of water/air phase transition kinetics of radon, experimental results for the three types of extraction units are presented. Quantitative suggestions for optimizing the detection setup by increasing the water/air interface and by reducing the air volume circulating through the degassing unit and radon detector are made. It was shown that the flow-through spray chamber and flow-through membrane perform nearly similarly, whereas the submersible membrane tubing has a significantly larger delay in response to concentration changes. The flow-through spray chamber is most suitable in turbid waters and to applications where high flow rates of the water pump stream can be achieved (e.g., where the power supply is not constrained by field conditions). The flow-through membrane is most suited to radon extraction from clear water and in field conditions where the power supply to a water pump is limited, e.g., from batteries. Finally, the submersible membrane tube is most suitable if radon is to be extracted in situ without any water pumping, e.g., in groundwater wells with a low yield, or in long-term time series, in which short-term variations in the radon concentration are of no relevance.

Utility of 222Rn as a passive tracer of subglacial distributed system drainage

“…Continuous (hourly) measurements were made using a RAD7 (Durridge Inc.) radon-in-air monitor in series with a desiccant chamber and a RAD7 water probe, a submersible air-filled gas-permeable membrane coil made from Accruel ® tubing (henceforth the term "water probe" will refer to the 222 Rn extraction unit). River water 222 Rn equilibrates with air in the membrane coil through passive diffusion; the air is continuously circulated in a closed loop through the RAD7 radon-in-air monitor system (Hofmann et al, 2011;Schubert et al, 2012). Radon-in-air activities were then converted to radon-in-water activity via the temperature dependent airwater partition coefficient as described by Schubert et al (2012 (Schubert et al, 2012).…”

“…River water 222 Rn equilibrates with air in the membrane coil through passive diffusion; the air is continuously circulated in a closed loop through the RAD7 radon-in-air monitor system (Hofmann et al, 2011;Schubert et al, 2012). Radon-in-air activities were then converted to radon-in-water activity via the temperature dependent airwater partition coefficient as described by Schubert et al (2012 (Schubert et al, 2012). To assess the magnitude of this offset, we analyzed field equilibration times during instrument startup and performed laboratory experiments to estimate the water probe's response time to changes in 222 Rn activity.…”

“…To assess the magnitude of this offset, we analyzed field equilibration times during instrument startup and performed laboratory experiments to estimate the water probe's response time to changes in 222 Rn activity. Key factors that control the equilibration time include the air volume in the system (RAD7 unit, tubing, desiccant and membrane coil), the membrane coil surface area, and the 222 Rn activity gradient across the membrane water/air interface (Schubert et al, 2012). The equilibration time can be minimized when the air volume of the system is low (i.e.…”

“…The 222 Rn activity gradient at the membrane water/air interface has by far the largest effect on the equilibration and response time of the water probe; in stagnant water the response time is on the order of several hours (Hofmann et al, 2011;Schubert et al, 2012). In a flowing river, the activity gradient at the water/air interface is close to 100% such that the response time can be reduced to <1 h, similar to results obtained using a spray chamber equilibrator such as the RAD-Aqua (Durridge Inc.; Schubert et al, 2012). In our case, the equilibration time of the water probe in the proglacial river was determined to be no greater than 2 hours based on laboratory experiments and less than 1 hour in field observations.…”

Submarine Groundwater Discharge at a Single Spot Location: Evaluation of Different Detection Approaches

“…The two parameters show the advantage of straightforward on-site detectability [10,15]. The radionuclide radon-222 ( 222 Rn, hereinafter referred to as radon) is of particular suitability in the given context, since it is short-lived (half-life 3.82 d) and chemically inert, which makes it a preferable dissolved tracer in the field of hydrological sciences [13,[16][17][18][19][20].…”