Improved Automated Analysis of Radon ( 222 Rn) and Thoron ( 220 Rn) in Natural Waters NATASHA DIMOVA,* ,† WILLIAM C. BURNETT, † AND DEREK LANE-SMITH ‡ Department of Oceanography, Florida State University, Tallahassee, Florida 32306, and Durridge Co., Inc., 7 Railroad Avenue, Suite D, Bedford, Massachusetts 01730 Received July 9, 2009. Revised manuscript received August 30, 2009. Accepted September 1, 2009. Natural radon ( 222 Rn) and thoron ( 220 Rn) can be used as tracers of various chemical and physical processes in the environment. We present here results from an extended series of laboratory experiments intended to improve the automated analysis of 222 Rn and 220 Rn in water using a modiﬁed RAD AQUA (Durridge Inc.) system. Previous experience with similar equipment showed that it takes about 30-40 min for the system to equilibrate to radon-in-water concentration increases and even longer for the response to return to baseline after a sharp spike. While the original water/gas exchanger setup was built only for radon-in-water measurement, our goal here is to provide an automated system capable of high resolution and good sensitivity for both radon- and thoron-in-water detections. We found that faster water ﬂow rates substantially improved the response for both isotopes while thoron is detected most efﬁciently at airﬂow rates of 3 L/min. Our results show that the optimum conditions for fastest response and sensitivity for both isotopes are at water ﬂow rates up to 17 L/min and an airﬂow rate of 3 L/min through the detector. Applications for such measurements include prospecting for naturally occurring radioactive material (NORM) in pipelines and locating points of groundwater/surface water interaction. 1. Introduction A recently developed automated radon-in-water measure- ment system (1) based on the commercially available RAD7 radon-in-air monitor, product of Durridge Inc., has triggered extensive coastal oceanography studies using radon as a tracer. This variation has been used by oceanographers in two different modes. In one conﬁguration we record changes in the radon-in-water concentration over time at a ﬁxed point. We then estimate groundwater discharge using a simple radon mass balance box model to account for all sources and sinks in the seepage area (2-5). The same instrumenta- tion may also be combined with GPS navigation, depth sounding, and sensors for salinity, conductivity, and tem- perature and set up on a small boat for “mapping” the shoreline for groundwater sources (6, 7). Our mapping experience shows that the conventional RAD AQUA system does not respond quickly to sudden changes in the radon- in-water concentration in areas with multiple distinct sources such as submarine springs. Natural waters may be enriched in both radon isotopes if there is a source but their distributions are inﬂuenced by their respective half-lives and mixing of water masses. While the half-life of 222 Rn (t 1/2 ) 3.82 days) is long enough so that its concentration could be maintained during transport over relatively long distances, 220 Rn (t 1/2 ) 55 s) dissipates very quickly. This provides an opportunity to prospect for 220 Rn- enriched sources such as groundwater discharges using 220 Rn as a tracer. Just the presence of 220 Rn in coastal waters would indicate that one must be close to a source. Thoron may be used as prospecting tool for locating 228 Ra- bearing scale deposits in old drinking water systems. Such an approach includes 220 Rn measurements at points along a pipeline, or at a single site while varying the water ﬂow rate (8). Radium-bearing scale deposits have been widely reported over the past decade in both drinking water distribution systems (9-11) and in the oil- and gas-production industry (12). Determination of the precise location of radioactive scale could avoid expensive and unnecessary remediation. In this paper we present results from extensive laboratory experiments in an effort to optimize the parameters for simultaneous detection of both 222 Rn and 220 Rn while varying water and air ﬂow rates through two different variations of the conventional RAD AQUA system. 2. Experimental Section The designs for our experiments were driven by two main goals: (1) to shorten the response time for 222 Rn-in-water measurement, and (2) to increase the sensitivity of 220 Rn detection while maintaining high 222 Rn efﬁciency. The response of the system will be a function of the water-to-air and air-to-water exchange that occurs in the mixing chamber of our device (Figure 1). Higher water ﬂows will presumably deliver the radon gas faster to the system while higher airﬂows will promote mixing and delivery to the detector. This will be especially important for thoron detection because of its very short half-life. To evaluate the inﬂuence of the water and gas ﬂow rates we performed a series of experiments with a modiﬁed version of the commercially available RAD AQUA setup. This setup includes a radon-in-air monitor which uses a solid state passivated ion-implanted silicon (PIPS) detector, and thus has the ability to electronically determine the energy of each R particle. The RAD7 groups the spectrum’s 200 channels into 8 separate “windows”. Window “A” covers the energy interval of the 218 Po (E ) 6.00 MeV), the ﬁrst 222 Rn daughter, while the direct daughter of 220 Rn, 216 Po (E ) 6.78 MeV) appears in the range of window “B”. This makes a concurrent detection of 220 Rn and 222 Rn with the same instrumentation possible. Previous experiments (6) to evaluate these factors used water ﬂow rates up to 5 L/min. To be able to process water ﬂow rates larger than this through the system we employed a larger nozzle (BETE MP187W, BETE Fog Nozzle, Inc.) mounted directly to the head of a regular RAD AQUA exchanger and used a high-capacity Redi-Flo Variable Frequency Drive (VFD) submersible pump (Redi-Flo2, GRUNDFOS Pumps Corporation) to deliver the water. Because much larger water volumes were processed, the regular gas-mixing chamber base was replaced by a larger volume receiver. In addition, we also used an external air pump (UNMP850 KNDC-B, KNF Neuberger Inc.) to circulate the radon-enriched air between the mixing chamber and the RAD7. * Corresponding author e-mail: dimova@ocean.fsu.edu; tel: +850- 644 9914; fax: +850-644 2581. † Florida State University. ‡ Durridge Co., Inc. Environ. Sci. Technol. 2009, 43, 8599–8603 10.1021/es902045c CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 22, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8599 Published on Web 09/28/2009