Measuring Radon Concentration in Water Based on Bubbling Method


 Radon is soluble in water, and the radon in drinking water is one of the sources of indoor radon. The China national standard for drinking water (GB5749-2006) indicates that the radon concentration in drinking water cannot be higher than 300pCi/L. The widely used method for measuring radon concentration in water is to use the RAD7 and the H2O accessory which provided by the Durridge Company for bubbling measurement. There are two kind sample bottles: 40ml and 250ml; the measurement steps: bubbling for 5 minutes, and measuring for 20 minutes. For the 40ml sample volume the conversion coefficient is around 25. For the 250ml sample volume the conversion coefficient is around 4. Since the price of the accessory is more than one thousand dollars, and the drying tube of the accessory is small, it needs to be replaced after each measurement which makes the operation is troublesome. We used a big drying tube to perform the radon concentration in water measurement. A new model for description the radon concentration in the gas circuit is proposed, and the conversion coefficient in any measurement condition is obtained. Any volume of sample bottle and drying tube can be used, selecting the suitable bubbling time and measuring time, the radon concentration in water can be obtained.


Introduction
Radon (Rn-222) is a chemically inert and odorless radioactive gas from the 30 uranium (U-238) decay chain (Tan et al. 2020). And its half-life is 3.8 days. It is one 31 of the most important naturally occurring radioactive nuclides and contributes to more 32 than 50% of the ionizing radiation dose of humans (UNSCEAR 2000). In addition, 33 atmospheric particles containing Rn decay products are of high relevance to humans 34 since they may be deposited in the respiratory tract, causing high radiation hazards 35 (Eichler B et al. 2000). Radon is soluble in water, which allows radon to enter the 36 house through groundwater aquifers and indoor pipelines. The radon gas in water is 37 mainly derived from the decay and release of radium dissolved in water. It should be 38 noted that around 1-2% of radon in the air comes from drinking water (USEPA 1991). 39 The sanitary standard for drinking water ( GB5749-2006) stipulates that the 40 concentration of radon in drinking water cannot be higher than 300pCi/L. Despite 41 being less of a health risk than its airborne counterpart, radon carried in water is often 42 a major transportation mechanism leading to high airborne radon concentration 43 (Welch LE. 1994). As a result of its noble gas configuration radon behaves 44 biologically and chemically inert. Due to that property, it is applied as environmental 45 tracer in hydrology, geochemistry, and oceanography for tracking of transport 46 pathways in aquifers and surface water systems and for the quantification of 47 admixture rates (Axel Schmidt et al. 2008). Especially for seismology, earthquakes 48 and other similar geological activities like volcanic explosion will affect the 49 concentration of radon in groundwater, which is usually measured for earthquake 50 prediction (Igarashi G et al.1995. Pradeep Talwani et al. 1980. Liu et al. 1984.

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Therefore, the measurement of radon concentration in water is a very meaningful 52 work. Based on different purpose and requirement, the radon-in-water measurement  The widely used device for bubbling to measure radon concentration in water is 66 to use the RAD7 and the H2O accessory which provided by the Durridge Company.

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For the 40ml sample volume the conversion coefficient is around 25. For the 250ml 68 sample volume the conversion coefficient is around 4 (https://durridge.com). Since 69 the price of the accessory is more than one thousand dollars, and the drying tube of 70 the accessory is small, it needs to be replaced after each measurement which makes 71 the operation is troublesome. We used a big drying tube to perform the radon   surface of the water sample to be tested.

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(2) Start the air pump and control the flow rate of the air pump to make air 91 bubble into the water sample through the intake pipe of the water sample bottle. The 92 generated bubbles will fully contact the water sample during the ascent process, and 93 the radon in the water will enter the bubbles through the diffusion effect. At the same 94 time, the radon in the water sample is carried out, and enters the drying tube through 95 the air outlet pipe of the water sample bottle, and then passes through the air pump, 96 and then enters the internal cell of theRAD7 radon monitor.

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(3) After several periods of measurement, the water radon concentration can be 98 obtained from the measurement value of RAD7.

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Assuming that the initial radon concentration in water is C0, the initial 100 concentration of radon and Po-218 in the air circuit of the device for measuring radon 101 concentration in water by the bubbling method is 0. Thus, the water sample measuring bottle was replaced with a bottle filled with charcoal before the experiment to adsorb the radon in the air loop. Using the pump of the RAD7, the air is continuously 104 circulated in the loop, and the radon concentration in the loop will reduce until it 105 drops to 0.

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Calculation process 107 Assuming the water radon concentration in the water sample measuring bottle is 108 C1(t), the radon concentration in the air circuit is C2(t), and the Po-218 concentration 109 in the internal cell of RAD7 is C3(t).

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Assuming that the radon concentration in the bubble has reached equilibrium 111 with the radon in the water during the process from the inlet pipe of the water sample 112 measuring bottle to the water surface, the following relationship exists between C1(t) 113 and C2(t): where V1 is the water sample volume; L is the flow rate of the pump. And X is 116 the ratio of radon in air to radon in water at equilibrium.
where T is the temperature of the water sample (ºC). So , the 120 value of X is related to temperature.
where V2 is the volume of air circuit.

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According the measurement principle of the RAD7, C2(t) and C3(t) have the Assuming the Sampling time is 1 t , and the internal cell of the RAD7 radon If L is large, If L is large enough, The pump flow rate of RAD7 is 0.7L/min. The conversion coefficient and the radon concentration in water can be obtained by 184 Eq. (10). Table 2 lists the radon concentration in water and the conversion coefficient  The results listed in Table 3 show that the radon concentrations in water in two