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Naturally occurring 222Rn is increasingly recognized as a powerful environmental tracer in hydrology. Radon-in-water concentrations can be measured in the field by stripping radon from a water sample into a gas volume and measuring the respective radon-in-gas concentration using a portable radon-in-gas monitor. Alternatively, radon is firstly extracted from the water body by diffusion through a radon exchanger such as polypropylene (PP) tubing and then is measured using a radon-in-gas monitor, connected in closed-loop to the PP membrane. The paper discusses results of field experiments in which the Radon-in-Water Probe (Durridge co.), a 2.2 m long PP tubing, connected to a RAD7 monitor (method A), is used to determine dissolved radon concentration in four water bodies characterised by different water flow velocity and radon concentration. The efficiency of this method is validated by comparison with two established methods, gamma-ray spectrometer + charcoal canister (method B) and RAD7 monitor + Big Bottle RAD H20 accessory (method C). Relative efficiency of method A is directly proportional to water flow velocity, ranging from about 0.50 ± 0.05 at 0.01 m/s to about 0.92 ± 0.08 at 0.57 m/s. A minimum of 2–3 h are needed to collect enough records to asymptotically fit radon-in-gas data and obtain equilibrium radon concentration, which is then converted into radon-in water concentration, considering the temperature-dependency of radon partition coefficient between water and air. Equilibrium condition is reached after about 6–8 h. No correlation was found between relative efficiency and radon concentration. An equation is proposed to correct radon data as a function of water flow velocity, even for poorly moving water bodies. The DURRIDGE Water Probe is useful to monitor radon-in-water levels, without the potential risk of radon loss during water sampling and sample handling. However, it must be pointed out that duplicate or triplicate sampling using other methods similarly permit to evaluate whether radon loss is an issue.

Glass bottles are generally employed for water sampling because glass is impervious to radon and is not lost during sample storage. On the other hand, glass is fragile and may break, so 1 L High Density PolyEthylene (HDPE) bottles (Thermo Scientific Nalgene) are tested in place of glass vessels employing Big Bottle RAD H2O device (Durridge Company) coupled with RAD7 monitor. The purpose of this calibration is to quantify radon loss during storage in HDPE bottles, evaluate possible radon uptake by known volume of desiccant (Drierite, granular CaSO4) and quantify radon interaction with the rubber and plastic parts of the experimental circuit. These processes have been separately investigated, performing proper experiments for the assessment of their influence on resulting radon data using seven series of solutions at known activity concentrations in the range from 27 to 194 Bq/L. Percent radon loss during storage in 1 L HDPE bottles has been estimated at 0.0045 min−1. Radon absorption by desiccant, expressed as ‘equivalent’ volume of Drierite is 0.673 ± 0.092 L and is somehow independent, within errors, from i) the amount of water already absorbed in Drierite, ii) a recirculation time greater than 30 min and iii) radon concentrations. Radon absorption/desorption from rubber and plastic parts of the experimental device has been assessed as a function of concentration gradient between the inner volume of the circuit and the pores of polymer's. A final algorithm accounting for the above described physical processes has been developed for long runs (2–3 h). A simplified calculation method for short measurements (30 min) is also provided.