Radon in water

This application note introduces a fast and simple method to determine the Radon activity concentration of water samples. Nothing else is required than a Radon monitor like RTM 1688-2, RTM 2200, or RTM 2200 Soil Gas from SARAD GmbH and a bubbling flask.

Physical Basics

The measurement of the Radon activity concentration of a water sample is based on the equilibrium state between the Radon air and water activity concentrations which takes place within a sealed system after a certain time.

The ratio between the activity concentrations depends only on the temperature of the water sample. This dependence can be expressed by the so-called Oswald coefficient which points out that the solubility of the Radon in water decreases if the water temperature increases. That means, higher water temperatures result in higher Radon concentrations within the air.

1) K_Oswald = C_Rn(Water)/C_Rn(Air)

The Oswald-coefficient can be approximated for the temperature range between 0 to 40°C by the following function:

2) K_Oswald = 0,425 * EXP(0,05 * -Temperature in °C) + 0,1

The absolute concentrations within the sealed system are dependent on the original concentration of the water sample and on the ratio between the water and air volumes.

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