Radon-222 as a groundwater tracer. A laboratory study

Abstract

Radon-222 (²²²Rn; half-life 3.8 days) was used in a laboratory study for determining residence times and velocities of water flowing through an open-topped sand box. The quartz sand in the box contained natural trace amounts of ²²⁶Ra, the progenitor of ²²²Rn. The radon method is based on the increase of the activity of ²²²Rn in interstitial water with increasing residence time and assumes that the ingrowth can be described by the firstorder kinetics of secular radioactive equilibrium. The validity of this assumption was checked in two experiments. The results of the experiments showed (a) that the ingrowth of radon in a flowing water system can be adequately described and (b) that the radon method might also be applied to field studies in sandy aquifers with groundwaters of a relatively slow flow velocity, i.e., ≥0.2 m day^-1. Spatial differences in the radon production were observed and explained by a model of inhomogeneous distribution of ²²⁶Ra in the sand. Spatially different steady-state activities were used in the calculations which matched the observed data of the first three outlets slightly better than the first-order ingrowth approach.