RN-222 AS A GROUNDWATER TRACER - A LABORATORY STUDY

Radon-222 (Rn-222, 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 Ra-226, the progenitor of Rn-222. The radon method is based on the increase of the activity of Rn-222 in interstitial water with increasing residence time and assumes that the ingrowth can be described by the first-order 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., greater-than-or-equal-to 0.2 m day-1. Spatial differences in the radon production were observed and explained by a model of inhomogeneous distribution of Ra-226 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.