Natural radon as a limnological tracer for the study of vertical and horizontal eddy diffusion

Radon-222 (half-life 3.8 d) has been used successfully as a geochemical tracer for vertical near bottom mixing in the ocean. The parent nuclide radium-226 (half-life 1600 a) occurs in far greater quantities in sediments than in the water column, thus providing a boundary source for emanation of radon. Vertical mixing in lakes may be of central importance for the evolution of chemical and biological processes. Most lakes pass through a stagnation period during which the euphotic zone continuously loses nutrients by sedimentation of plankton through the thermocline. The return flux of nutrients from the sediments through the hypolimnion and thermocline to the euphotic layer can only be understood and quantified if vertical mixing processes are known. The traditional means by which vertical eddy diffusion is calculated is the temperature method. However, temperature changes near the bottom of deep lakes are often too small to be measured. Among various (natural or man-made) geochemical tracers radon-222 seems to be especially suitable for the study of vertical mixing since its 'memory' of about one week very often allows measured activities to be interpreted in terms of a relatively simple steady-state model