Use of radionuclides in the study of contaminant cycling processes

Abstract

Of all the geochemical boundaries, the sediment-water interface an exert the greatest control on the cycling of many elements in shallow aquatic environments such as lakes, rivers, estuaries and coastal embayments and, to a lesser extent in the deep sea. Across this interface, the gradients in physical properties (i.e. density), in chemical conditions (i.e. pH, pE, ligand concentrations), biota abundance (i.e. fauna and flora living near the interface) are large, thus producing potentially large fluxes. Some of the physical, chemical, biological and sedimentary controls operating at or near these interfaces can be deciphered from the measurements of natural radioisotopes (e.g. U/Th series or cosmic-ray produced), bomb fallout isotopes or, most recently, fallout from the Chernobyl-reactor accident. Commercially available, reactor-produced isotopes are most often used in enclosures to elucidate the coupling of the various processes at the sediment-water interface, while the former are used both as geochronological tools in the sediments and as tracers to measure or calibrate the rates of exchange across this interface of nutrients or trace elements associated with water or particles.

Applications of radioisotopes for studying biological, physical, chemical and sedimentary processes near the sediment-water interface are discussed. In particular, multitracer approaches to study the dynamic coupling of physical, chemical and biological transport processes in lakes are emphasized. Examples from two hard-water lakes in Switzerland, Lake Biel and Lake Zürich, give evidence for the resuspension of fine (‘rebound’) particles, radionuclides and trace metals from the horizontal boundaries focussing them to their final repositories in the interior of the lake.