TRACERS AS ESSENTIAL TOOLS FOR THE INVESTIGATION OF PHYSICAL AND CHEMICAL PROCESSES IN GROUNDWATER SYSTEMS

In complex environmental systems, tracers are indispensable tools for the investigation of various physical, chemical, and biological proces ses. From the large variety of tracers employed by EAWAG in the aquati c environment, we present some examples relevant to groundwater resear ch. Some tracers (e.g., Rn-222, H-3/He-3, chlorofluorocarbons) allow t he time since groundwater infiltration to be determined; this informat ion can be used to quantify process rates such as flow velocities, rec harge and reaction rates. Other tracers (e.g., conductivity, isotopes of oxygen or boron) can be used to quantify mixing ratios between wate rs of different origin, for instance, to study the admixture of leacha te from a landfill to an aquifer. In contrast to these inert tracers, chemically reactive tracers (e.g., nitroaromatic compounds) can be use d to study subsurface biogeochemical transformation processes, e.g. th e reduction of contaminants under anaerobic conditions. The successful use of tracers in the study of environmental systems requires highly developed analytical facilities for a broad palette of tracer measurem ents and a careful choice of appropriate tracers for each specific pro blem.