0 1994, by the Am&can Society of Limnology and Oceanography, Inc. Potentials of photolytic rhodamine WT as a large-scale water tracer assessed in a long-term experiment in the Loosdrecht lakes

The dispersal of 0.13 kg of water-tracer rhodamine WT in the Loosdrecht lakes was surveyed during 19 months. Solid-phase extraction and liquid chromatography were used to measure the very low con-centrations of rhodamine WT in the experiment. The detection limit was-2 x lo-l1 kg m-3. The photolysis constant of rhodamine WT was measured in a laboratory experiment. A numerical model for hindcasting the dilution of a photolytic tracer in the lake system was developed. Model results compare well with observations showing that rhodamine WT is well suited for tracer experiments on large time and space scales, provided that photolysis is taken into account. In densely populated and industrialized regions, those responsible for the conservation of the natural environment encounter various water-quality problems. These problems in-volve not only sudden contaminations of aquatic systems by industrial accidents but also more gradual deterioration (e.g. due to contin-uous releases of nutrients and toxic chemicals). Appropriate measures require knowledge of the transport mechanisms applying to both sud-den and continuous releases of various chem-ical substances. Tracer experiments are a pow-erful tool for investigating these mechanisms and validating transport models (e.g. Carter 1974; Leibundgut 1982; Van Dam 1982). Be-cause of the limitations of earlier fluorometric detection methods (e.g. imposed by high back-ground fluorescence) and the strong dilution of tracers in the surface water, useful obser-vation periods of fluorescent tracers have gen-erally been restricted to a few days. Studies of l Present address: IWACO B.V., Consultants for Wate