Dynamic behavior of fluorescent whitening agents in Greifensee: Field measurements combined with mathematical modeling of sedimentation and photolysis

The dynamic behavior of the two fluorescent whitening agents (FWAs) that are currently used in laundry detergents in Switzerland (DAS 1, a diaminostilbene, and DSBP, a distyrylbiphenyl) has been evaluated quantitatively during 1 year for Greifensee, a small lake in Switzerland, by using a one-box model and simulation software (MASAS) for modeling organic pollutants in lakes. Both the one-box model and the one-dimensional simulation were based on (i) independently evaluated processes affecting the fate of FWAs and (ii) measured FWA concentrations in the lake. The relevant processes were derived from literature data and from measurements in the lake and in its tributaries. Besides loading from tributaries, only photodegradation (responsible for 53% and 81% of the elimination of DAS 1 and DSBP, respectively), sorption/sedimentation (20% and 9%), and flushing (27% and 10%) were found to be relevant for FWAs in Greifensee. In particular, no evidence was found for other degradation processes such as biodegradation or hydrolysis. During 1 year, concentration depth profiles were measured above the deepest point of the lake in intervals of 4 weeks. The measured RNA concentrations were between 50 and 120 ng L-1 (DAS 1) and between 10 and 110 ng L-1 (DSBP), with maximum Values in the thermocline during summer. Evidence was found that these maximum Values originated from subsurface loading occurring in summer at levels of 4-8 m depth. This study is one of the first field validations for photochemical degradation rates measured in the laboratory.