Distribution and fl uxes of fl uorescent whitening agents discharged from domestic wastewater into small rivers with seasonal changes of fl ow rates Abstract Distributions of fl uorescent whitening agents (FWAs) in the waters of a small river system in Japan were surveyed in summer when the water volume rose following high precipitation and in winter when it subsided as precipitation declined. The main source of FWAs in the river system was domestic wastewater, and fl uxes in the tributaries depended on the size of the residential population and the elimination rates of sewage treatment systems in their catchments, although FWA concentrations in the river itself fl uctuated, largely as a consequence of dilution by heavy precipitation and unstable water fl ows in the small-sized river system. The FWA concentrations in the river water during summer decreased not only by the dilution of river water but also by photodegradation because of more prolonged exposure to sunlight.
To understand the behavior of fluorescent whitening agents (FWAs) in a lake environment, we measured the quantities of two FWAs, DSBP, and DAS1, in water samples collected monthly from six depths of the water column, in sediment trap sample, and a sediment core sample from Lake Biwa, the largest lake in Japan, and in sewage, effluent, and river water in the lake's catchment. We conducted a sunlight exposure experiment and developed a method to estimate the degree of photodegradation by using DSBP/DAS1 ratio in environmental samples. The observed seasonal pattern of the vertical distributions of the FWAs in the water column can be explained by stratification of the water, photodegradation in the euphotic zone, the subsurface loading of river water, and their seasonal changes. The DSBP/DAS1 ratio was much lower in the lake water (0.12-0.52) than in sewage (6.4 ± 1.1), indicating intensive photodegradation in rivers and the lake. A mass balance calculation and DSBP/DAS1 ratio demonstrated that ∼95% of DSBP and ∼55% of DAS1 supplied in sewage were photodegraded in inflowing rivers and the lake, and that sedimentation to the lake bottom is insignificant for DSBP and ∼35% for DAS1. More intensive photodegradation of FWAs, especially more photodegradable DSBP, in Lake Biwa than in Greifensee, a lake in Switzerland, was suggested, attributable to the longer residence time of water in and the larger size of Lake Biwa. These results demonstrate that photodegradation is important to the fate of FWAs in lacustrine environments, and that FWAs and the DSBP/DAS1 ratio are useful markers for understanding the role of direct photodegradation in the behavior of water-soluble chemicals in aquatic environments.
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