Separation of Photochemical and Non-Photochemical Diurnal In-Stream Attenuation of Micropollutants

For a better process understanding of in-stream attenuation of trace organic contaminants (TrOCs), quantitative comparisons between field studies under different environmental conditions and controlled laboratory experiments are important to separate different processes. However, this is hampered by the challenge to transfer kinetics from the laboratory to different field conditions due to the lack of good quantitative measures to account for different boundary conditions. For phototransformation, in situ light conditions in a river are difficult to determine because light is reduced, for instance, by absorption, scattering on suspended particles, and shading effects. In this study, we present an approach to separate photochemical from non-photochemical diurnal in-stream attenuation based on rate constants relative to diclofenac, as a reference compound, to account for the difference in the in situ light conditions combined with laboratory experiments. 12 out of 45 detected target TrOCs showed a diurnal attenuation at a selected river stretch. A non-photochemical process, potentially biotransformation, was responsible for the diurnal attenuation of bisoprolol, metoprolol, O-desmethylvenlafaxine, tramadol, and venlafaxine. Attenuation of amisulpride, flufenamic acid, hydrochlorothiazide, naproxen, and xipamide can be quantitatively explained by phototransformation, partially for sotalol. Attenuation rate constants of hydrochlorothiazide at different field sites from this study and from published data range over 2 orders of magnitude. Differences can be quantitatively explained by different light exposures but not by water chemical parameters.

Automated summary

In order to understand the in-stream attenuation of trace organic contaminants (TrOCs), comparing field studies under different environmental conditions and controlled laboratory experiments is crucial, but transferring kinetics from the lab to field conditions is challenging due to the lack of quantitative measures for different boundary conditions. This study presents a method to differentiate between photochemical and non-photochemical diurnal in-stream attenuation, with biotransformation potentially responsible for the diurnal attenuation of certain TrOCs and phototransformation explaining the attenuation of others, showing a range of attenuation rate constants at different field sites which can be explained by light exposure.