Retrospective trend analysis of biocides in suspended particulate matter of major German rivers

Due to their intrinsic biological activity biocides can pose an unintended threat to various aquatic organisms. Monitoring data on the spatial distribution and temporal trends are needed to evaluate potential risks and the effectiveness of mitigation measures, but these are scarce for biocides in aquatic environments. In particular, even though many biocides tend to sorb to particles, there are only few studies investigating the contamination of suspended particulate matter (SPM). The aim of this study was to obtain an overview of the temporal trends of selected biocides in SPM using German rivers as an example. For this purpose, SPM from the German Environmental Specimen Bank was used for a retrospective trend assessment of a broad spectrum of biocides in integrated SPM samples (yearly composite) in six large German rivers between 2008 and 2021. Overall 16 of 23 analyzed biocides were found, whereof 10 substances were detected in all samples. Highest concentrations were found for quaternary ammonium compounds (QACs, the sum of four analyzed QACs were up to 8.7 µg/g) and methyl-triclosan (up to 280 ng/g), a transformation product of the bactericide triclosan. Considerably lower concentrations in the range of 0.08 to 88 ng/g and < 0.03 to 13 ng/g were detected for azoles and triazines, respectively. The pyrethroid permethrin, which is highly toxic to aquatic organisms (invertebrates: NOEC = 0.0047 µg/L; fish: NOEC = 0.41 µg/L) as well as to sediment-dwelling organisms (Chironomidae: LC50 = 2.1 mg/kg and NOEC 0.1 mg/kg), was detected at several sampling sites (up to 11.2 ng/g). Concentrations of the other analyzed pyrethroids were below the respective quantification or detection limits. In general, for most compounds, concentrations were higher for locations with higher wastewater proportion, but overall no clear differences in biocide concentration pattern between the different sampling locations were observed. For cybutryne and triclosan significant decreasing concentration trends were observed. This is consistent with regulatory use restrictions and confirms their effectiveness. For benthic organisms a toxicological risk from the individual azole fungicides and QACs seems to be low. Explicit differences between sampling sites and temporary changes in local concentrations indicate regional variations of biocide emissions which hamper identification of long-term concentration trends. Moreover, time trends could be affected by remobilization of legacy contamination from contaminated sites. Hence, for biocides a continuous long-term monitoring is crucial to identify the effectiveness of recent restrictions and mitigation measures.