Legacy pollution of floodplain soils with quaternary ammonium compounds - Insights into vertical distribution, historical trends and suspected microplastic carriers

Quaternary ammonium compounds (QACs) are antimicrobials and cationic surfactants used since the early 20th century but increasingly under scrutiny because of their biocidal properties and potential to induce antimicrobial resistance. Although recognized as aquatic contaminants, little is known about the entry, persistence and effects of QACs in floodplain soils. Due to their sorption to suspended particulate matter (SPM), we hypothesized that floodplains may have acted as sinks for QAC contamination in the aquatic-terrestrial interface for decades. Thus, we expected vertical QAC distributions in dated floodplain soil profiles to reflect historical emissions and flood deposits. Moreover, we hypothesized particle-associated entry with SPM and microplastics (MPs) to be a key input pathway. We therefore assessed the depth distribution of 31 QACs in two dated floodplain soils of the German river Lahn by high performance liquid chromatography-mass spectrometry after ultrasonic extraction. Correlation analysis between QAC and MP (>500 μm) concentrations and cluster analysis of QAC homologue patterns in SPM and different German soils were used to identify probable entry routes. QACs were detected down to 90 cm depth, corresponding to the 1920s-1930s, with peak concentrations in soil layers related to a recent hundred-year flood. Highest concentrations up to mg kg^-1 for the semi-quantified dimethyldioctadecylammonium tentatively exceeded ecotoxicological effect thresholds by twofold. Concentrations of several QACs correlated positively with MP contents while QAC homologue distributions showed similarity between SPM and floodplain topsoil segments, highlighting the importance of particle-associated entry. These findings confirm QACs as potentially persistent contaminants of ecotoxicological concern in floodplain soils.