Transport, retention and degradation of the urban biocides terbutryn, diuron and octylisothiazolinone and the herbicide acetochlor in urban surface-soil interfaces

Urban biocides, including terbutryn, diuron, and octylisothiazolinone (OIT), commonly leached from facade coatings during rainfall-runoff events, can infiltrate subsurface environments through permeable urban surfaces, posing a risk to groundwater quality. Despite frequent detection of biocides in urban groundwater, the transport and transformation processes occurring at the surface-soil interface remain insufficiently characterized. This study combines outdoor lysimeter experiments with reactive transport modeling and compound-specific isotope analysis (CSIA) to assess biocide leaching, retention, and degradation across three typical urban interfaces: gravel strips, permeable pavements, and vegetated soils. Leaching of biocides differed among lysimeters, with 10-17 % of terbutryn, 40-47 % of acetochlor, and 5-40 % of diuron released from the gravel and pavement systems, whereas leaching from vegetated soil was minimal (<0.5 %). OIT was not detected in any leachates, likely due to rapid degradation. Eight-year modeling further indicated limited retention in gravel and pavement, with up to 36 % and 22 % of the applied terbutryn mass, respectively, migrating to depths greater than 30 cm. In contrast, vegetated soils retained over 99 % of terbutryn mass, primarily through sorption and biodegradation. CSIA indicated higher biodegradation rates in pavements than in gravel. Measurements and modeling showed substantial transformation of terbutryn into transformation products. These results underscore the critical role of biogeochemical processes at surface-soil interfaces in regulating biocide transport to groundwater. Permeable, non-vegetated surfaces receiving facade runoff were identified as significant contributors to subsurface contamination. The study highlights the limited and variable retention capacity of different urban surfaces and stresses the need to integrate biocide control into sustainable stormwater management practices.