Tracing multi-pathway transport of progestins, glucocorticoids, and antiprogestin in pilot-scale soil-water systems: Surface runoff, interflow, and leachate dynamics during simulated rainfall events.

The occurrence of highly potent steroid hormones in soils raises ecological concerns due to their endocrine-disrupting effects on nearby aquatic ecosystems. This study systematically investigated the multi-pathway transport dynamics of seven progestins, glucocorticoids, and antiprogestin using a pilot-scale soil-water system through six sequential rainfall simulations. Post-rainfall analysis revealed vertically stratified hormone distributions within soil profiles, mediated partially by hydrophobic partitioning. Initial rainfall triggered substantial hormone redistribution ("flushing effect"), while subsequent rainfalls maintained stable distributions through adsorption-desorption equilibria and stabilized transport pathways. A significant positive correlation emerged between hormone octanol-water partition coefficients and normalized loss rates in surface soil. Additionally, several hormones exhibited unexpectedly high aqueous-phase transport rates despite their high hydrophobicities, confirming colloid-mediated transport mechanisms. Mass flux analysis identified interflow as the dominant aqueous-phase transport pathway (50.7 % of total flux), followed by runoff (41.0 %) and leachate (8.30 %). Particulate-phase transport primarily occurred via runoff (47.4-81.0 %), followed by interflow (7.21-33.2 %) and leachate (0-19.4 %). Mass balance analysis after six rainfall events identified biotransformation as the major fate pathway (46.0-89.6 %, half-lives = 8.90-32.6 days) for these hormones, surpassing the fraction retained in soil (10.3-53.9 %) and transport losses (aqueous: 0.007-0.530 %; particulate: 0.015-0.038 %). Seventeen transformation products (TPs) were identified during transport, generated through hydrogenation and dehydrogenation pathways, with several retaining androgenic or progestagenic activities. These findings highlight the importance of incorporating both interflow and surface runoff pathways in environmental risk assessments. Additionally, despite substantial transformation, the slow transformation kinetics combined with bioactive TP generation implied persistent ecological impacts.