Micropollutant mixtures

Groundwater quality is of increasing concern due to the ubiquitous release of micropollutants, often originating from surface waters. Stream-groundwater interactions near agricultural and urban areas represent an important entry pathway of micropollutants into shallow aquifers. Here, we evaluated the biotransformation of a micropollutant mixture (i.e., caffeine, metformin, atrazine, terbutryn, S-metolachlor and metalaxyl) during stream-groundwater interactions dominated by lateral stream water flow to adjacent groundwater. We used an integrative approach combining concentrations and transformation products (TPs) of micropollutants, compound-specific isotope analysis (CSIA), sequencing of 16S rRNA gene amplicons and conceptual modeling. Duplicate laboratory aquifers (160 cm x 80 cm x 7 cm) were fed with stream water and subjected over 140 d to three successive periods of micropollutant exposure as pulse-like (6 mg/L) and constant (0.6 mg/L) injections under steady-state conditions. Atrazine, terbutryn, S-metolachlor and metalaxyl persisted (<10% dissipation) in both aquifers during all periods. Metformin dissipation (up to 14%) was only observed from 90-140 d, suggesting enhanced degradation over time. In contrast, caffeine was dissipated (>90%) during all injection periods, agreeing with fast degradation rates (t1/2<3 d) in parallel microcosm experiments and detection of TPs (theobromine and xanthine). Significant carbon stable isotope fractionation was observed for caffeine in both aquifers, whereas no enrichment in 15N occurred. A concentration dependence of caffeine biotransformation was further suggested by model simulations following Michaelis-Menten kinetics. Bacterial diversity and community composition varied and increased over time, suggesting long-term bacterial adaptation to micropollutant exposures. Altogether, our results highlight the variability of micropollutant degradation in aquifers subjected to a micropollutant mixture and underscores the interest of an integrative approach to understand the interplay of subsurface hydrochemistry, bacterial adaptations and micropollutants biotransformation during stream-groundwater interactions.