Mobilization and plant uptake of antibiotics in Mexican soils during transition from irrigation with untreated to treated wastewater

Treatment of wastewater before its reuse for irrigation is increasingly implemented to reduce health and environmental risks. We hypothesized that the transition from irrigation with untreated wastewater (UWW) to irrigation with treated wastewater (TWW) mobilizes previously accumulated antibiotics in soils, thereby increasing their plant uptake and leaching. Soil columns of Vertisols, Phaeozems and Leptosols with a long history of irrigation with UWW were planted with cilantro and irrigated with either UWW or TWW (both unspiked or spiked with antibiotics and quaternary alkylammonium compounds). While irrigation with TWW reduced the uptake of trimethoprim and sulfamethoxazole by cilantro plants, the leaching of clindamycin and sulfamethoxazole was higher compared to irrigation with UWW. Irrigation with TWW resulted in a stronger net mobilization of sulfamethoxazole from Vertisols and Phaeozems, and of ciprofloxacin from Vertisols and Leptosols. Antibiotics spiked to irrigation water were largely retained or transformed in soils. Antibiotic concentrations in soils were highest along preferential water flow paths and not affected by water treatment. Implementation of wastewater treatment in long-established irrigation systems using untreated wastewater reduces the risk that antibiotics enter the food chain, but increases the risk of groundwater contamination by leached antibiotics at least transiently.