Concentrations and mobility of human pharmaceuticals in the worldʹs largest wastewater irrigation system, Mexico City–Mezquital Valley

Concentrations and retention of pharmaceutically active substances are crucial for assessing the environmental risk of medication of humans. We hypothesize that environmental introduction concentrations (EICs) of drugs in the Mexico City–Mezquital Valley wastewater irrigation system can be estimated using information on water consumption, sales data, and excretion rates. EICs of six acidic and five basic drugs were calculated and compared with concentrations measured in wastewater, irrigation water, soil drainage, and springs by liquid chromatography electrospray ionization tandem mass spectrometry (LC–ESI–MS/MS). EICs of trimethoprim, erythromycin, naproxen, ibuprofen, and diclofenac in sewage equaled or exceeded the US FDA action limit of 1 μg/L for detailed environmental risk assessment (ERA). Concentrations of clarithromycin, clindamycin, metoprolol, sulfasalazine, bezafibrate, and gemfibrozil were smaller. Calculated EICs of all compounds except metoprolol and clarithromycin were comparable to measured concentrations if excretion rates were considered. Whereas concentrations of basic compounds with positive or neutral charges were effectively reduced during reservoir storage and soil passage, acidic, anionic compounds were hardly retained. Though realistic EICs can be predicted for most substances, large deviations between EICs and measured concentrations in the case of metoprolol illustrate that estimated concentrations cannot substitute for monitoring programs.