A LC tandem MS method was developed for the simultaneous determination of four iodinated X-ray contrast media (ICM) and 46 ICM biotransformation products (TPs) in raw and treated wastewater, surface water, groundwater, and drinking water. Recoveries ranged from 70% to 130%, and limits of quantification (LOQ) varied between 1 ng/L and 3 ng/L for surface water, groundwater and drinking water, and between 10 ng/L and 30 ng/L for wastewater. In a conventional wastewater treatment plant, iohexol, iomeprol, and iopromide were transformed to >80%, while iopamidol was transformed to 35%. In total, 26 TPs were detected above their LOQ in WWTP effluents. A significant change in the pattern of ICM TPs was observed after bank filtration and groundwater infiltration under aerobic conditions. Predominately, these TPs are formed at the end of the microbial transformation pathways in batch experiments with soil and sediment. These polar ICM TPs, such as iohexol TP599, iomeprol TP643, iopromide TP701A, and iopromide TP643, were not or only partially removed during drinking water treatment. As a consequence, several ICM TPs were detected in drinking water, at concentration levels exceeding 100 ng/L, with a maximum of 500 ng/L for iomeprol TP687.