Chlorothalonil, a fungicide applied for decades worldwide, has recently been banned in the European Union (EU) and Switzerland due to its carcinogenicity and the presence of potentially toxic transformation products (TPs) in groundwater. The spread and concentration range of chlorothalonil TPs in different drinking water resources was examined (73 groundwater and four surface water samples mainly from Switzerland). The chlorothalonil sulfonic acid TPs (R471811, R419492, R417888) occurred more frequently and at higher concentrations (detected in 65-100% of the samples, <= 2200 ngL(-1)) than the phenolic TPs (SYN507900, SYN548580, R611968; detected in 10-30% of the samples, <= 130 ngL(-1)). The TP R471811 was found in all samples and even in 52% of the samples above 100 ngL(-1), the drinking water standard in Switzerland and other European countries. Therefore, the abatement of chlorothalonil TPs was investigated in laboratory and pilot-scale experiments and along the treatment train of various water works, comprising aquifer recharge, UV disinfection, ozonation, advanced oxidation processes (AOPs), activated carbon treatment, and reverse osmosis. The phenolic TPs can be abated during ozonation (second order rate constant k(O3) similar to 10(4) M(-1)s(-1)) and by reaction with hydroxyl radicals ((OH)-O-center dot) in AOPs (k(OH) similar to 10(9) M(-1)s(-1)). In contrast, the sulfonic acid TPs, which occurred in higher concentrations in drinking water resources, react only very slowly with ozone (k(O3) <0.04 M(-1)s(-1)) and (OH)-O-center dot (k(OH) <5.0 x 10(7) M(-1)s(-1)) and therefore persist in ozonation and (OH)-O-center dot- based AOPs. Activated carbon retained the very polar TP R471811 only up to a specific throughput of 25 m(3)kg(-1) (20% breakthrough), similarly to the X-ray contrast agent diatrizoic acid. Reverse osmosis was capable of removing all chlorothalonil TPs by >= 98%. (C) 2020 The Authors. Published by Elsevier Ltd.