The Aalst city authorities say there is no danger of an explosion and there's no immediate threat to public health, but they are urging residents who smell a nasty odour indoors to ventilate their homes.
Soil, air and water samples have been analysed. Increased levels of hydrocarbons were found in sewage.
The Mayor of Aalst told reporters that a door-to-door had not yet allowed the police to identify the culprit. The mayor wants prosecutors to carry out an intense investigation. The pollution, probably a discharge, may have occurred on purpose or happened accidentally. Aalst firefighters and civil protection officers are continuing to flush and ventilate the city's 70 km of sewers.
With temperatures on the rise the stench could get worse at the weekend.