The Alzette river was the subject of an investigation on Sunday morning after emergency services detected pollution in its waters around Schifflange.
The Grand Ducal Fire and Rescue Corps set up a temporary dam in the area around the Brill nature reserve and placed clarifying cubes in the river to help halt the pollution.
An exact source for the pollution is yet to be determined, while the nature of the substance itself has not been confirmed. An investigation is ongoing.
The intervention was reported to have ended by 11.30am on Sunday morning, but the temporary barrage remains in place at the time of writing.
On Saturday, a soapy foam was spotted in a stream near Bissen, with the cause yet to be confirmed.