Hundreds of barges of illegal miners dredging for gold were navigating along the Madeira River in the Brazilian Amazon on Friday, and researchers said they posed a threat of pollution — including toxic mercury — for the broader environment.

The barges were spotted this week by the municipality of Autazes, some 120 kilometers (70 miles) from Manaus, the capital of Amazonas state.

Smaller gatherings of barges are common along rivers in the region, but the latest collection drew international attention this week when Greenpeace and news media published images of several rows of rafts.

Brazilian Vice President Hamilton Mourão announced an imminent police operation in the area, prompting the miners to depart early Friday and head elsewhere along the river.

Miner Thiago Bitencourt Gomes, wearing just a pair of shorts and some flip flops, told The Associated Press on Friday that about 400 barges – some 3,000 people – congregated in the area after one miner found gold there and alerted the others.

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