According to a report published by the Water Witness International, popular fast fashion brands could be causing pollution that has dyed some rivers in Africa blue or turned their waters as alkaline as bleach.

The report focuses on the polluted rivers in Lesotho in southern Africa and Tanzania to highlight the risks posed as global brands increasingly source garments from contractors in Africa, attracted by cheap labor and tax incentives.

Global brands could force better practices, but so far their presence in Africa has done little to stem rife pollution, water hoarding by contracting factories or even ensure adequate water and sanitation for factory staff, says Nick Hepworth, Director, WWI and Author

In Lesotho, researchers found a river visibly polluted with blue dye for denim jeans. Meanwhile, samples taken from Tanzania's Msimbazi river in Dar es Salaam tested a pH of 12 – the same as bleach – near a textiles factory, the report adds.

It identifies 50 international brands that source or have sourced their clothes from African nations, including Inditex's Zara, ASOS and H&M, but didn't tie the pollution to any company's supply chain.

Brands can and do make environmentally sustainable clothing, and consumer pressure was key to encouraging more, adds Katrina Charles, Expert-water Security and Quality, University of Oxford.