The union said that the run-off and waste water from refuse lorries that are hosed down and cleaned should be put into sealed barrels and disposed of.
It claims Serco instructed staff to instead pour the waste water into public drains and has called on both the Health and Safety Executive (HSE) and the Environment Agency (EA) to investigate the allegations.
The disposal of waste into the drains would breach the Environmental Protection Act 1990, the union said.
Serco – “We can confirm we are now conducting a full and detailed investigation into the claims. As a company we take personal and environmental safety very seriously and strive to provide and safeguard the highest standards of safety for our people and the residents we serve.”
The union also claims that no risk assessment was performed, no safe system of work was in place and no personal protective equipment was provided for the task, all of which are potential breaches of the Health and Safety at Work Act.
GMB said it had “significant concerns not just in relation to the health and safety of our members but to the members of the public living in the area due to the potential exposure to prohibited substances.
“This unregulated disposal of potentially hazardous waste is unacceptable and GMB members and the public must be protected from such unscrupulous practices.
“If Serco are unable to carry out this £140m contract in a legitimate way, then Hammersmith and Fulham should take it back in-house.”
A spokesperson for Serco, which has run the £140m waste management and street cleaning contract in Hammersmith and Fulham since 2008, said: “Serco can confirm we were concerned to receive news in relation to an alleged incident at Hammersmith and Fulham and treat all such incidences extremely seriously.
“We can confirm we are now conducting a full and detailed investigation into the claims. As a company we take personal and environmental safety very seriously and strive to provide and safeguard the highest standards of safety for our people and the residents we serve.”