The mishap took place in Odhav locality late Saturday night when a worker went into the well of the civic-run Ambikanagar pumping station and fell unconscious after inhaling a toxic gas, he said.
The deceased were identified as Sarvjeet (25), Sunil (25), Ravji Chauhan (38), and Lalsinh Marwadi (26), he said.
The bodies were sent for postmortem and a case of accidental death was registered, the official said.
The storm water pumping stations are cleaned as part of the pre-monsoon preparedness for which the Ahmedabad Municipal Corporation has outsourced work to some private contractors, a source in the civic body said.
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