Jacky-Éric Salvant said residents on Montreal's South Shore have approached his firm complaining about how the city handled the situation.
Salvant said it took too long from the time the city found out about the tainted water to the time they told residents not to drink it.
"It was mismanaged," he told CBC.
He is planning to file the paperwork this morning to get authorization to launch the suit.
The spill was caused by a generator at the Longueuil waste-water treatment plant on Wednesday morning at around 4 a.m.
Quebec’s Environment Ministry was only informed of the situation more than five hours later, at 9:30 a.m.
Nearly 300,000 affected
The city originally issued a statement saying the water was safe to drink.
It took more than 12 hours for the city to send out an updated advisory telling residents they shouldn’t drink the tap water.
Nearly 300,000 residents were affected by the water consumption ban, which lasted two days.
Salvant said he will demand a symbolic $100 per resident, which would translate to close to $29 million.
For her part, Longueuil Mayor Caroline St-Hilaire has called for an inquiry into what happened.
She said people affected deserve to know what happened and has promised a full investigation.