Drive first to the mayor's office to start the mission. Next, drive to the quarantine zone. Once inside, you'll have to submerge the containers in the water to neutralize them. As you work your way to the containers, I suggest switching to your conventional weapons to keep the undead away. Take note that you should be careful not to blow the containers with your guns or its mission failed. Use the Sonic Boom to push the containers and keep them intact. The zombies are also fast so you incendiary weapons will do wonders here.
You can start by taking one of the Enforcers in the roadblock behind your starting point so you have faster and meaner means of travel. Push the first container to the river then head to Viola's position. Kill them then head to the second container. There's a brute there so prioritize in taking it out. After pushing the second container out to the water, you need to head to the nearby Rim Jobs to fix your mask's damaged hose. You can also find friendlies there you can recruit for additional firepower.
Once there, protect Viola as she searchers for the parts to fix you up. You'll notice how shaky your controls will become, as an effect of the gas. Don't worry about the timer since its only there to freak you out. After you're patched up, head to the third container and push it off to the water.
Fight your way to the chemical truck and from there you'll have to choose what you want:
Deliver the virus to Oleg = Zombie Homies
Destroy the virus for Mayor Burt = Homie - Burt Reynolds and SWAT Backup
The zombie homies are nice and all but you'll be better off with Burt Reynolds who comes with SWAT backup and Mayor's Favor (Police notoriety wipe, similar to Police Forgiveness).
Reward: $16000, Item – Gas Mask, Notoriety Wipe - Law