You're locked out for the time being, so go through the little hole in the back wall. Your squad stays behind because of the toxicity. The water here is toxic and eats right through your suit. It does about 5 damage a second. Don't fall into it. If you do, just start jumping and get on top of something in the water as quickly as you can. Use the spools and boxes to get through the water. Watch out once you pass the truck though. There are zombies along the side that spawn out of the water. Keep your eyes open and be ready to pull out the shotgun. Also, try to avoid the center platform if you can, by aiming for the flaming truck. If you step on the

click to enlarge
center platform, it will spawn about 7 zombies from the water around you. Your limited movement will make the fight very difficult.
Your target is the broken pipe lying across the overturned tanker truck. Get to the white car at the back and jump onto it. Then climb the pipe to get into a vent. Break another grate in the vent to drop down and start moving down the hallway. Open the door to let your squad back through. Go up the stairs and to the street. Stop right now though.
They are shelling the streets and a mortar is incoming at the end. It will just about kill you if you are anywhere near the end of the street. Just stay where you are and wait for the rocket to hit at the end of the street. Kill the six fast headcrabs that pop out of it. Then go to the gate to see that the rebels are in a bit of trouble.