Dirty Water - Find the hidden entrance into the Wolvark Barracks - Trophy/Achievement.
As you are making your way to the Wolvark Barracks, keep progressing through the level as normal until you get to the 2nd bridge. Take out all the Wolvark's and activate the switch opening the 2nd bridge. Once you have opened the bridge, turn left and jump off the bridge into the water. Swim up to the rope and climb up. From there you will see a Wolvark near the switch which leads to the hidden entrance to the Wolvark Barracks. Take out the Wolvark and activate that switch. Once through the opening through the sewer pipe will open. Head through and you will enter the corridor entering the hidden entrance. The Dirty Water Trophy/Achievement will unlock while you are on your way going through the hidden entrance to the Wolvark Barracks.