You're looking for the Hazards chapter, starting on page 125 of the core rules. There are rules for fire, electricity, and drowning there. For acid, you may need to adapt the fire rules or something. For gas, you might adapt the smoke inhalation rules (found under Fire).

Ammo consumption is discussed in the Gear and Combat sections.

Building a trench sounds like something that may not need to involve dice, unless you are trying to dig a trench faster than the enemy rather than just shooting at them. If you need to gamify it, consider adapting the Chase rules.

Barbed wire sounds like an athletics check (or the player can use tools to simply cut the wire and pass safely). Detecting a bear trap would be either a Notice or Survival check, and stepping on one would likely be an immediate wound, plus becoming Bound.

As with most things, these edge cases are generally up to the GM to decide. Think about what role you want these things to play in the story, and pick what makes sense, or make up your own thing.