The Western Front ran from the North Sea down to the Swiss border and lasted for four years. There are still areas where the mustard gas is of a high enough concentration to cause harm. French and Belgian farmers frequently find munitions and when trenches are discovered they often still have their occupants inside.
Sadly the British army used brass or "fibre" to make ID tags and these almost never survive. Any bodies found are buried by nationality as "Unknown Soldier".
The Chemical warfare that took place was hideous. The German army "opened the ball" and we and the French copied the idea instantly... Nasty business and became quite an art form.
My Grandfather was a trench mortar operator in '17 and '18 and used gas shells. Apparently the idea was to use a variety of gasses that had differing effects to cause the most damage. For example You would start with Chlorine to get the enemy vomiting and unable to wear their masks and then hit them with something nastier. Mustard gas blisters your skin and rendered gas masks inneffective.
As an interesting side note apparently Churchill wanted to use Gas against the German cities in 1944 when the V-1's started coming in, the only thing that stopped us was that the firestorm attacks were more destructive and considered more terrifying.
Will