”However, there are some big ammunition depots in north of France with all these WWI shells collected over the years”
Normal shells are fairly easy to dispose of using explosives, so those might be poison gas shells. I know Belgium had an enormous backlog in destroying such shells in Houthulst even as recent as 15 years ago (http://www.greatwar.nl/frames/default-houthulstn.html), but don’t know what happened with it (at the time, there were talks on the urgency of cleaning it up)
Normal shells are fairly easy to dispose of using explosives, so those might be poison gas shells. I know Belgium had an enormous backlog in destroying such shells in Houthulst even as recent as 15 years ago (http://www.greatwar.nl/frames/default-houthulstn.html), but don’t know what happened with it (at the time, there were talks on the urgency of cleaning it up)