It depends on what is worse, a slow leakage that can poison the life in the immediate vicinity for decades, or a quick explosion that is immediately lethal to l

It depends on what is worse, a slow leakage that can poison the life in the immediate vicinity for decades, or a quick explosion that is immediately lethal to life in a large volume but is quickly diluted. Also, I don't believe that arsenic-containing chemical agents were used in WWI. Lewisite production started in 1918, too late for it to enter the war. And other chemical weapon agents are not persistent polluters.