Not really, most chemical weapons degrade after a few hours in the sunlight, its environmental impact is almost none.
The problem with this type of weapons is mostly that they are indiscriminate and kill a particularly horrible way and how big its area of effect can be (thus they are WMD's), those are the major reasons those weapons are banned.
Being very blunt the whole issue is about international law and status quo, if the united nations doesnt enforce the bans in such weapons via political/economic/military means it will lose credibility which will then make it lose its status as a mediator, and that can have many (negative) consequences, at least for the western powers.
The civil war has been going on since early 2011 and has killed hundreds of thousands, international concern isnt on the lives lost, its in the politics of international law.