For the CW stuff its actually pretty complicated. In general, the land (and sea) has absorbed much of the toxic material, as anticipated by our for-fathers. Where the problems lie is not particularly from the battlefields, where the items had somewhat even distribution, but from the burial (land and dump (sea) sites. In these locations you are looking at sometimes thousands to millions of munitions in a concentrated area, so your source strength is much higher.
The problem then is which type of chemical agent and which type of munition, and how it was left. Some agents break down quickly and easily in soil or water, others do not. The WWII Clark munitions were heavy in arsenic, as was Lewisite. Arsenic does not break down further, so once you have a concentration in one area it becomes a problem.
Which munition you have determines (in part) how quickly the round is broken through and the agent is released. A mixture of different munitions means different breakthrough times and a gradual release. All the same type of munitions means a similar rate of corrosion and a greater release at one time.
Likewise if the munitions were just buried/dumped, or if they were overpacked or left in ships. Whether the water is cold or warm, soil conditions, etc.
There have been many studies in areas like the Baltic (over 100K tons), but the results are never completely clear as well. Most studies are in theory mostly, ie based on types of bacteria found and attempting to state that the amount of bacteria identifies the amount of agent from an earlier period. What is clear is that much of this is about money.
Burial sites are not common, but do exist in substantial numbers. Much of the history of these sites is lost and they are only discovered when groundwater contamination is detected. At that point most countries investigate and recover.
Identifying a problem is comparatively easy. Doing something about it is extremely expensive. In the Baltic particularly, a couple countries made the stuff which is there, others dumped it, the potential and ongoing problems affect still others, who pays? For obvious reasons few are terribly interested in supporting this line of questioning. It was done in good faith at the time, to the standards of the time. Regardless, the technology isn't there to do much about it yet, with very limited underwater recovery of CW done so far and moderate land recovery.
The largest land burial site is still under investigation, recovery has not yet begun although it has been studied for the past 13 years. It has been estimated at 350,000 to 750,000 buried WWII CW.
