I've heard that mass-produced chocolate tends to source cocoa from the Ivory Coast, which has an alarming amount of heavy metals in their soil. This is just hearsay though, I don't have any specific sources that compare between Africa and South America.
There's a Geochemical Atlas of Europe: http://weppi.gtk.fi/publ/foregsatlas/ and a "work in progress" Global Geochemical Baselines database under development that will reference the available glonal surveys in the public domain, buuuut ...
If you want solid usable data from across the globe you probably want to access the pooled transnational mega mining companies database of surface geochemical results, back in 2008 the <redacted> companies portion was some 16 TB of raw sample results from every conutry and territories across the globe. Not complete of course, but once you mosaic that with other collections of a similar size .. that's something pretty neat.
> High cadmium levels are almost exclusively found in cacao beans grown in South America, with beans grown in West Africa showing little contamination.
This is just conjecture but, some of the other comments here have said that the lead contamination come from environmental contamination during the harvesting and fermenting process. If non-ethically sourced cocoa is coming from farmers earning below poverty level incomes for their crops, they may not have the ability or incentive to avoid this contamination.
