My experience has shown me that shredded paper and cardboard work well, especially when well layered with grass clippings for air, nitrogen, and hence . composting.
Soils are also a big factor: sand will be less likely to turn it into gley due to porosity in soil. Clay will be opposite.
We are also working with inoculating the carboard with stropharia mycelium. This seems to be the biggest missing element- overall soil health. especially if you are sheet mulching on tilled or disturbed soil.
As far as contaminants go- watch for shiny and glossy surfaces. compounds will be broke down by mushrooms, but heavy metals will not. grow a mustard crop??
Soil temp is extremely affected by sheetmulch and will not work in northern climates for vegetables other than cold loving crops.
My latest thoughts have been to use a hammer mill to shred the cardboard and place shredded cardboard on top of myceliated cardboard.
check out our east coast mushroom lab:
Pix will come in october.