Nothing is perfect.

The previous owners used the edge of the garden to burn trash now and then. found lots of ashes with half burnt plastic, batteries, paint, pieces of some kind of chalkboard, maybe even asbest. I have tried to remove as much as I could. Then filled the hole with compost infused with EM (effective micro-organisms) and wood. Micro-organisms have the capacity to eat pollutants and render them inert or even break them down into unharmful elements.
The book "Earth Repair" gives wonderful insights in those processes.