Forage at Roots and Shoots – contamination of soil

Segen and I are going to do a forage of the site on Sunday. I had a very interesting chat with Linda, Director of roots and shoots in preparation for this day and as part of the risk assessment. This is something I have to do to check the quality of the soil in the places we are foraging in. The present site of roots and shoots was a wartime Meccano factory, manufacturing parts for fighter aircraft. In 1983, when Roots and shoots began working the land to create the gardens, they found tin, broken glass and other metal objects in the ground. Soil testing showed heavy lead and cadmium in the soil. 20 Tonnes of top soil were brought in to remedy this. Across the road in the open green space, there used to be early Victorian cottages until around 1984/5 when they were allowed to fall into disrepair and then demolished. So all this means, that the gardens aren’t ideal for foraging also Linda said that some topsoil has been put on the wildlife garden.
A question that I need answering and I call on any soil experts, permaculturalists and other interested people out there:

how long does it take for contaminated soil to repair itself?

is this indeed possible?

What difference is there between the heavy metals, lead, cadmium etc?

What plants can aid the process?
On the day, we’ll be doing some plant ID on the wild plants in the wildlife garden and I know there’s some chickweed hitching a ride in the raised beds around the garden. Then we’ll cook up a wild leaf risotto. We look forward to seeing you there between 2 and 4pm