Thank you everyone for your replies. The mean arsenic level is approx. 160ppm but some areas are 1200ppm and the plan is to re soil to 600mm and line with a permeable membrane, would this stop the arsenic moving and re contaminating the fresh soil ? The area is approx 7 acres with pH varying from 4.2 to 9.1. This is in Cornwall UK with an old tin mine within 150m but it is not clear if the contamination is from mine spoil or underlying rock. Borehole samples to 1m show random contamination from the top to the bottom of the samples with no clear pattern. At the higher levels does this pose a problem to health if you are in contact with the soil ?