High levels of arsenic and lead have been found in the soil at a school and allotments in York.
The soil samples were taken as part of the planning process for alterations to Danesgate pupil support centre off Fulford Road in Fulford.
Further tests are being carried out as a matter of urgency in the surrounding area, including Fulford Cross Nature Reserve, the council said.
Allotment gardeners are being advised not to eat their produce.
The council has also written to local residents and parents of children at the Danesgate pupil referral unit and Steiner School to alert them to the situation.
Patrick Scott said the former use of fire ash as a fertiliser meant finding higher than average concentrations of heavy metals in old gardens and allotments was quite common.
"I understand that local residents will be concerned and we will be carrying out more detailed tests as a matter of urgency to find out whether the levels of contaminates in the area pose a risk," he said.
"This could take a number of weeks. In the meantime, we have contacted the Health Protection Agency and will be working with them to ensure that local people have as much information as possible as soon as possible."
People in the area are advised to wash their hands thoroughly before eating and after handling soil, children should not be allowed to play in the soil and gardening should be stopped or kept to a minimum.