Ground can be contaminated by landfills, gasworks or other polluting activities that existed previously. Some landfills date back to the 1800s. Waste in these dumps was usually spread randomly and this means that some places are very contaminated and others less so. Dumped industrial waste used to contain heavy metals as well as petroleum and tars. The oil has usually disintegrated so generally only small amounts of oil in the soil are found.
Tars decompose very slowly and are still found in soil after many years.
Tars have been found in many allotment gardens. Heavy metals among these lead and cadmium do not disintegrate and so remain in the soil. Therefore these contaminants do not disappear. Contaminants have been found both on the surface and down to half a meter deep. Soil is not stationary and is mixed when you dig.