Study highlights magnitude of diffuse and point metal pollution in urban soil

Volumetric flasks
As part of a nationwide mapping of chemical elements in the surface environment a study of soil chemistry within and around the City of Sheffield has highlighted our long-term impact on the urban environment. A comprehensive survey undertaken by the British Geological Survey (BGS), and results studied in collaboration with Rothamsted Research, has shown that average lead concentrations in urban soils are approximately twice that of the surrounding rural soils, equating to a cumulative load of around 20 tonnes of lead per square kilometre across the City. The dominant sources of lead include historical industrial activity and vehicle exhaust emissions. Despite our reliance on urban soil to filter water and as a medium to grow vegetables, the magnitude of diffuse pollution has not been established before. The survey provides a basis for investigating the impact of this extra lead on the urban population and the extent of its effect on human health. The study involved collecting a total of 569 samples of soil from a range of land use types across the City and measuring their total metal concentrations. Given the historical legacy, particularly that of steel manufacturing in the City, the study focussed on three metals; lead, nickel and chromium. Those sites with unusually large metal concentrations due to point contamination were identified using a robust statistical procedure and treated separately from the majority of sites at which the concentrations reflect the natural soil composition plus diffuse pollution. Point contamination of chromium and nickel, presumably related to steel manufacture and processing, was common in former industrial areas to the north-east of City. Thirty-five hotspots also occurred in residential gardens. These require further investigation before any potential threat to human health can be assessed. The BGS has surveyed the soil of other towns and cities in the UK but the results have not yet been subjected to the same detailed statistical analysis as those from Sheffield. Such analysis will put the level of point and diffuse pollution in Sheffield into context, and indicate whether the magnitude of pollution reflects its long industrial legacy. The full results of the current study are published in the journal.


16 January 2006