Normal background concentrations of contaminants in the soils of Wales : exploratory data analysis and statistical methods

The British Geological Survey (BGS) has been commissioned by the Department for Environment, Food and Rural Affairs (Defra) to give guidance on what are normal levels of contaminants in soils in support of the Part 2A Contaminated Land Statutory Guidance revision. The work was initially focused on England but has been expanded here to look at Welsh soil. This work explores contaminant spatial variability and population distributions in topsoils for six contaminants, namely, arsenic (As), cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni) and lead (Pb). A seventh contaminant, benzo[a]pyrene (BaP), was previously explored for British soil because of a paucity of available data and so is not further explored here. Normal levels of contaminants are quantified as normal background concentrations (NBCs). These are attributed to different regions of Wales for each contaminant, based on factors that are seen to contribute to higher concentrations in some areas, referred to here as domains. The three most important contributing factors are: the underlying parent material upon which the soil has formed; non-ferrous metalliferous mineralisation and associated mining activity; and urbanisation/industrialisation.