The land surface of England has been divided into domains for purposes of defining background soil concentrations of arsenic (As), lead (Pb) and benzo[a]pyrene (BaP). Within any domain for a particular substance there may be one or more factors (anthropogenic or geogenic) which guide us to expect elevated background concentrations of a substance. In the case of As, the Ironstone and Mineralisation Domains have elevated concentrations from geogenic sources. Other sites, with no particular factors causing elevated concentrations, constitute the Principal Domain. In the case of Pb there are elevated concentrations from anthropogenic sources in the Urban Domain, geogenic sources in the Mineralisation Domain, and a Principal Domain where elevated concentrations are not expected. Because there are much less data for BaP, both England and Britain are divided into just two domains: Urban and Principal (i.e. non-urban).udFor each contaminant, the normal background concentration (NBC), for each of the three contaminants within their domains, have been determined in a systematic and robust statistical manner which is summarised in a methodology flow diagram. First, the statistical distributions of contaminant concentrations are characterised for each domain. Histograms or density plots and summary statistics (the skewness coefficient and the octile skewness coefficient) are used to judge whether the data can assumed to come from a Gaussian variable and whether outliers are present in the data set. If the distribution is not Gaussian then the data are transformed so that the distribution becomes Gaussian by either taking the natural logarithm or in some instances a Box-Cox transform of the data. The NBC is set by taking the upper 95% confidence limit of the 95th percentile of the distribution.
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