Assessing natural contents of hazardous metals in soils by different analytical methods and its impact on environmental legislative measures

A variety of analytical techniques are commonly used to assess natural metal concentrations in soils. Although the choice of the analytical procedure may appear to be an unimportant issue, through an example from the Po Plain we document that for specific hazardous metals the arbitrary choice between separate analytical techniques may have contrasting legislative implications. Comparative analyses of 160 soil samples obtained with X-ray Fluorescence (XRF) and aqua-regia Inductive Coupled Plasma Mass Spectrometry (ICP-MS), respectively, reveal a systematic lower efficiency of chromium determination by ICP-MS (45-60% of XRF determinations), irrespective of the laboratory chosen. Maximum permissible concentrations of chromium for residential and recreational sites were invariably exceeded by sediments deriving from chromium-rich parent rocks after XRF analyses. In contrast, replicate determinations following aqua regia digestion were generally below the threshold values, defining the same soils as unpolluted. This study thus emphasises the need for strictly standardised procedures of metal determination.