Establishing background variation and threshold values for 59 elements in Australian surface soil

During the National Geochemical Survey of Australia over 1300 top and bottom sediment samples (including ~10% field duplicates) were collected from the outlet of 1186 catchments covering 81% of the continent at an average sample density of 1 site/5200 km2. The <2 mm fraction of these samples was analysed for 59 elements by ICP-MS following an aqua regia extraction. Results are used here to establish the geochemical background variation of these elements, including potentially toxic elements (PTEs), in Australian surface soil. Different methods of obtaining threshold values, which differentiate between background and those samples with unusually high element concentrations and requiring attention, are presented and compared to Western Australia's 'ecological investigation levels' (EILs) established for 14 PTEs. For Mn and V these EILs are so low that an unrealistically large proportion (~24%) of the sampled sites would need investigation in Australia. For the 12 remaining elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sn and Zn) very few sample sites require investigation and most are far removed from human activity centres, potentially highlighting minor local contamination or mineral exploration potential rather than pollution. No major diffuse source of contamination by PTEs visibly affects Australian soils at the continental scale. Of the statistical methods used to establish threshold values, the most pertinent results come from identifying breaks in cumulative probability diagrams, the Tukey inner fence and the 98th percentile. Threshold values for over 59 elements, including emerging 'high-tech' critical elements such as lanthanides, Be, Ga or Ge, for which no EILs exist at present, are presented. Citation: Clemens Reimann, Patrice de Caritat, Establishing geochemical background variation and threshold values for 59 elements in Australian surface soil, Science of The Total Environment, Volume 578, 2017, Pages 633-648, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2016.11.010