copper: 7440-50-8, toxin, soil pollutant, pollutantlead: 7439-92-1, toxin, soil pollutant, pollutantzinc: 7440-66-6, toxin, soil pollutant, pollutantchromium: 7440-47-3, toxin, soil pollutant, pollutantnickel: 7440-02-0, toxin, soil pollutant, pollutantthorium: 7440-29-1, toxin, soil pollutant, pollutantsoil qualityspatial variationanthropogenic processgeostatistical predictionremediation effortnational-scale soil monitoring network