Diagnostic screening of urban soil contaminants using diffuse reflectance spectroscopy

Abstract

There is increasing demand for cheap and rapid screening tests for soil contaminants in environmental consultancies. Diffuse reflectance spectroscopy (DRS) in the visible-near infrared (vis-NIR) and mid infrared (MIR) has the potential to meet this demand. The aims of this paper were to develop diagnostic screening tests for heavy metals and polycyclic aromatic hydrocarbons (PAH) in soil using vis-NIR and MIR DRS. Cadmium, copper, lead, and zinc were analysed, as were total PAH and benzo[a]pyrene. An ordinal logistic regression technique was used for the screening and predictions of either contaminated or uncontaminated soil at different thresholds. We calculated the rates of false positive and false negative predictions and derived Receiver Operating Characteristic curves to explore how the choice of a threshold affects their proportion. Zinc and copper had the best prediction accuracies of the heavy metals, with 89% and 85%, respectively. Cadmium and lead had the lowest prediction accuracies, with 68% and 67%, respectively. PAH predictions averaged 78.9%. With an average prediction accuracy of 79.9%, MIR analysis was only slightly more accurate than vis-NIR analysis, which had an average prediction accuracy of 77.5%. However, vis-NIR may be used in situ, thereby reducing cost and time of analysis and providing diagnosis in ‘real-time’. DRS in the vis-NIR can substantially decrease both the time and cost associated with screening for soil contaminants.