Application of spectral indices in the evaluation of soil-oil interaction and enhanced detection and quantification of oil contaminants in bare soils

Current methods to monitor oil contamination in bare soils rely upon geophysical techniques or direct sampling and testing, and their use amongst the global community is limited by their complexity and cost. Recent studies demonstrate that Visible-Near Infrared Spectroscopy (VNIRS) may be a reliable technique, but has found limited use due to effects of soil composition and moisture content. This study describes two spectral indices, the ‘Total Petroleum Hydrocarbon Index’ (TPH_ix) and ‘Oil Index’ (O_ix), and demonstrate their application in enhanced detection of oil contaminants and the evaluation of soil-oil-water interaction in bare soils. To validate their applicability, soil-oil experiments were performed on 26 samples comprising of 12 artificial and 14 natural soils in dry states, and 10 sub-samples in wet condition. Analyses of TPHix demonstrate that this index provides a better estimate of low oil content in different soil classes, while O_ix versus % Oil regression plots with a discrete inflection point termed the ‘Critical Point’ (CP) give an indication of soil-oil-water interaction as CP varies with soil composition and moisture content. This research demonstrates that VNIRS indices can improve oil contaminants detection in soils and provide insight into soil-oil-water interaction where there are few established techniques available to researchers.