Application of near infrared spectroscopy in sub-surface monitoring of petroleum contaminants in laboratory-prepared soils

This study was conducted to determine the suitability of near-infrared (NIR) spectroscopy for subsurface monitoring of crude oil contaminants in different soil types under different moisture conditions. Calibration tests, carried out in both wet and dry soils, with crude oil contents 5–30% indicate an inverse relationship between spectra and oil content with R² values ≥0.98 for dry soil mixtures. A derived index termed “Oil Index” was generated by calculating a spectral ratio between the main oil peak in the NIR region and a spectrally inactive part of the spectrum within the visible region. It was used in a series of experiments designed to test whether NIR probes could yield useful information about the movement of oil through different soils. Synthetic crude oil was dropped into a series of soil samples of different particle size distribution and moisture content. Analyses of the 3D distribution of values of the Oil Index demonstrate that it is possible to estimate and map synthetic crude oil concentration in the subsurface of the soil samples. Results showed that while the Oil Index provided a reasonable estimation for oil concentration in dry samples, this was not the case for wet samples, although the Index was useful in understanding the pattern of movement of oil contaminant in wet soils. The work indicates that this technique may enhance field investigation of oil contamination, providing an accurate in-field technique.