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

Figures & data

Table 1. A table showing soil mixtures and a range of moisture and oil content used for the experiments.

Samples	PSD Mix	% clay	Added Volume of Water (mL)	Added Volume of Oil (mL)	Oil content
Dry soil	SAND100	0	0	255075100125150	5%10%15%20%25%30%
	SAND90	10	0
Wet soil	SAND100	0	255075100	150	30%
	SAND90	10	255075100	150	30%

Figure 1. Distinctive spectra of various mixtures of sand, clay, water and oil. Absorption peak at 1720 nm distinguishes oil-contaminated vs uncontaminated soil samples. Spectral signatures with broad peaks around 1400 nm and 1900 nm are indicative of water, while sharp peaks at 1400 nm characterize soil samples.

Figure 2. (a) Calibration plot showing inverse relationship between Oil Index vs oil content for dry SAND₁₀₀. (b) calibration plot for dry SAND₉₀. (c) A polynomial regression plot of Oil Index vs oil content in dry SAND₁₀₀ showing good correlation. (d) A plot showing good correlation for Oil Index vs oil content in SAND₉₀ (e) A good correlation for Oil Index vs oil content in wet SAND₁₀₀ (f) A plot showing poor correlation for Oil Index vs oil content in SAND₉₀.

Figure 3. (a) Slice-map showing a 3D outline of oil contaminant plume in SAND₁₀₀, delineated area (white dotted line). Average Oil Index = 7.7. (b)Slice-map of wet SAND₁₀₀. Spatial variation along slice surfaces may be due to relative displacement of water and oil. Note the lower index values at the bottom slice, indicating fluid accumulation.

Figure 4. (a) Slice-map of dry SAND₉₀ sand showing the spatial distribution of oil in the subsurface. The white dotted lines on the first and third slice delineate oil pool formed on the surface in the picture above. Average Oil Index = 6.6. Blue arrows indicate possible oil accumulation in minor fissures. (b) Slice-map of wet SAND₉₀ soil showing spatial distribution of oil (and water). Note the reduced Oil Index values and accumulated fluids toward the bottom.

Figure 5. Slice map of wet SAND₉₀ mixture showing points where spectral visualization were done at various depths. Oil peak occur at 1720 nm. Brown spectra (F1, F3, G2, H1, H4) are indicative of relatively high oil concentration, blue spectra (G3, H2, H3) indicate low concentration while green spectra (F2, G1) show areas where water preferentially accumulated relative to oil.