Oil spills can cause severe and persistent soil hydrophobicity, which alters soil structure, reduces water infiltration, and increases surface runoff. While most mechanistic research has focused on soils rich in organic matter, soils with low organic content, such as those found in arid regions, have been less studied. Organic matter is a key binding agent in temperate soils yet cannot explain the phenomena in arid soils. This study investigates the binding mechanism of degraded oil pollution in arid calcareous soils dominated by CaCO₃. It is hypothesized, that in these soils long-chain fatty acids, derived from crude oil degradation, may interact with Ca²⁺ ions found on the surface of carbonate minerals, forming a hydrophobic monolayer that sustains water repellency.
To test this hypothesis, soils from Avrona Nature Reserve, impacted by the 2014 crude oil spill, were studied. These are sandy loam soils dominated by quartz with minor amount of calcite. Currently, despite in-situ remediation, ~50% of total petroleum hydrocarbons (TPH) remained, and hydrophobicity persists.
In this study, soil samples were collected from three distinct sedimentary belts along the polluted stream. Hydrophobicity was quantified using the molarity of ethanol droplet (MED) test, and TPH concentrations were measured via solvent extraction and GC-FID analysis. Polluted soils exhibited extreme hydrophobicity (5M–13M MED) and high TPH concentrations (1,290–14,550 mg/kg). After TPH removal, residual hydrophobicity (4M–5M MED) persisted. The TPH residue was re-extracted with organic solvents following by GC-MS analysis which identified the presence of long-chain fatty acids. To further test our hypothesis, synthetic fatty acids were added to pure CaCO3, clean natural soil (19% CaCO₃) and CaCO₃-free soil. While the pure CaCO3 and CaCO₃-containing soil developed hydrophobicity (14M and 7.5M MED, respectively), the CaCO3 free soil sustained its hydrophilic nature. Thus, confirming the major role that CaCO₃ may play in sustaining hydrophobicity.