Soil water repellency at old crude oil spill sites

My doctoral research focussed on soils that develop severe and persistent water-repellent character several years or decades following contamination by crude oil. These water-repellent soils no longer look, feel or smell like they contain any oil, but they remain dry even under heavy rainfalls and support sparse plant growth. The underlying objective of my research was to identify the causative agents and the processes leading to the development of soil water repellency at old crude oil spill sites.;Characterization of three water-repellent soils revealed that they are not salt-affected, nitrogen-deficient, nor sterile. Viable soil microorganisms subsist in them, although they are likely inactive due to limited access to water and dissolved nutrients. An extraction procedure had to be found that would remove soil water repellency completely and not destroy causative agents in the process. Soxhlet extraction with isopropanol/15.7 M NH4OH (7:3, v/v) (IPA/NH4OH) met these criteria and was used to isolate putative causative agents for further characterization. Isolated putative causative agents were identified as consisting predominantly of homologous series of straight-chain and polycyclic aliphatic hydrocarbons (C12 to C34), including petroleum biomarkers (i.e. hopanes). From this evidence, I concluded that causative agents of soil water repellency are petroleum residues and not products of microbial origin. I subsequently found that sequential extraction alternating between amphiphilic and nonhydrogen-bonding solvents eliminates and in turn restores soil water repellency.;My research indicates that soil water repellency is not simply caused by the presence of hydrophobic organic molecules in soil. My data and literature reports on the conformational flexibility of insoluble organic molecules lead me to conclude that soil water repellency is a function of the packing density and tail conformation of amphiphilic organic molecules forming the outermost layer of soil particle surfaces. Intact or partially oxidized amphiphilic molecules of petroleum origin appear to be the causative agents of soil water repellency at old crude oil spill sites. Two possibly synergistic mechanisms are proposed to explain how the sorption of vapour- or liquid-phase petroleum hydrocarbons may have initiated the development of soil water repellency at old crude oil spill sites.