2013 Residual Saturation, Fire/Explosion Hazard and Hydrophobicity

This PTAC project had two main objectives. The first, which was executed by Millennium EMS Solutions Ltd. was to investigate how far the roots of Alberta Green Zone trees penetrate into the ground, and hence identify a depth below which the ecological direct contact pathway may not be a primary concern. The second, and the subject of this document was to provide scientific evidence to support the management limits that may be appropriate at remote forested Green Zone sites on public land for non- mobile petroleum hydrocarbons (i.e., fractions F2 and F3) in soil.

Management limits currently exist for petroleum hydrocarbon (PHC) fractions F1 to F4. These management limits were calculated based on a range of considerations including: free phase formation, vapour exposure of workers in trenches, fire/explosion hazard, effects on buried infrastructure, aesthetics, and technological factors. These considerations were primarily developed with urban settings in mind.

Some of the considerations noted above are relevant to a remote Green Zone setting, and others are much less so. The Technical Steering Committee for this project agreed at their January 16, 2012 teleconference that the primary considerations of concern relevant to developing green zone management limits for F2, F3, and F4 were:

  • Free phase formation;
  • Fire/explosion hazard;
  • Hydrophobicity; and,
  • Upward migration.

These considerations include some from the existing management limits (free phase formation and fire/explosion hazard) and some new considerations (hydrophobicity and upward migration). The main objective of this study was to evaluate hydrophobicity, flammability and residual NAPL saturation of coarse and fine grained soil.

To determine residual saturation, the difference between free NAPL (non-aqueous phase liquid) and mobile NAPL must be quantified. Mobile NAPL was defined as being continuous in the pore space and flows under a pressure gradient or gravitational force. Residual NAPL was defined as immobile, non- water entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous (White et al., 2004). Below the residual saturation, NAPL becomes discontinuous and is immobilized by capillary forces (Mercer and Cohen 1990, Brost and DeVaull 2000). Residual NAPL concentrations in soil depend on NAPL properties including liquid density, surface tension and viscosity and soil properties such as porosity, organic carbon fraction, moisture content, relative permeability, moisture wetting history and soil heterogeneity (Brost and DeVaull 2000). Residual NAPL concentration in soil decreases with increasing particle size and is reduced at higher moisture contents (Brost and DeVaull 2000).