Linking changes in coal tar NAPL physical properties and compositional chemistry

Density, interfacial tension, viscosity, and wetting angle are properties that control coal tar NAPL migration. Analysis of these parameters is not widely available, but we hypothesise they are related to NAPL composition. If so, it could be possible to estimate certain parameters on the basis of simpler compositional analyses. We collected 21 coal tar NAPLs from a former gasworks site in east London, UK, and subjected them to physical and compositional characterisation. We found linear relationships between NAPL density and both surface tension and viscosity. The NAPLs were also subjected to serial dissolution in small batch systems, and became progressively enriched in PAHs and asphaltenes as more soluble species were leached. As dissolution progressed, some of the NAPLs increased in density. In some cases, LNAPLs became sufficiently dense to become DNAPLs. However, at a certain weight fraction, the asphaltenes precipitated out of the NAPL. This resulted in a sharp drop in NAPL density – in some cases lower than their starting density. Correlations between asphaltene content and density, surface tension and viscosity show that simple chemical analyses can give an indication of NAPL mobility, which should allow for better risk assessment and optimisation of free-phase recovery