Non-aqueous phase liquids
Spreading of non-aqueous phase liquids in heterogeneous geological media
Many contaminants entering the subsurface soil and groundwater due to anthropogenic activities are immiscible with water and move as their own phase. These so-called Non-Aqueous-Phase-Liquids are divided to those lighter than water (LNAPLs) or denser than water (DNAPLs). Common LNAPLs are gasoline and diesel fuel and typical DNAPLs are chlorinated hydrocarbons such as trichloroethylene (TCE), a common solvent. While preferentially present as their own phase, NAPLs also undergo phase transformations i.e. vaporize into the gas phase, dissolve into groundwater and adsorb onto soil particles. Further complication in predicting NAPL spreading is that they often are mixtures of chemicals that volatilize into the soil gas and dissolve into the aqueous phase at different rates, thus changing the composition of the NAPL mixture with time. The different phase interaction and phase transformation phenomena in combination with natural geological heterogeneity give rise to highly heterogeneous transport and retention behavior.
Figure: Fagerlund et al. (2008); LNAPL spreading in heterogeneous system
In our research we develop new methods and models that allow an improved description of NAPL spreading and immobilization in porous and fractured rocks, with particular emphasis on the effect of geological heterogeneity. Confidence in model predictions for NAPL transport in heterogeneous systems is as of to date limited. The constitutive models have not been sufficiently validated, mainly due to the lack of appropriate experimental data. We recently presented a set of well-controlled laboratory data on NAPL spreading in heterogeneous water saturated media that allows such validation (Fagerlund et al, 2007a,b). These data have been used as basis for model validation, showing that continuum based multiphase models can in general capture the overall behavior (Fagerlund et al, 2008). The work has continued partly with experiments aiming at characterizing the behavior of immobile source zones of Dense Non-Aqueous Phase Liquids by means of Partitioning Inter-well Tracer Tests (PITT), where the breakthrough curves of tracers that partition into the DNAPL are compared to breakthrough curves of conservative tracers (Zandin et al, 2007) as well as by looking at NAPL distribution, immobilization and subsequent source emissions from rock fractures. The work is done in collaboration with Colorado School of Mines (Prof Tissa Illangesekare). We have recently built our own experimental facilities to observe NAPL spreading in rock fractures, which will allow improved model validation.
Figure: Comparison of model results from IP and modified continuum multiphase flow model to NAPL migration in heterogeneous fracture (Yang et al, 2011)