Lecture 5Non-Aqueous Phase Liquid (NAPL)Fate and TransportFactors affecting NAPL movementPartitioning processesWater to gas partitioning (volatilization)Henry’s Law ConstantNAPL to gas partitioning (volatilization)VolatilityExample: equilibrium with benzeneExample: equilibrium with gasolineGas to solid partitioningLiquid to solid partitioning (adsorption)Organic soil content and adsorptionOther adsorption mechanismsNAPL to liquid partitioning (dissolution)Solubility of mixturesSurface effects in dissolutionCo-solvent effects in dissolutionVapor densityRelative vapor densityExample Relative Vapor DensitiesVapor-phase transportFactors affecting NAPL movementNAPL movementNAPL movementTwo-phase flowRelative permeabilityRelative permeabilityRelative permeabilityObservation of relative permeabilityLecture 5Non-Aqueous Phase Liquid (NAPL)Fate and TransportFactors affecting NAPL movementFluid properties: 9 Density9 Interfacial tension9 Residual saturationPartitioning propertiesSolubilityVolatility and vapor densityPorous medium:PermeabilityPore sizeStructureGround water:Water content9 VelocityPartitioning processesNAPL can partition between four phases:NAPLAqueoussolutionGas(vapor)SolidWater to gas partitioning (volatilization)Aqueous ↔ gaseousHenry’s Law (for dilute solutions)Dimensionless (CG, CWin moles/m3)Dimensional (P = partial pressure in atm)HCCWG′=HCPW=Henry’s Law ConstantH has dimensions: atm m3/ molH’ is dimensionlessH’ = H/RTR = gas constant = 8.20575 x 10-5atm m3/mol °KT = temperature in °KNAPL to gas partitioning (volatilization)NAPL ↔ gaseousRaoult’s Law:CG= Xt(P°/RT)Xt= mole fraction of compound in NAPL [-]P° = pure compound vapor pressure [atm]R = universal gas constant [m3-atm/mole/°K]T = temperature [°K]VolatilityVapor pressure P° is measure of volatilityP° > 1.3 x 10-3atm → compound is “volatile”1.3 x 10-3> P° > 1.3 x 10-13atm →compound is “semi-volatile”Example: equilibrium with benzeneP° = 76 mm Hg at 20°C = 0.1 atmR = 8.205 x 10-5m3-atm/mol/°KT = 20°C (assumed) = 293°KAssume 100% benzene, mole fraction Xt= 1CG= XtP°/(RT) = 4.16 mol/m3Molecular weight of benzene, C6H6= 78 g/molCG= 4.16 mol/m3× 78 g/mol = 324 g/m3= 0.32 g/LCG= 0.32 g/L x 24 L/mol / (78 g/mol) x 106= 99,000 ppmvOne mole of ideal gas = 22.4 L at STP (1 atm, 0 C), Corrected to 20 C: 293/273*22.4 = 24.0 L/molGas concentration in equilibrium with pure benzene NAPLExample: equilibrium with gasolineGasoline is complex mixture – mole fraction is difficult to determine and variesBenzene = 1 to several percent (Cline et al., 1991)Based on analysis reported by Johnson et al. (1990):Xt= 0.0093 for benzene in gasolineCG= Xt(P°/RT) = 0.0093 (0.32) = 0.003 g/LConcentration in equilibrium with gasoline is 100-fold less than in equilibrium with pure benzeneCline, P. V., J. J. Delfino, and P. S. C. Rao, 1991. Partitioning of aromatic constituents into water from gasoline and other complex solvent mixtures. Environmental Science & Technology. Vol. 25, No. 5, Pg. 914. May 1991.Johnson, P. C., M. W. Kemblowski, and J. D. Colthart, 1990. Quantitative Analysis for the Cleanup of Hydrocarbon-Contaminated Soils by In-Situ Soil Venting. Ground Water. Vol. 28, No. 3, Pg. 413-429. May/June 1990.Gas to solid partitioningOnly a factor in very dry soilsIf relative humidity in soil gas > 90% (almost always the case in soils), then solid grains are coated with water filmGas to solid partitioning is highly nonlinear, not much studied, and not well understoodLiquid to solid partitioning (adsorption)Linear adsorption isotherm: CS= KdCWThere are also non-linear adsorption isothermExample: Freundlich isotherm CS= KfCW1/nMost often, linear isotherm is assumedOrganic soil content and adsorptionAdsorption of organic chemicals is strongly influenced by the organic content of the solidMeasured as fOC= fraction organic carbon (by weight)If fOC> 0.001 → Kd= KOCfOCfOCcan be measured by laboratory analysisKOCis tabulated, or predicted from KOWusing empirical formulae (e.g. KOC= 0.6 KOW)KOW= octanol-water partition coefficientOther adsorption mechanismsAdsorption also occurs on mineral surfaces, fracture faces, other inorganic solids – but with no similarly simple predictive rule as fOCAdsorption → retardation (see Lecture 3)Best to measure retardation in field or laboratory column studiesRetardation affects contaminant movement in both liquid and gas phasesNAPL to liquid partitioning (dissolution)Aqueous solubility – concentration in water in equilibrium with pure phase chemicalSolubility is function of size and polarity (smaller and more polar molecules are more soluble)Solubility correlates withLow KOW→ more solubleSolubility is affected by temperature, salinity, co-solvents waterin conc.octanol in conc.=OWKSolubility of mixturesFor compound mixtures, analog of Raoult’s Law applies:Using benzene and gasoline as examples:CW° = 1800 mg/L for benzeneCW= 17 mg/L for benzene in gasoline with Xt= 0.0093This approximate concentration is found in field measurements by Cline et al. (1991)phase pure withmequilibriu in ionconcentrat ==DDWWtWCCXCSurface effects in dissolutionFresh drop of coal tarAged three days, after fluid withdrawn from insideSee Luthy, R., A. Ramaswami, S. Ghoshal, and W. Merkel, 1993. Interfacial Films in Coal Tar Nonaqueous-Phase Liquid-Water Systems. Environmental Science & Technology. Vol. 27, No. 13, Pg. 2914-2918. December 1993.Co-solvent effects in dissolutionPresence of co-solvents may enhance solubilityExample: presence of alcohol increases solubility of organic compoundsMTBE (up to 11% by volume in gasoline) may enhance solubility of BTEXVapor densityVapor density is proportional to molecular weight (assuming compound is at boiling point)Want to know vapor density relative to air density to understand environmental behaviorDensity of dry air = 29 g/molRTMWPVdD=Relative vapor densityMore useful than Vapor Density is Relative Vapor Density (RVD)RVD = density of vapor-saturated air at 1 atm, 25°C to dry air:()2929760P760MW760PRVDDD−+=Vapor fractionAir fractionfor P° in mm HgExample Relative Vapor DensitiesCompound Molecular weightVapor densityRVDBenzene 78 2.7 1.17TCE 131.5 4.5 1.35PCE 165.8 5.7 1.12Gasoline ~100 to 115 ~3.3 ~1.02Vapor-phase transportDiffusion is enhanced in gas phase (D is higher)Differential pressure in soil gas → advectionAdvection in basements of buildings:Houses are under negative pressure due to advection of hot air out