ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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' Phase II Field ARCAQIS Investigation and LNAPL ' Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility ' Tracy, California 5.1.4 LNAPL Pore Entry Pressure Calculations ' The LNAPL plume is stable and not migrating when the observed LNAPL thickness at the plume periphery does not exceed the calculated pore-entry pressure expressed as a critical LNAPL thickness. When a LNAPL plume is deemed unstable or migrating as a result of this analysis,then a monitoring program needs to be developed to verify and assess LNAPL plume migration. ' Lateral LNAPL migration into an unimpacted aquifer matrix that is saturated with groundwater requires displacement of that groundwater. In most aquifer systems, water is the wetting fluid (preferentially coating soil particles)and LNAPL is the non- wetting fluid. The amount of force that is required for LNAPL to displace groundwater is known as the non-wetting, or LNAPL, pore-entry pressure. The Brooks and Corey capillary pressure model is used to calculate the LNAPL pore- entry pressure. The van Genuchten capillarity model used in the LNAPL pore velocity calculations assumes that there is no entry pressure associated with LNAPL flow, ' which is conservative and appropriate when considering LNAPL movement in LNAPL- smeared soils. The Brooks and Corey model assumes that some minimum force is required for non-wetting fluid migration into water-saturated soil where no non-wetting fluid is present. This phenomenon is commonly observed for entry of air(a non-wetting fluid) into water-wet media. ' The critical pressure (or LNAPL head)that must be exerted to force LNAPL into saturated soil can be calculated from physical properties of fluids in the system and laboratory-air-displacing-water pressure(capillarity)data. The laboratory capillarity data are fitted using the Brooks and Corey equation in RETC, a retention curve program for quantifying the hydraulic properties of unsaturated soils developed jointly by staff of the University of California Riverside, PC-Progress, s.r.o., and the Federal University of Rio de Janeiro. The Brooks and Corey model displacement-head fitting parameter is modified using the interfacial tensions of the fluids in the system to adapt the airlwater displacement head to the equivalent LNAPL/Water displacement head. The critical LNAPL thickness in a well is related to the displacement head (pressure data in cm of water)through a density correction. This mathematical relationship is shown in Equation 10. 1 1 rW uprr4racy-Inaplm�iayc11-29-2010).ca 23 i