ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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tPhase II Field ARCADIS Investigation and LNAPL ' Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility ' Tracy, California displaces LNAPL with air via centrifugal force.The free-product mobility test ' provides a very conservative residual saturation, because the test is conducted at a centrifugal force of 1,000 times gravitational force. ' When field saturation equals residual saturation,the LNAPL is immobile. If field LNAPL saturation in a soil core is greater than the residual LNAPL saturation,then LNAPL is mobile at the pore-scale, but may not be migrating on a plume-scale. Hydraulic gradient, aquifer permeability, and LNAPL viscosity are a few factors that influence whether or not the LNAPL above residual saturation is capable of migrating. The LNAPL pore velocity calculations described below consider these additional factors. ' 5.1.3 LNAPL Pore Velocity Calculations ASTM International's E2531-06el Standard Guide for Development of Conceptual Site ' Models and Remediation Strategies for Light Non-Aqueous Phase Liquids (ASTM International 2006)suggests that a LNAPL mobility of less than 1 x 10-'centimeters per second (cm/s) is indicative that LNAPL in the formation is functionally immobile. When mobility exceeds this criterion, it is an indication that LNAPL is mobile at the pore-scale and capable of moving vertically or laterally within the formation.Additional ' analyses are needed to assess whether the LNAPL plume footprint is expanding due to LNAPL migration. Site-specific LNAPL mobility can be characterized through the use of site data and ' advanced LNAPL mobility calculations developed through the joint work of a number of research universities and private organizations. One of the more comprehensive bodies of work in this area has been completed by the API and published in numerous technical bulletins (e.g.,API Publications 4682,4711, 4729, and 4760). This science- based mobility assessment approach was formulated to support analysis of long-term movement of LNAPL plumes in the subsurface and is referenced as a component of t site management/characterization in a 2005 USEPA document titled A Decision- Making Framework for Cleanup of Sites Impacted with Light Non-Aqueous Phase Liquids(USEPA 2005). ' ARCADIS created a mobility calculation spreadsheet based on equations provided in API technical documents, predominantly API Bulletin 4760(API 2007).The ' spreadsheet model uses step integration functions to calculate the expected LNAPL saturation and corresponding LNAPL relative permeability based on input of the physical properties of the product and aquifer matrix and assumed water/LNAPL ' saturation conditions in the aquifer. LNAPL saturation and relative permeability are 19 ,plagrLacylnµl mo RY11-232010I.d.