ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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Phase II Field ARCADIS Investigation and LNAPL ' Mobility Evaluation Report Union Pacific Railroad Company Former Maintenance Facility ' Tracy, California • Advancement of LNAPL across a portion of the monitoring well network ' previously lacking measureable LNAPL, suggesting that the LNAPL zone is expanding in that area ' 5.1.2 LNAPL Field and Residual Saturations The degree of fluid saturation within porous media fundamentally controls the mobility of fluids within that media. In a soil system,soil pores beneath the water table are completely filled with groundwater,which facilitates lateral groundwater flow in response to small groundwater gradients.Above the water table within the capillary ' fringe,the water saturation decreases in relation to the elevation above the properties of the soil.As the water saturation within the soil pores diminishes,connections between water-filled pores decrease to the point where the groundwater can no longer ' move laterally. At a site with LNAPL impacts,the relative saturations of three fluids(air, LNAPL,and ' groundwater) must be considered when evaluating fluid mobility. Of primary interest is the mobility of the LNAPL and the related fluid saturations where LNAPL is mobile or immobile. LNAPL is mobile where there is continuity between LNAPL-filled soil pores ' that allows for lateral LNAPL movement. The LNAPL saturation at which LNAPL is discontinuous to the extent that it cannot flow is termed residual saturation.As defined by the Interstate Technology and Regulatory Council (ITRC), LNAPL saturations at or below residual saturation are not mobile(ITRC 2009a). Residual LNAPL saturation is not a unique value for a given soil type.The residual ' LNAPL saturation of a soil is directly related to the maximum historical LNAPL saturation,which is spatially variable at an environmental site. Soil near the LNAPL release area(s)is expected to have higher maximum historical LNAPL saturations and, ' therefore, higher residual saturations. The residual saturation is also related to the physical properties of the LNAPL and can vary based on different LNAPL types or weathering of the LNAPL. For this reason, LNAPL residual saturation values are ' typically determined at several site locations through collection and analysis of undisturbed soil cores. Laboratory analysis of pore fluid saturations in site soil cores provides data on the field (initial)and residual LNAPL saturations. The fluid saturations are reported as the percentage of the soil porosity that is filled by the fluid. The field LNAPL saturation ' value represents the LNAPL saturation at static in situ conditions. Residual saturation of LNAPL was determined using the laboratory analysis free-product mobility,which