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li Particle Trackinq Organic contaminants such as TPHg and TPHd move slower than groundwater due to chemical-specific retardation and the calculated distribution coefficients. As a conservative measure, Geocon modeled inert particle movement to determine if shallow groundwater beneath the Site would be captured by the public supply water well when pumped continuously at a discharge rate of 2,000 gallons per minute (gpm). Note that particle migration is not affected by chemical partitioning and inert particles move at the same rate as groundwater. The three-dimensional particle tracking was simulated using MODPATH incorporated in the Visual Modflow Pro software package to determine the time inert particles in groundwater would take to get from the Site to the public water supply well,independent of contamination. Findings The MODPATH results indicate that inert particles originating in shallow groundwater (i.e., Layer 1) beneath the Site would reach the public water supply well in 29 years, assuming the well was pumped continuously for 29 years at 2,000 gpm. The particle tracking pathlines are depicted on Figures 13 (plan view)and 14(cross-section view). Selected MT3DMS model output times for TPHg (as benzene) are shown on Figures 15 to 17. As depicted on the figures,TPHg is not projected to migrate beyond site boundaries, and predicted TPHg concentrations in groundwater decrease to below 100 pg/l within three years. Selected MT3DMS model output times for TPHd (as naphthalene) are shown on Figures 18 to 22. As depicted in the figures,TPHd is not projected to migrate beyond site boundaries, and predicted TPHd concentrations in groundwater decrease to below 100 pg/l within 20 years.- l Stockton Yard Maintenance Station,TO No.1B Caltrans Contract No.06A1895,EA No.43-910206 Geocon Project No.59800-01-OIB -8- September 30,2013