ARCHIVED REPORTS PHASE II FIELD INVESTIGATION AND LNAPL MOBILITY EVALUATION REPORT

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1 r,_;i it Phase II Field ' Investigation and LNAPL Mobility Evaluation Report Union Pacific Railroad Company ' Former Maintenance Facility Tracy, California MW-06A because it was thought the observed increase in product may increase t the product yield enough to make a skimmer system affective. The skimmer system removed only 6.1 gallons of product between August 1992 and May 1996,when it was removed and replaced with two bilge bags (ERM 2006). Passive diesel product ' recovery using bilge bags and absorbent socks was conducted until August 2007. During that time, only a limited volume of product(approximately 32 gallons)was recovered and passive product recovery was discontinued in August 2007. Based on ' this information,the skimmer system was not effective for removing LNAPL. The dual-phase extraction system operated from December 2000 to July 2002. UPRR ' shut down the dual-phase extraction system to evaluate rebound of VOCs and TEPH-d concentrations in soil vapor and groundwater. The results of this study were presented in ERM's report titled Rebound and Remediation Effectiveness Evaluation(ERM ' 2003). As concluded in the rebound evaluation report, the dual-phase extraction system was not effective for remediation of residual VOCs and diesel product remaining in the source area,therefore, the dual-phase extraction system continues to ' be maintained in shutdown mode. The dual-phase extraction system removed approximately 53 gallons (380 pounds)of free-phase product. t2. Hydrogeology Groundwater has been designated into three zones (the A-, B-, and C-Zones)based ' on depth below ground surface(bgs). Shallow groundwater(referred to as the A-Zone), occurring at approximately 10 to 25 feet bgs,was the focus of this investigation. Relatively low COC concentrations have historically been detected in samples ' collected from the B-and C-Zones. Groundwater generally flows toward the north- northeast. ' Soil at the Site,to a depth of approximately 100 feet bgs, is composed primarily of clay and silt with varying amounts of sand and fine gravel.A more permeable sand layer was identified in most of the Phase II field investigation CPT logs from 10 to 15 feet ' bgs. This sand layer was not identified at locations to the north and northeast(i.e., HP-2, HP-3, HP-4, HP-7, HP-20, HP-21, HP-23, HP-24, and HP-27). Copies of CPT and LIF boring logs from this investigation are included in Appendix A. Table 2 presents the depth-to-groundwater measurements and groundwater elevations from 2005 to the present. A March 2010 groundwater elevation contour map for the A- ' Zone is presented on Figure 3. The March 2010 data indicated the gradient in the A- Zone is toward the northeast. Depth to groundwater ranges from 8 to 17 feet bgs 1 mwpmesov.mapi m.'Wll 2x2010)d c 5