SITE INFORMATION AND CORRESPONDENCE_PRE 2019

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Stantec April 30, 2010 Page 5 of 14 Reference: First Quarter 2010 Site Status, Groundwater Monitoring and Remedial Summary Report-Stockton Terminals Technical Committee Facilities the certified laboratory analytical reports, chain-of-custody documentation, and results of quality control analyses are presented in Appendix D. 3.5 GROUNDWATER QUALITY DATA - FIRST QUARTER 2010 Recent groundwater analytical data are presented in Tables 6 and 7, while historical groundwater analytical data are presented in Appendices H and I. First quarter 2010 "A", "B", "C", and "D" zone isoconcentration contour maps for TPHg, TPHd, benzene, MTBE, and TBA are included as Figures 8 through 12, 13 through 17, 18 through 22, and 23 through 27, respectively. As illustrated in the concentration versus time plots (Appendix F) many of the wells exhibit long-term decreasing concentration trends. Concentrations in the remaining wells are generally within historical ranges. Analyte concentrations have decreased significantly from pre-injection averages in remedial observation wells located downgradient of the East System (Section 3.5). Historically high concentrations of petroleum hydrocarbons in BP/ARCO wells may be related to a release of fire-foam that occurred during the weekend prior to the fourth quarter 2008 sampling event. At the request of the RWQCB, first quarter samples collected from 39 ARCO terminal wells were analyzed for MBAS (Table 6 and Appendix H). MBAS concentrations in 16 wells increased during first quarter 2010. Most of these wells have only been analyzed for MBAS four times and results may be normal data fluctuation. BP/ARCO wells will continue to be monitored for MBAS to determine trends. Summary tables for each hydrostratigraphic zone are presented in Appendix D and provide a concise overview of groundwater quality and cleanup progress by comparing analytical results with Water Quality Objectives (WQOs). The tables also present average and median concentrations as well as a comparison to the maximum detected historical concentrations for each analyte. Groundwater analytical data for each hydrostratigraphic zone is summarized below. 3.5.1 "A" Zone Groundwater Quality Data Groundwater samples were collected from 44 "A" zone wells (Table 1 and Appendix Table D-1). The overall trend in the "A" zone has been decreasing concentrations across the site (Appendices H and 1). Consistent with this trend, TPHg, TPHd, benzene, toluene, ethylbenzene, and total xylenes (BTEX), and MTBE concentrations in the "A" zone were decreasing or within historical ranges during first quarter 2010. Several noteworthy decreases are mentioned below. • Well AR/MW-9A - TPHg, TPHd, BTEX, and MTBE concentrations decreased by more than an order of magnitude from fourth quarter 2009; MTBE, toluene, and xylenes were not detected (<0.50 pg/L). • Trigger Well OW-4A - Tertiary butyl alcohol (TBA) concentrations have displayed a long-term decreasing trend in this well and during first quarter 2010 reached 5.5 pg/L (below the WQO for TBA). Since August 2004 (prior to startup of East System), TBA concentrations in this well have decreased by 97 percent.