SITE INFORMATION AND CORRESPONDENCE_PRE 2019

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Stantec April 30, 2010 Page 7 of 14 Reference: First Quarter 2010 Site Status, Groundwater Monitoring and Remedial Summary Report-Stockton Terminals Technical Committee Facilities • The Site's first quarter maximum TBA concentration (1,800 pg/L) decreased 81 percent from the historical maximum (9,400 pg/L in 2008), while the first quarter maximum MTBE concentration (1,300 pg/L) decreased 75 percent from the maximum historical concentration (5,100 pg/L in 2007) (Appendix Table D-2). • Over 55 percent of the wells are below the WQOs and/or MRLs for the respective analytes, except TPHd. 3.5.3 "C" Zone Groundwater Quality Data Groundwater samples were collected from 14 "C" zone wells (Table 1 and Appendix Table D-3). The overall trend in the "C" zone has been decreasing concentrations across the site. Consistent with this trend, dissolved TPHg, TPHd, BTEX, and MTBE concentrations in the "C" zone were decreasing or within historical ranges during first quarter 2010. Several noteworthy decreases are mentioned below. • Well AR/MW-13C - TPHg, TPHd, and BTEX concentrations all decreased by more than an order of magnitude from fourth quarter 2008; including non-detectable concentrations of TPHg (<50 pg/L), toluene, ethylbenzene, xylenes and MTBE (<0.50 pg/L) during first quarter 2010. • Well PS/MW-20C - TPHg and MTBE concentrations decreased to non-detect while TPHd concentration decreased by more than an order of magnitude from fourth quarter 2009. • The Site's first quarter maximum TBA concentration (570 pg/L) decreased 90 percent from the historical maximum (5,600 pg/L in 2008), while the first quarter maximum benzene concentration (1,500 pg/L) decreased 88 percent from the maximum historical concentration (13,000 pg/L in 2002) (Appendix Table D-3). • Over 64 percent of the wells are below the WQOs and/or MRLs for the respective analytes, except TPHd and benzene. 3.5.4 "D" Zone Groundwater Quality Data Groundwater samples were collected from nine "D" zone wells (Table 1 and Appendix Table D-4). The overall trend in the "D" zone has been decreasing concentrations across the site. Consistent with this trend, dissolved TPHg, TPHd, BTEX, and MTBE concentrations in the "D" zone were decreasing or within historical ranges during first quarter 2010, with the following exception: • Well AR/MW-14D - Increased concentrations of toluene (14 pg/L), ethylbenzene (7.2 pg/L), and xylenes (31 pg/L) were reported first quarter, however all of these concentrations remain well below the respective WQOs. All other analytes were within historical ranges.