SITE INFORMATION AND CORRESPONDENCE_PRE 2019

Home Browse Search

Stantec April 30, 2010 Page 6 of 14 Reference: First Quarter 2010 Site Status, Groundwater Monitoring and Remedial Summary Report-Stockton Terminals Technical Committee Facilities • Remedial Observation Well PS/MW-18 - TPHg and BTEX concentrations were at non-detectable levels, while concentrations of MTBE (7.7 pg/L) and TBA (22 pg/L) both decreased from fourth quarter 2009 (52 and 300 pg/L, respectively). MTBE and TPHd concentrations decreased by 91 percent since August 2004 (prior to startup of East System); TBA decreased by 96 percent. • Treatment Area Well TS/MW-1A - Toluene, ethylbenzene, xylenes, and TBA were non-detect during first quarter. Since August 2004 (prior to the startup of the West System), hydrocarbon concentrations have decreased by over 97 percent for TPHg and over 99 percent for benzene and MTBE. • The Site's first quarter maximum MTBE concentration (9,700 pg/L) decreased 97 percent from the historical maximum (290,000 pg/L in 1999), while the first quarter maximum benzene concentration (8,000 pg/L) decreased 82 percent from the historical maximum (45,000 pg/L in 1993) (Appendix Table D-1). • Over 56 percent of the wells are below the WQOs and/or MRLs for the respective analytes, except TPHg and benzene. 3.5.2 "B" Zone Groundwater Quality Data Groundwater samples were collected from 18 "B" zone wells (Table 1 and Appendix Table D-2). Well AR/MW-14B was inaccessible due to terminal operations and could not be sampled. The overall trend in the "B" zone has been decreasing concentrations across the site. Consistent with this trend, dissolved TPHg, TPHd, BTEX, and MTBE groundwater concentrations in the "B" zone were decreasing or within historical ranges during first quarter 2010, with the following few exceptions: • Well AR/MW-5B - Benzene (81 pg/L) increased from fourth quarter 2009 (69 pg/L). Benzene concentrations in this well have been gradually increasing. Most other analyte concentrations from this well have decreased significantly since fourth quarter 2008. • Well AR/MW-9B - Benzene (720 pg/L), toluene (5.2 pg/L), TPHg (1,900 pg/L), and MTBE (15 pg/L) concentrations exceeded fourth quarter 2009 concentrations of 590 pg/L, 3.2 pg/L, 1,600 pg/L, and 9.1 pg/L, respectively. • Well TO/WC-2B - MTBE (300 pg/L) exceeded the fourth quarter 2009 concentration of 250 pg/L. MTBE concentrations in this well have been gradually increasing. All other analytes are within historical ranges. Several noteworthy decreases are mentioned below. • Treatment Zone Well TS/MW-1B - Since August 2004 (prior to the startup of the West System), hydrocarbon concentrations have decreased by over 99 percent for TPHg, benzene, and MTBE; while TPHd decreased by over 93 percent and TBA decreased by 91 percent.