SITE INFORMATION AND CORRESPONDENCE_PRE 2019

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Stantec April 30, 2010 Page 10 of 14 Reference: First Quarter 2010 Site Status, Groundwater Monitoring and Remedial Summary Report-Stockton Terminals Technical Committee Facilities BTEX and MTBE were not detected in any trigger wells during first quarter 2010. As illustrated on concentration versus time plots included in Appendix F (Figures 9, 13, 15, 34, 35, and 38), detected concentrations in the remedial observation wells have generally decreased with time and show noticeable decreases following installation of the East and West Systems. The table below further illustrates the decreases in compliance analytes benzene and MTBE following installation of the East and West Systems. MTBE Remedial Observation Wells Benzene (N9/L) N9/L) A Zone AR/MW-21A Historical Average 0.58 281 2008 Average <0.50 3.5 First Quarter 2010 <0.50 <0.50 PS/MW-18 Historical Average 72 492 2008 Average 2 60 First Quarter 2010 <0.50 7.7 PS/WC-1S Historical Average 116 0.99 2008 Average 6.9 <0.50 First Quarter 2010 <0.50 <0.50 B Zone AR/MW-18B Historical Average 2.4 0.85 2008 Average <0.50 0.63 First Quarter 2010 <0.50 1.5 AR/MW-21 B Historical Average 1.8 516 2008 Average 0.54 3 First Quarter 2010 <0.50 <0.50 PS/WC-1M Historical Average 3,168 38.7 2008 Average 39 3.9 First Quarter 2010 <0.50 <0.50 Historical averages include historical data through December 2005. Bold concentrations are below WQOs or MRLs. 5.0 CONCLUSIONS Overall hydrocarbon concentration trends in groundwater beneath the Site and down-gradient across Port of Stockton property appear relatively stable to decreasing. Offsite hydrocarbon concentrations present no significant threat to human health, sensitive receptors, or drinking water quality. Long-term decreasing concentration trends are apparent in many wells as illustrated in concentration versus time plots in Appendix F. The highest petroleum hydrocarbon concentrations continue to be reported in wells situated in or near the tank farm areas of the terminal facilities. Results from the ozone injection monitoring program indicate the East and West remedial systems are effectively mitigating off-site migration of petroleum hydrocarbons. Results also indicate compliance analyte concentrations are generally reduced from historical (pre-system installation) averages in remedial observation wells, some by several orders of magnitude. Compliance analytes were below reporting limits in all trigger wells.