ARCHIVED REPORTS_QUARTERLY GROUNDWATER MONITORING REPORT 1ST QUARTER 2016

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Quarterly Groundwater Monitoring Report - First Quarter 2016 L&M Operable Unit- West Weber Avenue and North Lincoln Street,Stockton, CA April 29, 2016 • 5.0 Conclusions and Recommendations 5.1 GROUNDWATER SAMPLING RESULTS A-Zone Trends: Depth to water measurements presented this quarter appear consistent with previous trends. A-Zone groundwater analytical data reported are within Site historical limits and appear to be stable. • In general, reported TPHg, TPHd, TPHo, benzene, MTBE, and TBA concentrations in samples from the shallow zone wells were within historical ranges during 1Q16 (Figure 3 through Figure 8). • The extents of shallow petroleum hydrocarbon constituents appear to be generally delineated with the exception to the north where the Stockton Deep Water Channel appears to be influencing groundwater quality. The UOU and MOU wells were not . sampled by the City of Stockton during 1Q16; therefore, shallow petroleum hydrocarbon constituents could not be delineated to the east. TPHg, TPHd, TPHo, Benzene, MTBE, and TBA appear to be stable throughout the shallow zone. • Although Stantec currently calls out data gaps to the east of the L&M OU which limits the .'current" definition of the shallow groundwater plume due to the City of Stockton discontinuing their groundwater well sampling program, Stantec does have historical data from this area, ending in April of 2008, which may be used to help define this plume. Historical data, previous to April 2008, typically showed low concentrations of heavy end hydrocarbons (TPHd and TPHo) within some of these shallow zone groundwater wells, while all other analytes were typically reported below laboratory method reporting limits (Table 9 - Appendix D). B-, C-, D-Zone Trends: • In general, reported TPHg, TPHd, TPHo, benzene, MTBE, and TBA concentrations in samples collected from the deeper zone wells were within historical ranges during 1 Q16 (Figure 9 through Figure 15 for B-Zone, Figure 16 through Figure 22 for C-Zone, and Figure 23 through Figure 29 for D-Zone). Concentrations during 1Q16 remained relatively stable or were reported as non-detect. • Dissolved phase TPHg, TPHd, TPHo, benzene, and MTBE in B-Zone were generally consistent with historical trends for the Site. Wells located to the east (UOU and MOU wells) were not sampled, and therefore, data is not available to contour. ® Stantec 15