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Stantec
<br /> L & M OPERABLE UNIT- FOURTH QUARTER AND ANNUAL 2012 SITE STATUS,
<br /> REMEDIAL SUMMARY, AND GROUNDWATER MONITORING REPORT
<br /> Groundwater Monitoring and Sampling
<br /> February 1, 2013
<br /> concentrations remained stable or non-detect in a few wells from C- and D-Zone wells,
<br /> respectively, during 4Q12. TPHd concentrations in groundwater samples collected from B- and
<br /> C-Zone wells generally increased during 4Q12. TPHd concentrations in groundwater samples
<br /> collected from D-Zone wells generally decreased during 4Q12. Benzene, MtBE, and TBA
<br /> concentrations in groundwater samples collected from all deep zone wells (B-, C-, and D-Zone)
<br /> generally remained stable or were non-detect during 4Q12. TPHg concentrations were at a
<br /> historic high in B-Zone well MW-59 (2.3 mg/L), and D-Zone wells MW-68 (0.69 mg/L), MW-69
<br /> (0.067 mg/L), and MW-74 (0.075 mg/L), during 4Q12. TPHd concentrations were at a historic
<br /> high in B-Zone wells MW-42 (0.63 mg/L) and MW-59 (0.95 mg/L), and C-Zone well MW-67
<br /> (0.53 mg/L), during 4Q12. TPHo concentrations were at a historic high in B-Zone wells MW-42
<br /> (0.4 mg/L) and MW-57 (0.32 mg/L), C-Zone well MW-67 (1.1 mg/L), and D-Zone wells MW-55
<br /> (0.66 mg/L) and MW-69 (0.32 mg/L), during 4Q12. Benzene concentrations were at a historic
<br /> high in B-Zone well MW-42 at a concentration of 0.0024 mg/L, during 4Q12. Xylenes
<br /> concentrations were at a historic low in C-Zone well MW-64 (0.0012 mg/L), and D-Zone well
<br /> MW-70 (0.0045 mg/L), during 4Q12. TBA concentrations were at a historic high in C-Zone well
<br /> MW-64 at a concentration of 0.028 mg/L, during 4Q12.
<br /> Dissolved TPHg in B-Zone groundwater was delineated to the southwest, west, and northwest,
<br /> during 4Q12. Dissolved TPHd was delineated to the northwest, west, southwest, and south,
<br /> during 4Q12. Dissolved TPHo in B-Zone groundwater was delineated to the north only, during
<br /> 4Q12. Benzene in B-Zone groundwater was delineated to the southeast, south, southwest, and
<br /> west, during 4Q12. Dissolved MtBE and TBA in B-Zone groundwater samples had no
<br /> detections at or above the MRL; therefore, was considered delineated during 4Q12.
<br /> Dissolved TPHg in C-Zone groundwater was delineated to the west and northwest, during
<br /> 4Q12. Dissolved TPHd in C-Zone groundwater was delineated, except to the southeast, during
<br /> 4Q12. Dissolved TPHo in C-Zone groundwater was delineated to the northeast and east,
<br /> except for a detection in well MW64, during 4Q12. Dissolved benzene in C-Zone groundwater
<br /> was delineated, except to the east and a detection in well MW-66, during 4Q12. Dissolved
<br /> MtBE and TBA in C-Zone groundwater were delineated, except to the east during 4Q12.
<br /> Dissolved TPHg in D-Zone groundwater was delineated to the south only, during 4Q12.
<br /> Dissolved TPHd in D-Zone groundwater was delineated during 4Q12. Dissolved TPHo in
<br /> D-Zone groundwater was delineated, except to the north and two detections in wells MW-73
<br /> and MW-74, during 4Q12. Dissolved MtBE in D-Zone groundwater was delineated during
<br /> 4Q12. Dissolved benzene and TBA in D-Zone groundwater were delineated to the west,
<br /> southwest, and south during 4Q12.
<br /> The extent of petroleum-affected deeper (B-, C-, and D-Zones) groundwater has not been
<br /> investigated by the City of Stockton at the UOU and MOU to the east.
<br /> 2.8 ARSENIC
<br /> Arsenic concentrations in selected shallow (A-Zone) wells within the hydrocarbon plume and
<br /> within perimeter wells are evaluated for historical trends on an annual basis during the third
<br /> quarter. Arsenic concentrations, since sampling for arsenic started on Site, appear to be
<br /> relatively consistent with the exception of A-Zone wells MW-25, MW-31, and MW-45 where a
<br /> I:\Tidewater\TW Stockton(P2A)\QMRs\2012\4Q12-Ann ual\4Q12 and Annual QSSR QRSR QMR Stockton_FINAL.doc 2.18
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