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3. Groundwater Monitoring and Sampling Program Evaluation GHD reviewed historical and current groundwater analytical data to evaluate the current GWM&S program. Currently 26 groundwater monitoring wells are monitored and sampled on a semi-annual basis during the first and third quarters of each year. All groundwater samples are analyzed for the following constituents: • Total petroleum hydrocarbons, gasoline range (TPHg) by LUFT GC/MS according to Environmental Protection Agency (EPA) Method 8015B • BTEX (benzene, toluene, ethylbenzene, total xylenes, fuel oxygenates (methyl tert-butyl ether[MTBE], tertiary butyl alcohol [TBA], di-isopropyl ether[DIPE], ethyl tert-butyl ether[ETBE], tert-amyl methyl ether [TAME]), and lead scavengers (1,2-dibromoethane [EDB] and 1,2-dichloroethane [1,2-DCA]) by LUFT GC/MS according to EPA Method 8260B The monitoring wells are organized into the following water-bearing zones. • A-zone: MW-1 through MW-4 and MW-22 (5 wells) • B'-zone: MW-18 through MW-21 and MW-24 (5 wells) • B-zone: MW-5 through MW-14, MW-23, and MW-26 (12 wells) • C-zone: MW-15 through MW-17 and MW-25 (4 wells) A-zone TPHg and BTEX are not typically detected in A-zone monitoring wells, except for a few seemingly anomalous detections over the last decade. However, 1,2-DCA is regularly detected in MW-1 and MW-4, and MTBE is detected in all 5 A-zone wells. GHD compiled concentrations versus time graphs for A-zone wells to evaluate MTBE and 1,2-DCA A-zone concentration trends (Appendix B). Trends indicate that MTBE concentrations are stable or decreasing over time in wells MW-1, MW-2, and MW-22 located in the northern portion of the site property and are slightly increasing over time in wells MW-3 and MW-4, located on the southern portion of the property. 1,2-DCA concentrations are generally decreasing over time in A-zone wells. B'-zone Elevated TPHg and moderate BTEX concentrations are detected in B'-zone well MW-21, 1,2-DCA is detected in wells MW-18 through MW-20, and MTBE is detected in all 5 B'-zone wells. MTBE concentration versus time graphs indicate that MTBE concentrations are generally stable to decreasing over time. B-zone B-zone wells are mostly devoid of TPHg and BTEX concentrations, however MTBE and 1,2-DCA are widespread. Further B-zone assessment is currently being planned. C-zone No TPHg, BTEX, or fuel oxygenates are typically detected in C-zone wells, however 1,2-DCA is detected in all 4 wells. Further C-zone assessment is currently being planned. Proposed GWMBS Program Changes Since groundwater samples are currently analyzed only for EPA Methods 8015B and 8260B there no efficiencies to be gained by discontinuing any constituent analyses. Given that further assessment is planned for B-and C- zone intervals, changes to the existing GWM&S schedule in these zones are not warranted. GHD proposes the following changes to the current GWM&S program: GHD I Phillips 66 112594700 1 Offsite Investigation Work Plan 2