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SECOR Groundwater Remedial Observation Mid-Point Observation Trigger Wells Zone Wells Wells A PS/MW-18, PS/WC-1S, None AR/MW-21A OW-2A, OW-4A B AR/MW-18B, AR/MW-21 B, PS/MW-20 PS/WC-1M OW-2B, OW 4B Wells located immediately downgradient of the East System are designated as remedial observation wells (Figure 1A). The remedial observation wells will be used for concentration trend analysis to determine the effectiveness of the ozone system downgradient of the ozone array in the "A" and "B" Zones. Wells located immediately downgradient of the remedial observation wells are designated as mid-point observation wells(Figure 1B). "B" zone well PS/MW-20 will be used as a mid-point observation well to determine the effectiveness of the "B" zone ozone remediation system downgradient between the remedial observation wells and the trigger wells. Due to the location of the POSDEF COGEN plant there is no mid-point observation well for the "A" Zone. Well PS/MW-20 will be monitored and sampled quarterly to monitor "B" zone concentrations for an early warning that the trigger wells may be impacted in the future. • Wells located downgradient of the mid-point observation wells, at the downgradient boundary of the well network, are designated as trigger wells (Figure 1A). The above-designated wells are sampled quarterly to insure concentrations are not increasing after the anticipated initial spike in concentrations from the ozone remediation system. The initial spike of concentrations are anticipated to last up to one year, and concentrations of petroleum hydrocarbons and MTBE within observation wells is anticipated to increase by one to two magnitudes over historical concentrations. After the initial spike period, trends of concentrations will be evaluated for three quarters. If concentrations reported for the remedial observation wells have increased by one order of magnitude over historical concentrations for three consecutive quarters, the contingency plan presented in SECOR's June 27, 2003 Response to RWQCB Letter Dated May 9, 2003 And Addendum to Corrective Action Plan Dated April 30, 2003 will be implemented. Although the CRWQCB-CVR initially concurred with evaluating chemical concentrations of the remedial observation wells, the mid-point observation wells, and the trigger wells against the California Maximum Contaminant Levels (MCLs), the CRWQCB-CVR indicated in electronic correspondence dated January 6, 2005 that these levels are no longer acceptable for the trigger wells. Below is a revised list of compliance concentrations approved by the CRWQCB-CVR for the trigger wells, and the initially approved list of compliance concentrations that will be used for the remedial observation wells and mid- point wells: • STTC 1Q06 Final Report 770T.43673.12.0002 18 May 1,2006