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Fortner Beacon Station No 12419 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan 7.2 Distribution of Petroleum-Impacted Groundwater The quarterly monitoring data (Table 1) through fourth quarter 2001 indicate the highest concentrations of gasoline constituents in groundwater have been regularly detected in air sparge wells AS-1 and AS-2, and monitoring wells MW-3 and MW9 Groundwater analytical data indicate concentrations of TPHg ranging up to 150,000 micrograms per liter (parts per billion or ppb), and benzene ranging up to 12,000 ppb The lateral extent of TPHg in groundwater, as shown on Figure 7, is delineated to the west and south by monitoring wells MW-1, MW-4, MW-5 and MW-10 The proximity to the lateral extent of TPHg in groundwater to the north and east is indicated by near-reporting limit TPHg concentrations in groundwater samples from monitoring well MW-6 and air sparge well AS-3 The distribution of benzene concentrations in groundwater is well delineated as shown on Figure 8 Reported benzene concentrations range from 1 3 to 12,000 ppb The lateral extent of benzene concentrations in groundwater, as reported for fourth quarter 2001, is defined to the west, south and east by monitoring wells MWA, MW-4, MW-5, MW-6, MW-7 and MW-10 The lateral extent of benzene concentrations in groundwater to the north has been defined in past quarters (fourth quarter 2000 to second quarter 2001) in monitoring well MW-11 when water was present in the well The groundwater level dropped below the screened interval in the third and fourth quarters of 2001 Concentrations of MTBE were reported only in monitoring well MW-8 for fourth quarter 2001 Only monitoring wells MW-2 and MW-8 have had near-reporting Iimit MTBE concentrations ranging from 1 6 to 14 ppb in the past four quarters No MTBE concentrations have ever been reported in air sparge wells AS-1 and AS-2 in which the highest TPHg and benzene concentrations have been reported in quarterly groundwater monitoring 419PARCAP3-02 doc 13 Project No 1419 23 HORIZON ENVIRONMENTAL INC.