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E `Y.. ® II\ li <br /> • Mr Harlin Knoll <br /> October 28, 2005 <br /> Page 2 <br /> Number of Detections Minimum Maximum <br /> Constituents above the MDL of the 8 Concentration in yWL(Well ID) Concentration in yWL(Well ID) <br /> Total Samples <br /> TPHd 4 <50 380(MW-3) <br /> TPHg 2 <50 470(MW-2) <br /> Benzene 2 <0 5 36(MW-2) <br /> MtBE 3 <0 5 230(MW-3) <br /> Explanation <br /> MDL=Method detection limit <br /> ,og/L= Micrograms per liter <br /> Groundwater monitoring well MW-1 Is located downgradient of the former waste oil and <br /> underground storage tank (UST) excavations The maximum detected concentration of <br /> MtBE In monitoring well MW-1 has been 29 micrograms per liter (pg/L) over the past two <br /> years Figure 1a graphically presents MtBE concentrations in MW-1 over the past 10 years <br /> and clearly Indicates a consistently decreasing trend in MtBE with time <br /> . Monitoring wells MW-2, MW-3, and MW-5 are located to the southeast of MW-1 and <br /> downgradient of the former tank area and dispenser lines Although these wells have had <br /> consistent detections of TPHg, TPHd, benzene, and MtBE, Figures 2 through 5 clearly <br /> Indicate a consistently decreasing trend In the concentrations of these constituents with <br /> time Located down gradient of wells MW-2 and MW-3, concentrations of TPHg, TPHd, <br /> benzene, and MtBE In monitoring well MW-5 has approached non-detect levels over <br /> several of the past quarterly sampling events This implies that any residual Impact to <br /> groundwater In the vicinity of former Chevron service station 9-8632 Is localized to shallow <br /> groundwater in the vicinity of the former tank and product line area and is consistently <br /> decreasing with time <br /> Groundwater monitoring wells MW-6A and MW-6B are located down gradient and to the <br /> northwest of MW-1 and the former UST excavations Over the past three years, the <br /> maximum detected concentration of MtBE in well MW-6A has been 26,ug/L MtBE has not <br /> been detected In well MW-6A above Its primary Maximum Contaminant Level (MCL) of 13 <br /> ,ug/L In over a year MtBE has never been reported In well MW-6B at or above Its method <br /> detection limit (MDL) This indicates that petroleum hydrocarbon impact is confined to <br /> shallow groundwater Only sporadic detections of TPHg, TPHd, and BTEX have been 4 <br /> detected in wells MW-6A and MW-6B since sampling began in these wells Benzene has <br /> never been reported In these wells at or above its primary MCL Concentrations of these <br /> constituents have approached non-detect levels over several of the past quarterly events <br /> . a Figures 1 through 5 were plotted utilizing exponential regression with concentrations along y-axis In <br /> logarithmic scale <br /> N IFiles\Prolects\Chevron\Retaill#19-8632(Tracy)1Penodic Monitoring`200513Q2005 QMR 98632 doc <br />