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1 v <br /> a <br /> A S S O C I A T E S I N C <br /> the contaminant source TPHg and BTEX concentrations in water samples collected from <br /> MW1 have shown a decreasing trend from concentrations of 200,000 ug/l (TPHg) and <br /> ' 31,000 ugh (benzene) in December 1990 to concentrations of 410 ug/l (TPHg) and 5 9 <br /> ug/I (benzene) in June 1999 TPHg and BTEX concentrations exhibited an increasing <br /> ' trend from concentrations of 1,500 ug/l (TPHg) and 280 ug/l (benzene) in October 1999 to <br /> concentrations of 5,700 ug/l (TPHg) and 1,700 ug/l (benzene) in November 2000 and have <br /> generally shown a decreasing trend since the November 2000 peak concentration During <br /> the fourth-quarter 2003 quarterly groundwater monitoring event conducted on November <br /> 28, 2003, TPHg was detected at a concentration of 270 ugll This was the lowest <br /> concentration since sampling began and represents a 99 8% reduction in TPHg <br /> ' concentration Additionally, concentrations of BTEX constituents were below laboratory <br /> detection limits While the groundwater was sampled for petroleum hydrocarbons since <br /> 1990, quarterly groundwater sampling for MTBE was not initiated until December 1997 <br /> ' MTBE concentrations detected in the groundwater samples collected from MW 1 have <br /> ranged from 2 3 to 80 ugll and have shown a consistent decreasing trend A hydrograph <br /> depicting TPHg concentrations associated with MW 1 is shown on Figure 10 and a <br /> ' hydrograph depicting BTEX and MTBE concentrations associated with MW1 is shown on <br /> Figure 10 TPHg isoconcentration contours estimated for the fourth-quarter 2003 quarterly <br /> groundwater monitoring event conducted on November 28, 2003 are shown on Figure 8 <br /> • Groundwater samples have been collected from well MW2 and analyzed since Januar <br /> P Y Y <br /> ' 1994 Well MW2 is located approximately 50 feet to the north of the site, across Monte <br /> Diablo Avenue, in a hydrologically upgradient direction TPHg has never been detected in <br /> MW2 and BTEX compounds have only been detected on five sampling events at very low <br /> ' concentrations Benzene was only detected once at a concentration of 0 48 ug/l in June <br /> 1996 This was the same sampling event during which TPHg and BTEX concentrations <br /> reached a peak in well MW1 While the groundwater was sampled for petroleum <br /> t hydrocarbons since 1994, quarterly groundwater sampling for MTBE was not initiated <br /> until December 1997 No oxygenates (including MTBE) have ever been detected in MW2 <br /> ' • Groundwater samples have been collected from well MW3 and analyzed since January <br /> 1994 Well MW3 is located approximately 55 feet to the west of the site, across Buena <br /> Vista Avenue, in a hydrologically downgradient direction TPHg and BTEX concentrations <br /> ' in water samples collected from MW3 have shown an overall increasing trend from <br /> concentrations of 1,700 ugll (TPHg) and 21 ugll (benzene) in January 1994 to <br /> concentrations of 8,900 ug/l (TPHg) and 95 ug/l (benzene) in October 1999 and have <br /> ' generally shown a decreasing trend since the October 1999 peak concentrations During the <br /> fourth-quarter 2003 quarterly groundwater monitoring event conducted on November 28, <br /> ' 2003, TPHg was detected at a concentration of 1,700 ug/l Additionally, benzene was <br /> below laboratory detection limits While the groundwater`"wase sampled for petroleum <br /> hydrocarbons since 1994, quarterly groundwater sampling for MTBE was not initiated <br /> until December 1997 MTBE concentrations detected in the groundwater samples <br /> w 1271421repoM\closure doc 14 <br /> i <br />