ARCHIVED REPORTS XR0011745 - Laserfiche WebLink

Home Browse Search

Report.- Groundwater-quality Monitoring—July 30,2003: 7500 West Eleventh Street, Tracy, CA. Page 10 r -� Note: In the case of the analyses for Diesel, the sub-aliquot of the sample from each monitoring well was filtered before injection into the gas chromatograph. STL is certified to perform the specified analyses by the California Department of Health Services (DHS). r The results of the analyses of samples of groundwater are presented in Table 2, which also includes the results of analyses of groundwater samples recovered from the previous sampling rounds, Figures 2 through 5 were developed from the results of the groundwater-quality sampling conducted to provide an interpretation of the lateral and vertical extent to which the subsurface was affected by components of fuel hydrocarbons on July 30, 2003. 2.4.1 Groundwater Quality in the Near-Surface Unconfined Aquifer The results of analyses of samples of groundwater recovered on July 30, 2003 show that all of the samples on that date appeared to be affected by fuel hydrocarbons. However, as is noted where applicable below, SJC believes that, in several cases, the reported results are false positives. 2.4.1.1 Analytes of Concern in Groundwater Along Principal Plume Axis j The groundwater sample recovered from Monitoring Well MW-3, which is close to the location on the 7500 West Eleventh Street site where the fuel hydrocarbons were released, but where the groundwater quality has benefited from the remediation work performed when the underground storage tanks and piping were removed from the site, contained 1,800 µg/L of TPH quantified as gasoline and an apparent 300 µg/L of diesel. Of the BTEX compounds, Benzene was present 420 µg/L, ethyl benzene at 22 F.g/L, and total xylene isomers at 92 VLg/L, but no toluene was detected. The fuel oxygenate MTBE was present in the sample recovered from that well at a concentration of 1,200 pg/L, but no other fuel oxygenates were detected. In the case of Monitoring Well MW-4, the sample recovered contained gasoline at a concentration of 1,200 ptg/L and an apparent concentration of diesel at a concentration of 91 µg/L, together with MTBE at 1,700 µg/L. However, the hydrocarbon in the diesel range does not match the laboratory's standard for diesel and, because the concentration is very low and none was detected in the sample recovered from Monitoring Well MW-4 on April 15, 2003 that was pre-treated with silica gel before analysis, SJC believes that the analytical result is a false positive for diesel. Similarly, it is likely that the analytical result of 300 µg/L of diesel in the sample recovered from MW-3 on July 30, 2003 is also a false positive. Those concentrations of analytes of concern in samples recovered from Monitoring Wells MW-3 and MW-4, except for a significant increase in the concentration of gasoline in the sample recovered from Monitoring Well MW-4, are consistent with the previously- sic i 4 .