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Report:Groundwater-quality Monitoring-November 28,2'000: 7.500 West Eleventh Street, Tracy, CA. Page 10 2000 contained gasoline at a concentration of 3,300 µg/L compared to a concentration of J 11,000 µg/L in the sample recovered from the same well on May 11, 2000. Significant reductions also occurred in the concentrations of diesel and MTBE in groundwater from that well between the two sampling dates. The concentrations of diesel fell from 2,800 µglL to 1300 µglL and the concentrations of MTBE fell from 10,000 µg/L to 1,900 µg/L. There were correspondingly large decreases in the concentrations of the BTEX compounds. These reductions are compatible with dissipation of fuel hydrocarbons affecting groundwater in the area around MW-3 due to the successful removal of floating r' product and a large volume of soil affected by high concentrations of gasoline and diesel that was excavated, in December 1998, from an area a short distance up the groundwater gradient from that well's location. (The San Joaquin Company Inc. 1999.) Changes in the concentrations of the analytes of concern in samples of groundwater recovered from MW-7 are of considerable magnitude compared to the changes observed in samples recovered from the other monitoring wells over the same period and are more difficult to explain. The concentration of diesel in samples recovered from MW-7 rose from 1,300 pg/L to 36,000 pg/L. The concentration of gasoline rose from 1,400 µg/L to ?'= 47,000 µg/L, and there were commensurately large increases in the concentrations of BTEX compounds. However, while a trace of MTBE at 22 µg/L was detected in the sample recovered from MW-7 on May 11, 2000, none was detected in the sample recovered from that well on November 28, 2000. While the large increases in the concentrations of analytes of concern in samples recovered from MW-7 may reflect the - arrival of a bolus of severely-affected groundwater migrating across West Eleventh Street from the former fueling station site, the magnitude of the change is unusually large. Particular attention will be given to developing trends in concentrations of analytes of concern in samples recovered from this well during future rounds of groundwater sampling and analysis. i f . 2.5 Disposal of Purge Water When the results of the groundwater analyses were available, the monitoring well purge water held in 50-gallon drums on the 7500 West Eleventh Street property was segregated into two groups: drums holding purge water from wells in which water samples recovered were affected by petroleum hydrocarbons and those containing clean water. The clean purge water was disposed by decanting it from the drum onto the surface of the 7500 West Eleventh Street property. The drums containing water affected by fuel hydrocarbons were left stored on that property. The water will be shipped to a permitted recycling facility for disposal.when sufficient quantity has accumulated for that to be - done economically following future rounds of groundwater sampling and analysis. 3.0 FUTURE ROUNDS OF GROUNDWATER SAMPLING AND ANALYSIS i The SJCPHS has requested that, in addition to the round of groundwater sampling conducted in May 11, 2000 as part of the site characterization program, three additional rounds of groundwater sampling and analysis be.conducted at the 7500 West Eleventh Street site. Of those three, the first was the November 28, 2000 sampling round, the sic