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WWI Report: Groundwater-quality Monitoring—May 30, 2001: 7500 West Eleventh Street, Tracy, CA. Page 7 lr<�i trend seen since groundwater in that well was first analyzed in May 2000 when diesel was at a concentration of 2,800 µg/L and gasoline was at a concentration of 11,000 pg/L. ;t For the first time since groundwater-quality monitoring began in May 2000, a measurable concentration of diesel, at 81 µg/L, was detected in the groundwater sample recovered from MW-6. Although no gasoline or BTEX compounds were detected in the sample from that well on May 30, 2001, the fuel oxygenate TBA was present at a concentration of 140 pg/L and the concentration of MTBE, which in November 28, 2000 had been 160 { : pg/L had increased to 550 pg/L. MW-6 is located across West Eleventh Street from the area on the 7500 West Eleventh Street property at which fuel hydrocarbons had leaked into the subsurface. The fuel oxygenates, which were already present in the first groundwater samples recovered from that well in May 2000, are interpreted to be a leading edge of a plume of affected groundwater emanating from that source. The initial absence of any other analytes of concern can be accounted for by the propensity of MTBE to flow more rapidly than other fuel components through a given hydrostratigraphic environment (The San Joaquin Company Inc. 2001b). The first appearance of diesel in the groundwater at the location of MW-6 on May 30, 2001 would support that hypothesis, but insufficient data is yet available to predict future characteristics of that phenomenon with certainty. r , The most striking differences between the two rounds of groundwater sampling conducted on November 28, 2000 and May 30, 2001 were the changes in concentration in analytes of concern recovered from monitoring well MW-7. There were large decreases in the concentrations of diesel, gasoline and the BTEX compounds between those two dates. The concentration of diesel fell from 36,000 µg/L to 1,000 µg/L and the concentration of gasoline fell from 47,000 pg/L to 2,800 pg/L, with commensurate large reductions in the concentrations of the BTEX compounds. (The sample of groundwater from MW-7 continued to be free of any traces of fuel oxygenates, including MTBE.) Those lower concentrations are compatible with those initially measured in groundwater from MW-7 in May 2000. The unusually pronounced perturbation in the concentration of the analytes of concern that was observed from the November 28, 2000 sampling round was unexpected and may not be typical of groundwater quality at that location. It will be important to observe carefully future trends in concentrations of analytes of concern in MW-7, which is directly down the groundwater gradient from the area just to the south of monitoring well MW-3 where fuel products were released into the subsurface when the fueling station at 7500 West Eleventh Street was in operation. 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 sic '� w