ARCHIVED REPORTS XR0011745 - Laserfiche WebLink

Home Browse Search

Report, Groundwater-quality Monitoring—July 30, 2003: 7500 West Eleventh Street, Tracy, CA. Page 11 1 - recorded pattern in which increases and decreases in concentrations have occurred around >` a steadily declining trend that has been ongoing since groundwater affected by floating product was pumped from the large excavations opened on the 7500 West Eleventh Street site at the time the underground storage tanks were removed from that location. When groundwater in Well MW-7 was first analyzed in May 2000, it was affected by relatively moderate concentrations of diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 µglL and 1,400 µg/L, respectively), and MTBE (at 22 gg/L). However, over the next two years, the concentrations of both diesel and gasoline in samples recovered from that well exhibited a significant upward trend. As the concentration of those analytes increased, MTBE was no longer detected. That is almost certainly due to its presence being obscured in the chromatograms rather than by its actual absence. By April 11, 2002, it was evident that there was floating product in Well MW-7 and it was measured to have an apparent thickness of 0.30 ft. Since that time, the apparent thickness of floating product in that well has fluctuated in the expected manner in response to the rise and fall of the groundwater table. On July 30, 2003, the apparent thickness of floating product in Monitoring Well MW-7 was 0.33 ft. 4 What SJC believes to be a probable significant change has occurred in the hydrocarbon content of groundwater in the vicinity of Monitoring Well MW-11, as indicated by the : results of the analyses performed on the sample of water recovered from that well on July 30, 2003. Since groundwater monitoring began in the area at the down-gradient fringe of the plume, the groundwater in Monitoring Well MW-11 has contained a trace of MTBE, but, until July 30, 2003, no other analytes of concern were detected in samples recovered from that well. The sample recovered from Monitoring Well MW-11 on July 30, 2003, however, contained gasoline-range hydrocarbons at a concentration of 66 pg/L as quantified in the form of TPH(g). 69 �Lg/L of hydrocarbons in the diesel range were also detected in the sample, but neither matched the laboratory's applicable standard. Because no hydrocarbons in either the gasoline or diesel range had been previously detected in a sample recovered from that well, the July 30, 2003 data indicates that the principal plume of groundwater has by now advanced to the location of Monitoring Well MW-11 and that the plume fringe where groundwater is affected only by MTBE has advanced beyond that point. 2.4.1.2 Fuel Hydrocarbon and Oxygenates Around Fringes of Principal Plume As had been the case previously, the sample recovered on July 30, 2003 from Monitoring Well MW-5 which, as shown on Figure 2, is located at the extreme western perimeter of the plume of affected groundwater, contained a trace of MTBE (at the very low concentrations of 4.2 µg/L). The sample recovered from Monitoring Well MW-6, which is situated across the street and down the groundwater gradient from the location at which fuel hydrocarbons were discharged to the subsurface beneath the 7500 West Eleventh Street property, contained no detectable concentrations of any analytes of concern other than MTBE at a concentration of 250 µg/L, which concentration is also generally similar to the concentration detected in samples from that well recovered during earlier rounds of water-quality monitoring . sic