ARCHIVED REPORTS_XR0004383 - Laserfiche WebLink

Home Browse Search

' I This explanation is not fully compatible with the laboratory data from VM-3 and VM-4, however Both of these wells are located on the east side of the site, and should therefore be subject to the same downgradient migration of hydrocarbons as VM-2 The laboratory data from these wells does not show any clear pattern, rather, hydrocarbons have been detected randomly in VM-3, and even though contaminants (especially BTEX) have been detected regularly in VM-4, concentrations have fluctuated up and down with no apparent trend Both wells should be expected to show the same pattern as VM-2 under the prevailing eastward hydraulic gradient 5.2.4 Hypothesis. Additional Hydrocarbon Source A fourth possibility is that the trend observed in VM-2 records a contribution from an additional contaminant source near this well A nearby source could further contaminate this well without impacting other wells We have observed two possible sources that might be contributing hydrocarbons One possibility is the small storage tank that is buried beneath the sidewalk on Grant Street, approximately 30 feet northwest of VM-2 If there is also a buried pipeline from this tank, the potential source could be even closer to the well When last checked in mid-1998, the tank was holding fluid and did not appear to be leaking ' A second possible source is suggested by the strong "sewer" smell that is frequently noticed in the water that is purged from VM-2 during quarterly sampling This odor is not present in any of the other wells, and suggests that there may be a broken sewer line in the vicinity of VM-2 ,If gasoline is occasionally being transported in this sewer line, it would explain the large fluctuations in concentrations that are visible in Figure 4 The fluctuations cannot be attributed to seasonal changes in water depth, because such changes are minor at this site Other laboratory data collected earlier in this investigation also point to an additional, unidentified source In July 1996, methyl-tertiary-butyl ether (MTBE) was detected in water samples from the three monitor wells (Table 2) The concentration in VM-1 was particularly high, and suggests a rather large and/or recent release MTBE is a gasoline additive that was not in widespread use prior to the early or middle 1990's, and therefore would not likely be present in the UST's at this site, which were taken out of use in 1987 and removed in 1990 Also detected in 1997 were several chlorinated hydrocarbons (1,2-DCA, TCE, 1,2-DCE, chloroform, etc ) that are typical components of solyents and other products but are not normally found in gasoline As we concluded in earlier reports, these compounds strongly suggest one or more additional contaminant sources In summary, the concentration trends that have been observed in water samples from this site reflect multiple, independent processes Drilling additional monitor wells in order to develop a fuller understanding of the subsurface geology and construct a more complete map of the contaminant plume should enable us to better distinguish the relative importance of each process 5.3 Status of Encroachment Permit Progress was made in early December to obtain the necessary encroachment permit to drill in the City of Stockton sidewalks The City has indicated that it will approve the permit but has requested that the amount of the faithful performance bond be increased Valley Motors is 6