ARCHIVED REPORTS XR0010073 - Laserfiche WebLink

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• Field Methods Geophysical Survey Norcal Geophysical performed a surface geophysical on January 16, 2001 in the area of the former tank pit and in areas suspected of possible tank presence This work was suggested by PHS-EHD to rule out possible sources of petroleum hydrocarbons given the contaminant trends observed at well MW-9 The survey consisted of magnetometer and ground penetrating radar equipment sweeping the area (see attached report Norcal report) The magnetometer survey searches for buned metallic objects and ground- penetrating radar (GPR) uses microwaves to penetrate the subsurface to depths of about 5 to 10 feet The responses of buned objects to this equipment indicate possible targets for additional subsurface study and provides data to evaluate buned objects The results'of the survey showed that some metal debns and utilities were detected however, no UST was located A possible fill port was noted in front of the large building (former garage), however that screw cap did not appear to have been a fill port and no vent pipes were traced to that location, no hard evidence of a UST was noted (see Norcal report, page 2) GPR data indicated small metallic debris near the welding building but it did not represent an object such as a drum or UST The suspected vent line was traced to the east (toward the former tank pit) with a termination about 55 feet east of the building • A suspected water line was also noted, and the large imgation line was not observed, possibly indicative of a clay pipe or nonmetallic pipe construction Exploratory Borings Twelve exploratory bonngs were drilled at the site (Figure 1) on Apnl 25, 2000 and Apnl 19, 2001 Drilling permits were secured from PHS-EHD, and all utilities were cleared pnor to doing the fieldwork Representatives of the Westside Irrgation Distnct and City of Tracy met Wright in the field and directed where boreholes should be placed On the day of drilling, the City of Tracy requested that due to the presence of two large sewer lines, not to dell any bonngs tentatively scheduled in that vicinity The direct push bonngs were advanced at the available locations in the unpaved areas shown on Figure 1 The bonngs were dnlled into the uppermost aquifer strata to the contact of, and into the underlying confining strata field working agwtard (referred to as the aquitaard strata in previous reports, strata which are used to terminate bonngs as a conservative measure to prevent puncturing possible clayey beds and allowing vertical leakage, the term aquitard implies a leaky strata) Moderate to severe flowing conditions were encountered in some boreholes, which required dnlling methods to be alternated between push and auger dnlhng depending upon the seventy of the flowing conditions Severe flowing borehole conditions were encountered locally, some soil and groundwater samples could not be collected, and the driller halted the borehole advance Page 3