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Geological nclsnics!xc Page 3 Sinclair&Thompson Property LM Additional Soil Gas Survey Report June 18,2007 LO Soil Gas Borehole Sample Depths • All borings were advanced to a depth of 5.0-feet bgs. The rods were pulled back approximately 2-inches, exposing the screened interval, until flow was measurable in the syringe. The recorded sampling depth indicates the bottom of the exposed screen interval. LO Drilling Method A 9600 Series Power Probe, direct push drill rig, was used to advance rods to the selected sampling depth using a 135-foot pound hammer. x. The drive rod was driven to a depth of approximately 5-feet bgs and then pulled back approximately 2-inches to expose the inlets of the gas probe. A hydrated bentonite seal was k' placed around the drive rod at ground surface to prevent ambient air intrusion into the annual space of the borehole. Silicon tape was wrapped around the junction where the tubing entered the drive rod. v A lag time of 20 minutes, or more, was allowed for equilibration of subsurface conditions after probe insertion to the sample depth. A sampling tube was attached to the probe tip to prevent infiltration. Silicon tape was wrapped around the junction where the tubing entered the drive rod. After sample collection,both the drive rod and the tubing were removed. Each boring used independent tubing and rods, eliminating the need to decon between sampling boreholes. Purse Volume Test LO To ensure that stagnant or ambient air is removed from the sampling system, a purge volume vs. contaminant concentration test was performed as described below: La a The purge test was performed in SG-7 (Figure 2). This sampling point was chosen because it is the closest to SG-1 and thus the most likely to produce the contaminants of concern. ❑ The purge volume of the sampling system included the internal volume of the tubing and the annular space around the probe tip. o A step purge test of one, three, and seven purge volumes was performed on SG-7. The appropriate purge volume for sampling was selected based on the highest concentration for BTEX constituents detected during the purge volume tests. Benzene was not detected at any of the purge volume tests. The highest levels were detected during one purge volume and this was set as the default for the remaining soil gas sample points. Leak Test A two-part leak test was performed on each of the borings: u First, at each sample point an equipment vacuum test was performed after the drive 4.0 rod has been driven to the desired sample depth. A vacuum was applied to the tubing before the probe is opened and let stand for 30 seconds to confirm a tight seal. This test was successful at each location. The probe was opened and a 20-minute LW