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Foob-24-914 11 : 11A Port off Stockton HR Oopt _ 1 P- 03 Ms Carol Oz February 24, 1999 Page two PAhovgh this would cover the previous PHC detection in groundwater sampled from this depth at boring B3-2, it would not provide evidence for any potential contaminants at the same depth at the other two boring locations As such, the cost of a fourth well is not justified and we are recommending against installing a fourth well and instead recommending modifying the screened interval at each of the three proposed wall locations from 10 to 35 Feet bgs In sum, during the referenced conference call, we collectively agreed that this would be a preferred option for the following reasons W Screening from 10 to 35 feet provides a completion across each of the coarser-grained units identified beneath the site (see Figure 4 of Investigation Report) It also provides screens across from all of the sampled locations at which PHCS were detected in either soils or groundwater during the initial investigation o Lowering the top of the screened interval from 5 feet to 10 feet bgs meets the spirit of PHSIEHQ's rationale to have screen across from the likely maximum water table Historical water levels (1992 - 1998)from monitoring wells at other locations on the Port show that 1098 was a maximum year for water levels at the site This maximum corresponds to several above average rainfall years, which culminated in last year's El Nino events In light of this gear's La lana events, however, it is therefor unlikely that water levels will rise substantially above the 1998 level at this location (approximately 10 feet bgs) 4 The 1 O-foot top of screen also makes sense because there is no detectable or likely contamination above that level The bottom elevation of the tanks removed from this site was at 11 feet bgs, and it is therefore unlikely that contamination would be present above that level Additionally, soil samples obtained from 10 feet bgs at all three locations contained no detectable levels of PHCS e Screening above the 10-foot level could introduce contaminants into sails that previously were not contaminated if the groundwater beneath the site is under confined conditions (fine-grained, potentially confining soils are present beneath the site starting at 9 to 13 feet bgs), contaminated groundwater in deeper zones could potentially migrate upwards through the casing and into the surrounding formations Starting the screen at the 10-foot level would limit this potential i I I