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James L. Tjosvold,P.E. <br /> August 7, 1996 <br /> Page 6 <br /> Response: <br /> 2. DTSC and RWQCB approved workplans for Phase III and IV in which it was specified <br /> that wells would be installed in certain zones without installation of a nearby well in another <br /> zone. The purpose of MW-14 was to install a Zone C Well at what was thought to be (absent <br /> information on local Zone C groundwater gradient direction) a downgradient Zone C <br /> location. Although monitoring wells were not installed in Zones A and B in the vicinity of <br /> Zone C wells, MW-14 and MW-15, hydropunch samples of the groundwater in Zones A and <br /> B from these Zone C well locations provides valuable information indicating that PNAs and <br /> VOCs are present in Zones A and B upgradient of the site. <br /> The idea that historic groundwater flow reversals would account for impacted groundwater <br /> conditions in any direction in the vicinity of the PG&E site is highly speculative and can not <br /> be proven. MW-14 is an offsite Zone C well that has been upgradient of the site (during the <br /> water level monitoring of Zone C) and has had higher levels of PNAs at times than the <br /> onsite Zone C well which is located downgradient of source area 1. The issue of other <br /> petroleum release sites having impacted groundwater in the vicinity of the PG&E site is <br /> discussed in the response to the previous comment. With respect to recent changes in <br /> groundwater flow direction two items need to be reiterated at this time: <br /> -The groundwater flow direction in Zones A and C have not changed. The flow <br /> direction in Zone B has not varied widely (from NE to E), and in the last year and a half it <br /> has not varied. <br /> -The Zone C wells installed during Phase III were supposed to be installed <br /> downgradient of the site. Based on the researched"regional' gradient direction they were not <br /> installed downgradient of the site. After collecting local Zone C water level measurements <br /> for a year, PG&E installed two Zone C wells downgradient of the site during Phase IV. It <br /> appears, based on previous comments from the agencies,that there is a perception that the <br /> Zone C gradient varies,however that is not the case. The Zone C gradient direction does not <br /> conform with a"regional' gradient but does conform with the local Zone A and B gradient <br /> directions. <br /> Comment: <br /> 3. Page ES-2, Executive Summary <br /> The report states that there is no current risk from constituents of potential concern (COPCs) <br /> in soil or ground water since no complete exposure pathway exists as determined by a risk <br /> assessment(RA). This is not an accurate statement because the RA did not consider ground <br /> water as a receptor. Since ground water is a receptor and has already been impacted, a <br /> complete pathway does exist. <br /> Response: <br /> 3. It should be noted that the human health and ecological risk assessments were approved <br /> by DTSC and RWQCB on October 21, 1993. These risk assessments were performed in <br /> accordance with guidance and advisories developed by USEPA and DTSC as described on <br />