SITE INFORMATION AND CORRESPONDENCE

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Response to SJCEHD Letter*ed March 2,2009 CGT Stockton Railyard,1645 Cherokee Lane,Stockton,CA March 13,2009 Page 2 The lower static water level is particularly relevant in well MW-6 where the upper portion of the screened interval brackets a sand lense between the depths of 50—53 fbg. This permeable sand lense is currently above the static water level measured in well MW-6 which could explain in part why well MW-6 was purged dry during the Fourth Quarter monitoring event after only 2.5 gallons had been removed. Well MW-5 has always been slow to recover following purging, even during initial development and sampling and this is likely due to reasons stated above. Regarding the pH fluctuations and high pH values reported for wells MW-5 and MW-6; this is not considered an indication of a compromised well. Since wells MW-5 and MW-6 are very slow to recovery following purging,a representative range of formation pH values is difficult to obtain and the measured value during the initiation of well purging is not likely representative of the actual formation pH value. The initial high pH value reported during well purging for all Site wells is more representative of the stagnant water within the well casing. For example,during the Fourth Quarter 2oo8 sampling event,the pH value in well MW-7 was initially 10.27. However,the second two readings during purging,which would be more representative of actual formation water,were 7.94 and 7.49 and consistent with pH values reported during initial well development and sampling in June 2008. The sheen reported in the purge water from MW-6 and the high pH value could also have been related to the low volume of purge water removed. Basically,the purging process had not fully removed all the stagnant casing water from well MW-6 and the slow recharge of formation water had not brought in enough clean formation water to obtain a pH value indicative of the formation. Had it been feasible to purge additional water from MW-6 during the event,the sheen may have disappeared and the pH may have been more representative of the formation pH. The high pH values may also be related to the imported fill material used to backfill the excavation. If the fill material used to backfill the excavation had a characteristically higher pH than the native soils,the result could be higher formation water pH values between wells located within the area of excavation(MW-5)and outside the area of excavation(MW-7). In conclusion,high pH values encountered during truncated well purging are not believed to be representative of the formation and it is not believed that wells have been compromised. Z Regarding the issue of silica gel cleanup of samples collected during the Fourth Quarter 2008 sampling event,all future samples will be analyzed for total petroleum hydrocarbons as diesel using EPA Method 8015M,with no silica gel cleanup. Regarding GeoTracker uploads,historical reports obtained from the previous consultant have been uploaded per request. Within those reports are contained well survey data,field sampling,and laboratory analytical reports. However,to ensure compliance with the California Code of Regulations,Title 23, Chapter 30,Section 3890,field sampling forms,laboratory reports,and well survey reports were uploaded to GeoTracker separately. In the future,TRC will provide 48 hours notice to the SJCEHD of all scheduled field activities, including routine quarterly groundwater monitoring and sampling events.