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Stantec • April 13, 2009 Page 8 of 13 Reference: Response to RWQCB March 11, 2009 Letter samples. The method used was to construct the wells with a sand pack placed in the annular space to just above the screen followed by a one to two foot-thick bentonite pellet seal placed over the sand pack. The wells were purged until turbidity cleared prior to collecting the grab sample. On October 3, 2008, groundwater samples were originally collected from 145 feet bgs and 155 feet bgs. Stantec subsequently learned that these temporary wells were not constructed with the bentonite seal necessary to avoid drawing into the casing water from the overlying borehole water column. This rendered the analytical data suspect. Following collection of the 160-foot sample, bentonite pellets were placed to a depth of 155 feet bgs and allowed to set and seal off the bottom of the borehole, another temporary well was correctly constructed in the 150 to 155-foot depth interval and grab sample 155A was collected following adequate purging. The same process was used to collect grab sample 145A. After sample 145A was collected, bentonite pellets were placed in the bottom of the borehole to a depth of approximately 111 feet bgs and allowed to set and seal off the bottom of the borehole. Well TS/MW-3D was constructed. Because of the suspect nature of the initial analytical results from 145 and 155 feet bgs, the data was not included on Table 2 of the Assessment Report. A revised Table 2 is included in Attachment 2. 7 We concur with the terms of the Revised Contingency Plan. The section on Page 3 entitled Contingency Plan Language makes implementation contingent on confirmation from a monitored natural attenuation (MNA) study that natural biodegradation will not occur at a rate sufficient to destroy remaining petroleum hydrocarbons and MTBE downgradient of the A and B zone ozone sparge systems. After unsuccessfully attempting to locate the results of this MNA study, Regional Water Board contacted Stantec personnel who stated that the MNA study had already been conducted back in 2004 or 2005. An email correspondence in our file from about April 2004 discusses evaluating baseline data in anticipation of conducting the nitrate-sulfate injection, enhanced bioremediation pilot study at the BP property in 2005. Therefore, in the event that the Contingency Plan has to be implemented in the future, STTC's decision to proceed with the nitrate-sulfate injections serves as confirmation that the STTC and the Regional Water Board have reached a consensus that natural biodegradation is not occurring at a rate sufficient to destroy remaining petroleum hydrocarbons. A few points of clarification are appropriate. When Stantec was contacted by the RWQCB regarding the referenced MNA study, the author informed the RWQCB that the study, if already performed, would have been completed prior to his arrival at Stantec (formerly SECOR). Stantec indicated that based on references to the proposed MNA study in historical documents from around the date of the original contingency plan, the study would likely have been performed in 2004 or early 2005. The enhanced bioremediation pilot study referenced above was conducted by URS Corporation (URS) on behalf of Atlantic Richfield Company in May 26 through September 29, 2004 with approximately one year of subsequent evaluation monitoring. The results of the study were presented to the RWQCB in the September 28, 2008 URS Pilot Study Evaluation Report. It is noteworthy that the pilot study and its results are not germane to the referenced MNA rates downgradient of the ozone injection array, which are the MNA rates relevant to the implementation criteria in the Contingency Plan. The pilot study I:\STTC-Stockton\Reports\Response to Comments\RWQCB Letter 311-09\STTC Response to RWQCB 3-11-09 Letter Final.doc