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Stantec • <br /> April 13, 2009 <br /> Page 7 of 13 <br /> Reference: Response to RWQCB March 11, 2009 Letter <br /> 5 We do not concur with the STTC's proposal to make installation of TS/MW-4D <br /> contingent on having to mobilize a drill rig to the site to install TS/MW-3DR. As <br /> discussed during our 10 becember 2008 meeting, TS/MW-31) is screened about 35 <br /> feet shallower than it should have been. Therefore, installation of deeper <br /> remediation and/or monitoring wells to address and verify cleanup of the full <br /> vertical extent of the dissolved phase petroleum hydrocarbon plume in this <br /> vicinity is required. The installations must be performed regardless of whether the <br /> proposed helium-oxygen tracer test shows that there is sufficient communication <br /> between TS/MW-3D and TSIMW-2D. We are willing to delay installation of the <br /> deeper wells until after the tracer test, but are not willing to make the installations <br /> dependent on the outcome of the test. <br /> Two unrelated aspects of this comment warrant clarification. Based on the results of <br /> the TS/MW-3D groundwater vertical-delineation study, the RWQCB requested that an <br /> additional monitoring well (TS/MW-4D) be installed to confirm and monitor impacts <br /> suggested by the grab sample data. STTC did not intend to imply that the installation of <br /> well TS/MW-4D would be contingent on the results of the proposed test of hydrologic <br /> communication between wells TS/MW-3D and TS/MW-2D. STTC's intent was to comply <br /> with the request, in the most cost-effective way during the same mobilization potentially <br /> needed to install replacement well TS/MW-3DR at its original target depth of <br /> approximately 110 to 115 feet bgs. STTC agrees to install well TS/MW-4D at a <br /> approximate depth of 125 to 135 feet bgs as requested by the RWQCB. <br /> As outlined in previous documents, the objective of installing injection well TS/MW-3D <br /> ata target depth of 110 to 115 feet was to conduct a pilot study to mitigate the relatively <br /> high petroleum hydrocarbon concentrations in well TS/MW-2D. The screened interval <br /> was erroneously installed at 104 to 109 feet bgs approximately 6 feet above the target <br /> depth. The most effective installation depth for the injection well and its potential <br /> replacement is at a similar depth to the relatively high impacts in well TS/MW-2D. If the <br /> helium-oxygen tracer test indicates that TS/MW-3D needs to be replaced with a well <br /> screened at a slightly deeper depth, then TS/MW-4D would be installed with <br /> replacement well TS/MW-3DR. If a replacement well is not needed, then TS/MW-4D <br /> will be installed in a unique mobilization, as was agreed to during our December <br /> meeting. <br /> 6 Table 2 in the Assessment Report appears to contain a discrepancy regarding <br /> the dates on which groundwater sampling occurred. The lower portion of the table <br /> shows that the TS/MW-3D grab groundwater sample was obtained from a depth of <br /> 140 feet bgs on 7 October 2008, and that deeper samples from 155 feet bgs and <br /> 160 feet bgs were obtained on 6 October. STTC needs to clarify whether this is a <br /> mistake and explain how deeper sampling was performed before shallower <br /> sampling. <br /> The sample dates shown on Table 2 are correct. The sample depths listed are the bottom <br /> depths of the five-foot screened intervals in the temporary wells constructed to collect the grab <br /> I:\STC-Stockton\Reports\Response to Comments\RWQCB Letter 3-11-09\STTC Response to RWQCB 3-11-09 Letter Final.ooc <br />