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Stantec • • April 13, 2009 Page 6 of 13 Reference: Response to RWQCB March 11, 2009 Letter OW-81D is even deeper extending from 120 to 125 feet bgs. There are 17 vertical feet of clay-rich strata between the sands in TS/MW-2D and the screened interval in OW-51) as well as approximately 1,500 feet of low-permeability clay-rich units laterally between the two wells. The general premise suggested by the RWQCB is that the relatively high impacts in well TS/MW-2D channel sands are migrating to the east up through the stratigraphic sequence and across the low permeability clay-rich overbank deposits. From strictly a stratigraphic perspective, a better upgradient source for well OW-51D impacts would be well TS/MW-1 D, screened between 80 and 90 feet bgs. However, the analytical data do not support this correlation. Historical analytical data for well TS/MW-1D display significantly lower TPHg, TPHd, and benzene concentrations than those in well OW-51D. The RWQCB also cites similar detections of TPHg, TPHd, and benzene amongst the wells. Review of historical data in the "D" zone indicates flow directions show some variation and average east-southeasterly. Assuming that OW-51D impacts are not derived locally, this would suggest a more likely source area west-northwest of OW-51D in the BP terminal. The only "D" zone well in the BP terminal is AR/MW-14D, which has historically displayed much lower TPHg, TPHd, and benzene concentrations than those in well OW-51D. STTChas cited evidence for a local source to impacts in well OW-51D in several previous submittals. In its January 24, 2008 letter entitled Response to RWQCB Letter Dated June 14, 2007, STTC concluded... "a local off-site source may be responsible for the relatively high concentrations in OW-51D. The lack of additional "D" zone wells located between wells OW-81D and OW-51D constitutes a gap in the off-site monitoring well network." STTC recommended..."Because the several lines of evidence presented above suggest that impacts to well OW-51D may be derived from an off-site source and do not appear to be the result of migration of COCs from the terminal facilities, the STTC recommends the installation of an additional "D" zone monitoring well to be located between well OW-81) and well OW-51D." The RWQCB approved the recommendation and requested additional wells be installed near the location of PS/MW-20. STTC installed wells PS/MW-20C and PS/MW-20D (20D screened from 73 to 88 feet bgs) late in third quarter and the wells were first sampled in fourth quarter 2008. Only two quarters of data are now available for these wells and additional quarterly data should provide a more definitive data set. It is well known that analytical results for initial monitoring well sampling events are typically biased high. Local soil impacts are often entrained in the drilling process and impact deeper aquifers. The data for both of these wells are consistent with this pattern. Although the first quarter 2009 data were not available when the RWQCB prepared its March 11, 2009 letter, TPHg and BTEX constituents were not detected above normal method reporting limits and TPHd was detected just above the water quality objective in well PS/MW-20D. These data, although preliminary, do not support the RWQCB's premise that well OW-51) impacts are derived from the relatively high concentrations near well TS/MW-2D. I:\STTC-Stockton\Reports\Response to Comments\RWQCB Letter 3-11-09\STTC Response to RWQCB 3-11-09 Letter Ftnal.doc