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Stantec • • April 13, 2009 Page 5 of 13 Reference: Response to RWQCB March 11, 2009 Letter significant variation in lithologies occurs between borings TS/MW-2D and TS/MW-3D (located approximately 18 feet apart and both continuously cored). In TS/MW-2D, poorly-graded sand from 111 to 116 feet below ground surface (bgs) is replaced in TS/MW-3D by clayey sand (111 to 112.5 feet bgs) and clay (112.5 to 116 feet bgs).18 feet lateral distance. Another example is the two approximately 7-foot thick sands between 102 and 117 feet bgs in boring OW-3DR replaced by mainly clay and a 1.5-foot thick clayey sand in boring OW-8D at similar depths. The RWQCB suggests the coarse-grained unit at 125 feet in TS/MW-3D and OW-8D is laterally continuous. As shown in the boring logs, the data do not support lateral continuity of this unit. The TS/MW-3D sand extends from 124.5 to 137 feet bgs and consists mainly of clayey sand and gravel with silty sand and silt. The OW-81D sand consists of silty sand and poorly-graded sand and extends only from 117 to 121 feet bgs where sample recovery ends near the bottom of the boring at 125 feet. The RWQCB points out that..."the base of the cross-section schematic is truncated below PS/MW-20D where the coarse grained bed should be connected between OW-8D and OW-3101R." The base of the cross-section is truncated because boring PS-MW-20D extends to a depth of 107 feet and there is no data below this depth to support the purported continuity of the coarse-grained unit over this distance of nearly 800 feet. 4 The cross-section also shows that monitoring well OW-5D, which is located about 1,500 feet downgradient from source area well TS/MW-2D and screened in the same fine grained stratum, contains similar detections as TS/MW-2D. The three D zone monitoring wells (OW-8D, PS/MW-20D, and OW-3DR) located in between these two wells are not screened in the same fine grained unit but also contain similar detections of TPHg, TPHd, and benzene. The borehole data provide evidence that individual fine and coarse grained strata within the D--zone do not occur at uniform depths throughout the entire site. However, this geometry is commonplace since local fluvial processes that deposited the beds rarely, if ever, superposition individual strata within a bed like a layer cake. As discussed in the previous comment, the D zone unit as a whole is laterally continuous and transmits contaminants well downgradient of the source area. Therefore, contrary to statements in the Assessment Report, any wells screened within any depth of the D zone water bearing unit, including OW-8D and OW-5D, are in communication with each other. The similar contaminant concentrations observed in OW-5D and TS/MW-2D call into question whether the proposed communication test is warranted. It does show, however, that adequate communication exists within and between individual D zone strata. STTC disagrees with most of the specifics and the general conclusion presented above. RWQCB states that wells OW-5D and TS/MW-2D are screened in the same fine-grained stratum. This is not the case. As outlined above and in the Assessment Report, TS/MW-2D is screened from 111 to 116 feet bgs in poorly-graded sand. Well OW-5D is screened from 84 to 94 feet bgs in silty clay, silt, and sandy silt. The screen in well I\STTC-Stockton\Reports\Response to Comments\RWQCB Letter 3-11-09\STTC Response to RWQCB 3-11-09 Letter Final.doc