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stagy April 13, 2009 Page 4 of 13 Reference: Response to RWQCB March 11, 2009 Letter The comment continues with: "In addition, there is no evidence that any grab groundwater sampling was conducted in the other six boreholes" As discussed above, groundwater grab samples were never proposed to be collected in the 'other six borings" in any document submitted to the RWQCB. These wells were installed to replace the above cited wells that needed to be relocated due to nearby equipment operations. Competent, discrete groundwater samples were collected after the wells were developed and purged. The comment concludes with: "The lack of groundwater data from these boreholes and the petroleum hydrocarbons detected in the wells installed in the boreholes is evidence that the vertical and lateral extent of petroleum hydrocarbon pollution downgradient of the site remains undelineated." The conclusion from the December 15, 2008 Assessment Report states... "Groundwater grab sample data from boring TS/MW-3D indicate dissolved petroleum hydrocarbon impacts do not extend to depths of the 155 to 160-foot screened interval, with the exception of reporting-level detections of TPHd (56 ug/L). This indicates the vertical extent of impacts has been determined near boring TS/MW-3D (in proximity to well cluster TS/MW). No statements regarding the lateral extent of impacts was proffered. However, as discussed briefly below, first quarter 2009 quarterly monitoring data from the newly installed downgradient wells preliminarily support the premise that local sources are more likely responsible for impacts in well OW-5D. Therefore, these data provide preliminary evidence that adequate downgradient delineation has been achieved and additional quarterly monitoring data from the newly installed wells may corroborate this preliminary conclusion. 3 The cross-section in the Assessment Report shows there is a fine grained unit (assumedly means coarse-grained unit) at about 125 feet bgs in boreholes TS/MW-3D, OW-81), and OW-3DR, indicating that this D zone unit is laterally continuous for about 1,100 feet downgradient of the source area. The STTC arbitrarily has depicted this bed as a pinch-out layer between TS/MW-3D and OW-81). In addition, the base of the cross-section schematic is truncated below PS/MW-20D where the coarse grained bed should be connected between OW-813 and OW-3DR. STTC needs to explain why this D zone coarse grained unit was not interpreted as laterally continuous where similar data were used to show that shallower units (A, B and C) are continuous. Several points of clarification are appropriate. The cross-section is a generalized interpretation of the subsurface based on limited data collected over a significant distance (-0.4 miles) and logs prepared by different consultants with both discrete and more recently continuous sampling techniques. As outlined in the Assessment Report, based on previous data typical "D" zone strata consist of isolated, approximately 2 to 5-foot thick sand units, interbedded with silts, and clays. The lateral extent of the "D" zone sands has not been established; however, the sands are less than 5 feet thick, and occur at varying stratigraphic intervals. Previous data support more lateral continuity in the "B" zone and "C" zone sand units. The available data do not support lateral continuity of "D" zone units, let alone for 1,100 feet. A good example comes from the location with the best available data, where I1STTC-Stockton\Reports\Response to Comments\RWQCB Letter 3-1"MT-TC Response to RWQCB 3-11-09 Letter Final.doc