SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

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Mr. James L. Barton October 19, 2007 Page 2 Robinhood Drive and screened from 45 to 65 feet bgs, has a similar.monitoring history with 10 consecutive non-detect events(January 2007 to October 2004). The last detection of dissolved petroleum hydrocarbons in MW-17 occurred in the July 2004 monitoring event. The analytical data have established that the"B"zone, in the vicinity of the former MW-15 and MW-17, is no longer impacted by petroleum hydrocarbons. Thus, ENSR proposed a well screened across the water table as appropriate for replacing MW-15. The CVWB recommendation of 28 to 43 feet is valid based on the water level fluctuation range, and ENSR concurs with the proposed well screen of 28 to 43 feet. Comment No.2 The Workplan does not include labeled cross sectional views of the well construction and the lithological sequence to be monitored. Please submit two cross sections, a dimensionally labeled well detail, and a lithology of the monitored zone,with the revised Workplan. Response No.2 Figure 2 shows the locations of the requested cross-sections presented in Figures 3 and 4. Correlation of the soil types encountered in prior soil borings suggest that the screen of MWA 5R will be located within fine-grained materials, specifically silt to sandy silt between roughly 5 to 45 feet bgs, with the possibility of encountering clay at depths greater than approximately 38 feet bgs. Figure 5 depicts the proposed construction of MW-15R, as requested. The total well depth to 43 feet and placement of well screen from 28 to 43 feet bgs is shown on the well construction figure. Comment No. 3 The Workplan includes Attachment A, Field Methods and Procedures, which states that the borings will be hand augered from surface to 8 feet bgs, and then drilled with an 8-inch diameter HSA to total depth. The Workplan Attachment narrative varies from the Workplan text on page 2, which specifies using an air knife from surface to 8 feet bgs and a 6.5-inch HSA to the total depth of 40 feet. Note that the air knife or hand augering techniques are acceptable for starting the boring. However, the San Joaquin County Environmental Health Department and I recommend the use of an 8-inch diameter HSA for the remainder of the boring, due to the high potential for bridging of the filter pack or bentonite seal when 2-inch wells are constructed inside the smaller diameter(6.5-inch OD, 3.25-inch ID) HSA at depths greater than 25 feet bgs. Likewise, removing 40 feet of 6.5-inch HSA to construct the well in the open boring might result in a collapsed boring,which would necessitate re-drilling with a larger diameter auger. Please include the dimensions of the air knife and specify in the text, the use of an 8-inch diameter HSA (per Attachment A)in the revised Workplan. Response No.3 The reference to 6.5-inch OD hollow stem auger for the drilling is incorrect, and the Workplan text should have stated the use of 8-inch diameter hollow stem auger, consistent with Attachment A. ENSR will direct 01231-154 ENSR I AECOM