Laserfiche WebLink
6 0 <br /> Stockton facility. Mud rotary drilling will therefore be used below the <br /> water table in borings to be completed as groundwater monitoring wells. <br /> Soils above the water table will be drilled using hollow stem augers to <br /> allow collection of soil samples. After the completion of sampling, the <br /> hole will be completed to its final depth using the mud rotary drilling <br /> method. Because the shallow soils may be contaminated and to prevent <br /> possible cross contamination, a conductor casing will be used to install <br /> MW-5, the deeper well adjacent to MW-4. After augering through the <br /> unsaturated zone, a conductor casing will be grouted in place. The boring <br /> will be continued using mud rotary once the grout has set. Centralizers <br /> will be placed above and below the screened section and at 40 foot <br /> intervals on blank casing for all wells greater than 60 feet deep. <br /> Sieve analysis of water-bearing strata will not be used to design filter <br /> packs for wells. Instead, a standard slot size of 0.02 inches will be <br /> used for all wells. There are two main reasons for this approach. First, <br /> based on previous drilling at the site, the subsurface soils are <br /> predominantly silts and clays with minor amounts of sand. It would be <br /> difficult to sieve such materials in the field in any meaningful manner. <br /> Further, sizing a screen small enough to minimize turbidity in such <br /> lithologies would also tend to reduce well yield. Therefore, we propose <br /> to maximize yield by using the larger 0.02 inch slot size and reduce <br /> turbity by other methods. A discussion of these issues is provided with <br /> the details of well installation in the Work Plan in Section 5.3. <br /> - Target Constituents and Analytical Methods <br /> PG&E has proposed a comprehensive sampling program for soils and <br /> groundwater. The EPA analytical methods that will be followed are <br /> provided in Table 5-1 of the Work Plan. <br /> As stated in the July 31 letter, dilutions of samples will be performed <br /> according to accepted laboratory practices, and will be used in assessing <br /> the concentrations of only extremely elevated constituents. <br /> - Tentative Schedule for the Supplemental Remedial Investigation <br /> The tentative schedule for the Remedial Investigation, Feasibility Study, <br /> and Remedial Action Plan is provided in Section 9 of the Work Plan. It is <br /> anticipated that the proposed scope of work will take approximately 20 <br /> months. You will note that this schedule allows a one month Department <br /> review and approval of major documents. <br /> If you have any questions concerning these responses or any other aspect of the <br /> Work Plan, please call me at (415) 972-1116. Our proposed schedule anticipates <br /> receipt of your comments on this Work Plan by September 1, 1991. <br /> Sincerely, <br /> Yvonne J. Meeks, R.G. , C.E.G. <br /> Manufactured Gas Plant Project <br />