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05/27/2004 16:33 5306766 STRATUS NO CALIF PAGE 02/04 <br /> P.. <br /> v � Y 3330 Cameron Park Drive, Ste 550 <br /> Cameron Park, California 95682 <br /> (530)676.6004-Fax:(5M 676.6005 <br /> May 27, 2004 <br /> Project No. U6100 <br /> Mr. Nuel Henderson, Jr. <br /> San Joaquin County <br /> Environmental Health Department <br /> 304 East Weber Avenue, Third Floor <br /> Stockton, California 95202 <br /> Re: Work Plan Addendum <br /> ARCO Facility No. 6100 <br /> 25775 Patterson Pass Road <br /> Tracy, California <br /> Dear Mr. Henderson: <br /> On behalf of Atlantic Richfield Company (ARCO — a BP affiliated company), Stratus <br /> Environmental, Inc. (Stratus) has prepared this Work Plan Addendum for ARCO Facility No. <br /> 6100, located at 25775 Patterson Pass Road,Tracy, California. <br /> After completing exploratory soil borings adjacent to the site in February 2004, Suatus elected to <br /> complete two monitoring well clusters (MW-10/MW-13 and MW-11/MW-14) in the Patterson <br /> Pass Road right-of-way. On May 25, 2004, well MW-11 was installed to approximately 108 feet <br /> below ground surface (bgs). During the advancement of the boring (B-43) adjacent to well MW- <br /> 11 on May 27, 2004, the hollow stem augers used to advance boring B-43 came into contact with <br /> well MW-11, damaging the well. Apparently either well boring MW-11 or MW-14 were not <br /> advanced completely perpendicular to the ground surface. Since well MW-11 was damaged, and <br /> may not have been installed completely vertical, Stratus is recommending that the well be <br /> properly abandoned by overdrilling, and backfilling with neat cement. Stratus is also <br /> recommending that boring B-43 be properly abandoned by backfilling with neat cement. <br /> Stratus is recommending that new borings be advanced to complete the remaining well <br /> installations- The recommended locations for the remaining well installations are shown on the <br /> attached figure. A discussion regarding the placement of the remaining wells is presented below. <br /> The remaining shallow screened monitoring well will be completed as MW-14, and the <br /> remaining deeper screened monitoring well will be completed as MW-15. <br /> Between May 25 and May 27, 2004, Stratus monitored deeper screened wells MW-10, MW-11, <br /> and MW-12 for the presence of groundwater. Groundwater was observed in wells MW-10 and <br /> MW-12; however, well MW-1.1. was dry. The groundwater level observed in well MW-10 <br /> PASp-Arco for URS%100VWork Plans\6100-work plan addaadum.dm <br />