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f Dec-11-03 16:09 From-URS SAdkNTO VO T-426 P.02/05 F-410 [December 11,2003 Mr.Robert McClellan Supervisor R-E.H.S. San Joaquin County Environmental Health Department 304 East Weber Avenue,Third Floor Stockton,California 95202-2708 Subject= Addendum to the Work Plan to Install Gas Migration Monitoring Probes World Enterprises Site Stockton,California Dear Mr.McClellan: URS has prepared this addendum to the work plan(addendum)on behalf of RadioShack to install and monitor the gas migration monitoring probes at the World Enterprises site in Stockton,California. This addendum was prepared to address San Joaquin County Environmental Health Department's(SJCEHD) work plan review com=nts. This addendum was prepared based on our understanding of WCEM requirements following our discussions with Mr.Ted Tasiopoulos of your office. Portions of the work plan not affected by this addendurn will be followed. BACKGROUND URS prepared a work plan dated November 7,2003 for the Wold Enterprises site located in Stock-ion, California(Figure 1)to address SJCEHD concerns identified in the Closed Site Inspection Repair dazed July 17,2003 and in your letter to Tandy Corporation(currently known as RadioShack)chisel September 18,2003. Following the submittal of the work plant,several discussions with Mr.Tasiopoulos were completed. Based on these discussions with Mr.Tasiopoulos,it is our understanding that SJCEHD would like a minimum of 5 feet of seal at the top of the probes,to use pea gravel rather than sand,and complete the top of the probes within a stove pipe/monument(3 feet above the ground surface)rather than within a Christy Box(surface completion). The November 7,2003 work plan was prepared to meet SJCEM requirement for the screened interval of the probes to correspond to the same depth interval(from 15 feet below the ground surface[bgs]to 5 feet bgs)of the debris. To meet this requirement,it was initially proposed in the November 7,2003 work plan that there would be four feet of seal. Additionally,RadioShack has a concern that an aboveground completion may attract vandals. Hence,it was proposed in the November 7,2003 work plan to complete the probes within a Christy Box. REVISED SCOPE OF WORK URS plans to utilize a truck mounted drill rig equipped with hollow stem augers to advance soil borings to a depth of 15.5 feet bgs at each probe location. After each boring has been advanced to the desired depth,a gas migration monitoring probe will be constructed within the boring. Each probe will be screened from 6 feet to 15 feet bgs. The probes will be constructed with either a 1-inch or 2-inch PVC piping. Pea gravel will be placed between the annulus of the probe casing and the boring wall from 15.5 feet to 5 feet bgs. A one-foot of bentonite will be placed in the annulus from 5 feet to 4 feet bgs. The remainder of the annulus will be tilled with neat cement(from 4 feet bgs to the ground surface). The top