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Nov-29-01 12 :OOP P.03 <br /> SAN JOAQUIN COUNTY �J ; <br /> ENVIRONMENT HEALTH DIVISION " <br /> 3o4 E.Weber Ave.,Third Floor r CH24 HILL <br /> Stockton,CA 952M <br /> 0 155 Grand Avenue <br /> Phone:(209)400%M 0 <br /> Suite 1000 <br /> Approved <br /> VVltl1 Chan See Remarks Oakland,CA <br /> CH2MH isappr :S Rem /^ W12. <br /> Remarks GC F.O. Rox 12681 <br /> Oakland,CA <br /> 2681 <br /> November 28,200 el 51 .251.2426 <br /> Date - `�-to� Fax 5 .603,6205 <br /> 131420.Ai. approval of these plans extends on that which is <br /> -hown and described here on but not authorize <br /> c; c r approve any ommission or deviation of requirements <br /> Michael Inhima i <br /> San Joaquin CQuntX,_ _.. _. <br /> Public Health Services/Environmental Health Department <br /> 304 E.Weber Avenue <br /> Third Floor <br /> Stockton,CA. 95202 <br /> Subject: Modification of the Deep Monitoring Well Installation Procedure, <br /> Del Monte Disco Site <br /> 110 Filbert Street,Stockton,California <br /> Dear Mr. Infuma: <br /> As per your request during our November 28,2001 telephone conversation,this letter <br /> describes our proposed modifications for installation of deep monitoring well MW-DIS-11D, <br /> downgradient of the Del Monte Disco site.This well was designed to include a conductor <br /> casing,however,because the proposed well location was moved from the north side of <br /> Highway 4,where 1,2-DCA and petroleum hydrocarbons were detected at relatively high <br /> concentrations,to the south side,we believe a conductor casing is not necessary.Our <br /> rationale for not installing a conductor casing is summarized below: <br /> • 1,2-DCA was detected in borings CPT-11 and-12(on the south side of the freeway)at <br /> concentrations of 11 and 200 Lig/L,at depths of 104 and 88 feet bgs,respectively(an <br /> order of magnitude less than on the north side of the freeway). Conductor casings are <br /> most appropriate when installing monitoring wells through source zones with heavily <br /> impacted soil and/or groundwater,or in the presence of NAPL.These low <br /> concentrations pose a minimal risk of cross-contamination during well installation <br /> operations.The use of mud-rotary drilling methods will further reduce the risk of cross- <br /> contamination. <br /> • Circulation of a properly engineered drilling fluid will produce a controlled filter cake <br /> on the wall of the borehole as the drill string advances.The filter cake will provide <br /> consistent borehole stability,prevent drilling fluids from entering the contaminated <br /> formation,and prevent the drilling fluids from contacting impacted formation water. <br /> • The hydrostatic pressure exerted by the weight of drilling fluid column is sufficient to <br /> create positive pressures within the borehole that exceed aquifer pore pressures.This <br />