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M CALIFORNIA GEOPHYSICAL GROUP , INC . P . O . Box 578341 , Modesto, CA 95357 Tel : (209) 527- 1247 Fax : (209) 527-4810 April 18, 2006 ( � Environmental Health Department n C I \vl � !'=' San Joaquin County W 304 East Weber Avenue Third Floor Stockton , CA 95202-2708 Attn : Mr. Nuel Henderson " Re : 2nd Addendum of Monitoring Wells (MW- 17 & MW- 18) dated March 21 , 2006 and SVE Work Plan dated March 28, 2006. Beneto Inc, 749 East Charter Way, Stockton , CA Site No . 1060 In response to the San Joaquin County Environmental Health Department (EHD) letter dated April 11 , 2006 you have requested a clarification and corrections of the well design for both wells (MW- 17 & MW- 18) and further information on the collection of the vapor samples . Monitoring Well MW-17 will be built by placing a section of 0. 020 slotted PVC casing into the bottom 10 feet of the boring . Then 10 foot threaded sections of schedule 80 PVC blank well casing will be threaded together from 120 feet to the surface . Number 3 sand pack will be placed opposite the slotted casing and 5 feet above it and slowly surged and consolidated prior to placing the transition seal until the sand pack has stabilized at 5 feet above the top of the well screen . A transitional seal will be placed from 120 to 115 feet which will consist of hydrated bentonite chips. From 115 feet to the surface the casing annulus will be filled with hydrated cement. To finish the installation , a traffic rated monitoring well box will be cemented into place at surface level and the new monitoring well will be capped with a locking cap and lock. Monitoring Well MW- 18 will be built by placing a section of 0. 020 slotted PVC casing into the bottom 10 feet of the boring . Then 10 foot threaded sections of schedule 80 PVC blank well casing will be threaded together from 145 feet to the surface. Number 3 sand pack will be placed opposite the slotted casing and 5 feet above it and slowly surged and consolidated prior to placing the transitional seal until the sand pack has stabilized at 5 feet above the top of the well screen . A transitional seal will be placed from 145 to 140 feet which will consist of hydrated bentonite chips . From 140 feet to the surface the casing annulus will be filled with hydrated cement. To finish the installation , a traffic rated monitoring well box will be cemented into place at surface level and the new monitoring well will be capped with a locking cap and lock. As indicated in the field logs submitted from the previous CPT boring (B-4) , a discrete sand/gravel interval exists at approximately 132 to 135 feet (bgs) . As you had previously commented and we concur that this zone of interest from 132 — 135 ft. needs to be investigated and analyzed . The two monitoring wells MW- 17 and MW- 18 will be installed in an effort to complete this goal . The soil samples collected from this point of interest will inform us to the possibility of the previously encountered sand lens contributing to the lateral movement of contaminates . The depth of MW- 18 at 160 ft (bgs) will also be valuable in confirming the vertical migration of contaminates. We have reassessed the value of sampling the groundwater during the installation of these wells and will only collect soil samples at the pre defined depths we previously indicated in the work plan . The soil samples that will be taken and analyzed from wells MW- 17 and MWA 8 will help to evaluate and further define the contamination plume .