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+ 1 CALIFORNIA GEOPHYSICAL GROUP,IjNC„ , ,, P O Box 578341, Modesto, CA 95357 ''• ,I 91`' k "'`- Tel (209) 527-1247 September 23, 1998 P 46 Fax (209) 527-4810 Public Health Services San Joaquin County P O Box 388 Stockton, CA 95201 ATTN Ms Carol Oz RE Work Plan August 21, 1998 Addendum Darpetro, Inc USA Service Station 749 E Charter Way Stockton, CA Site No 1060 On September 16, 1998, 1 received a call from Ms Carol Oz requesting further information on the above referenced work plan She requested us to verify the radius of influence of the Vapor Extraction System and information of the Air Sparging equipment we plan to use On September 16, 1998, we measured the vacuum in each vapor extraction well by closing the vacuum to that well and keeping open the vacuum to all the other wells It was impossible to . measure the vacuum in the monitoring wells because the depth to water was 34 3 feet and the highest slotted casing in any of the monitoring wells is 35 feet Well Closest Well Vacuum in Hg" (Feet) VW-1 22 230 VW-3 22 3 20 VW-4 23 250 VW-5 23 360 VW-6 26 010 We have prepared a graph (Fig 1) which plots the distance to the closest well vs the vacuum in each well The result is a line through two points which ends at 26 feet Therefore, the radius of influence is 26 feet Originally the radius was calculated to be about 38 feet, however, we were able to measure some of the monitoring wells at that time There were some abnormalities in the readings, since two wells were 22 feet distant and two wells were 23 feet distant from the nearest well This is possibly due to the geological units Some of the units are more porous than others and allow the vacuum to extend farther After the water table falls below 35 feet we will again check the vacuum pressures in the monitoring wells