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I 440 West Charter Way, Stockton, California September 16, 1992 Texaco Environmental Services Page 2 blower Results from the pilot test indicate a conservative radius of influence of 25 feet Air samples collected from MW-4 and MW-5 contained total petroleum hydrocarbons as gasoline (TPH-G) of 71,000 micrograms per liter (,ug/l) and 150,000 Vgll, respectively Because the MW's are cross-screened through the upper clays and lower silts and sands, soil vapors may have been preferentially drawn from the capillary fringe in the lower sands Therefore, the results of the pilot test may not be representative of the conditions in the upper clay soil horizon where the majority of the adsorbed hydrocarbons exist 3 0 WORK PERFORMED 3 1 Vapor Extraction Well Installation On April 27 and 28, 1992, a Groundwater Technology field geologist supervised the drilling of four 12-inch diameter soil borings (VE-1, VE-2, VE-3 and VE-4), to a depth of forty feet These borings are located in the northeastern part of the site in the area defined as being impacted with petroleum hydrocarbons (Figure 1) The spacing between borings was determined by assuming that each vapor extraction well would achieve a minimum 20 foot radius of influence in the clay strata as indicated during the vapor extraction pilot test performed on the MW's All boring locations for vapor extraction well (VE) installation were selected to ensure that the area of impacted soil would be influenced by the applied vacuum during vapor extraction During drilling operations a field geologist continuously logged the soils encountered in accordance with the Unified Soil Classification System Sod samples were collected at five foot intervals during drilling using a 2-inch inside diameter (I D ) California split spoon sampler A portable phototonization detector (PID) was used to field screen samples for the presence of petroleum hydrocarbons No soil samples were submitted for laboratory analyses since the borings were drilled near the areas explored during the assessment phase of this project Two vapor extraction wells were nested in each boring, with the screened intervals positioned at approximate depths of 15 to 25 feet BGS (VE-1 A, VE-2A, VE-3A and VE-4A), and 30 to 40 feet BGS (VE-1 B, VE-2B, VE-3B and VE-4B) This design was chosen based on the site stratigraphy determined earlier, and to allow concentrations of vacuum pressure on specific impacted horizons The wells were designed to make the extraction system efficient i1115k GROUNDWATER r-- �'�L TECHNOLOGY