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1 <br /> E K MR <br /> Mr James L Barton <br />' April 12, 2005 <br /> Page 3 <br /> Former Unocal Site, 5400 and 5606 Pacific Avenue, Stockton, California," dated <br /> October 13, 2000 <br /> Site stratigraphy is segregated into four hydro-stratigraphic units that generally correspond to <br /> screened intervals of site wells The shallowest wells are screened in the hydro-stratigraphic <br /> unit designated as Unit A and the deepest wells are screened across hydro-stratigraphic Unit D <br /> A cross-section location map and cross-sections A-A', B-B', and C-C' are included in <br /> Attachment E First quarter 2005 analytical results for TPHg, benzene, and methyl tertiary <br /> butyl ether(MTBE) are presented on the respective cross-sections <br /> IRemediation System Description <br /> Residual and dissolved concentrations of petroleum hydrocarbons beneath the site are treated <br /> by air sparging (AS) and by soil vapor extraction (SVE) The AS system consists of an <br /> Ingersoll-Rand model SSR-EP25SE air compressor with a maximum positive displacement of <br /> 95 standard cubic feet per minute (scfm) at a pressure of 125 pounds per square inch (psi) <br /> The system displaces air through 17 sparge wells The sparge wells are connected to the <br /> system by Schedule 40 PVC pipe Each sparge well head is equipped with a ball valve for <br /> Iadjusting air distribution and ports for measuring pressure and flow rate <br /> During the first quarter of 2005 ENSR converted the vapor abatement component of the SVE <br /> system from a thermal oxidizer to activated carbon adsorption treatment The SVE system <br /> utilizes 18 vapor extraction points 14 soil vapor extraction wells and four modified groundwater <br /> monitoring wells Since the SVE system was restarted on February 17, 2005 the extracted <br /> vapors have been treated using activated carbon before being discharged to the atmosphere <br /> The system discharges treated vapors under the San Joaquin Valley Air Pollution Control <br /> District (SJVAPCD) Authority to Construct (ATC) N-5879-1-1 <br /> The wells and conveyance piping are constructed of schedule 40 PVC pipe and are equipped <br /> with ball valves for regulating air flow as well as flow and pressure monitoring ports Well <br /> construction details are in Attachment C and the system layout is shown in Figure 15 <br /> Soil Vapor Extraction System Status <br /> The SVE system was started in October 1999 as required by CAO No 5-00-707 The SVE <br /> system was shut down in October 2003 as a result of "a general downward concentration trend <br /> for all wells, including MW-11 and MW-12, with occasional spikes" as stated in the RWQCB <br /> letter dated October 23, 2003 The SVE system was restarted on September 24, 2004, with <br /> RWQCB approval, as a result of an increase in the size of the benzene plume in the B-zone <br /> between the fourth quarter 2003 and third quarter 2004 <br /> On November 30, 2004, ENSR observed that the thermal oxidizer combustion chamber wall <br /> had ruptured since the previous site visit On December 7, 2004, ENSR submitted a letter to <br />' J 1Proiects10894015098-2931WorkinglReports15098 1st Qtr 05 QMR doc <br />' Celebrating 35 Years of Excellence in Environmental Services <br />