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110 Sonora, Stockton <br /> Page 2 <br /> For example, the September 2005 results were 500 micrograms per liter (ug/1) of Total <br /> Petroleum Hydrocarbons as gasoline (TPH-g) and 54 ug/1 of benzene,versus 350 ug/1 of <br /> TPH-g and 73 ug/l of benzene from almost two years ago in January 2004. Similarly, the j <br /> TPH-g result in March 2005 was 2,700 ug/l,which was completely identical to the <br /> concentration of 2,700 ug/l of TPH-g from July 2003 when the ozone system was first ' <br /> installed. <br /> Table 1. Selected historic groundwater contamination concentrations and parameters of <br /> VW-2d <br /> Depth to TPH-g Benzene Dissolved Oxidation Reduction <br /> t <br /> Groundwater in ug/l in ug/1 Oxygen Potential (ORP) in <br /> in feet bs O in mg/1 -millivolts m <br /> an 2002 22.21 22,336 18 <br /> April 2003 23.12 51000 590 <br /> July 2003 23.58 2,700 640 0.56 <br /> Dec 2003 24.58 2,500 310 <br /> Jan 2004 24.05 350 73 0.28 -115.5 <br /> March 2004 23.43 1,900 270 <br /> May 2004 23.31 1;500 . 330 0.26 -153 <br /> June 2004 23.83 1;100 750 <br /> July 2004 24.18 700 60 <br /> Aug2004 24.59 1;400 200 6.54 -136.7 <br /> Se t 2004 -- 1;200 180 -123.2 <br /> Dec 2004 25.18 910 57 0.63 -138 <br /> March 2005 23-00 2;700 190 0.6* -149.5 ` <br /> June 2005 22.26 360 <0.5 1.18 -102.8 <br /> Se t 2005 23.50 1 500 1 54 1.37 -95.1 <br /> * DO of VW-2d was not taken in March 2005 during the regular groundwater monitoring. <br /> DO of VW-2d listed here was from February 2005 after OZ-1 was installed. <br /> Following the installation of the micro-sparger in February 2005, the level of dissolved <br /> oxygen (DO) in VW-2d has approximately doubled compared to the levels during-the <br /> previous two years (with the exception of the August 2004 number). The increase in DO. <br /> indicates that ozone and its decomposition product oxygen are being delivered to the area <br /> around VW-2d.' Rising DO concentrations in VW-2d could be a result of direct oxidation <br /> on the targeted;contaminants by ozone (03+ RC = CR-> RCOCR + Oz), or simply due to <br /> decomposition of ozone,which half life in groundwater is approximately 30 minutes at <br /> normal temperatures and pressures (Guidance for In Situ Chemical Oxidation of Contaminated Soil <br /> and Groundwater;Second Edition,prepared by Interstate Technology &Regulatory Council). <br /> Since there has been no significant reduction trend in the level of contamination in V�V-2d, <br /> the delivery of ozone onto the targeted contaminants <br />