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EDL-It® W12120TIATIM.",M, • Ms. Margaret Lagorio ' May 4, 2005 Page 3 ' • Benzene concentrations increased in zone'A well U-2, zone B wells U-14, U-16, U-24 and piezometer NP-1-65, and in zone C wells U-10, U-23, and U-25 since the last sampling ' event. • Benzene concentrations remained below laboratory reporting limits in the remaining wells and zone A piezometers NP-1-22 and NP-2-22, and in zone C piezometers ' NP-1-111, and NP-2-116. MTBE Concentration ' • MTBE was not detected above laboratory reporting limits in any of the samples. ' Ozone Microsparne System Description The Ozone Microsparge system consists of groundwater treatment via two C-SpargeT"' systems installed at the site. Each C-SpargeTM system includes an ozone generator, air compressor, ' and a programmable timer/controller. The C-SpargeTm systems utilize ozone microsparging, a process where ozone is entrained in air and introduced into groundwater. The ozone/air mixture is sparged at rates of 2-6 cubic feet per minute (cfm) through specially designed sparge points '• to create "microbubbles." As these microbubbles rise within the column of water, dissolved volatile organic compounds (VOCs) including petroleum hydrocarbons are oxidized. The system cycles among 16 micro-sparge points and sparges an ozone/air mixture into wells SP-1A&B, SP-1C, SP-2A, SP-2B, SP-3A, SP-3B, SP-3C, SP-4A, SP-4B, SP-5A, SP-5B, SP-5C, SP-6A, SP-6B and SP-6C. Ozone sparge wells are located in the areas of highest dissolved VOC concentrations near the eastern property of the Unocal Site. Each sparge point ' has a dedicated line and is'connected to the system by low density polyethylene (LDPE) tubing. The tubing runs through Schedule 40 polyvinyl chloride (PVC) pipe. The system layout is shown on Figure 9. Ozone Microsparge System Oaeration With the exception of scheduled downtime for maintenance, the microsparge system operated +' continuously during the first quarter 2005. Effectiveness of Ozone Microsparsae System ' System effectiveness is monitored by assessing groundwater analytical data and concentrations of dissolved oxygen (D.O.) from single-completion zone A well U-3, from zone C well U-10, and ' from nested piezometers NP-1 and NP-2 screened in zones A, B, and C. (Figure 2). Each of these wells/piezometer is within the radius of influence of one or more sparge points. D.O. readings are collected periodically, with groundwater samples, from wells U-3, U-10, and ' piezometers NP-1 and NP-2. Well construction details and D.O. readings are summarized in Table 7 and Table 8, respectively. The microsparge system was operated continuously during the first quarter 2005. J:lProjects10694010187-3251WorkinglReports1018715t Qtr 05-QMR RSR.doc Celebrating 35 Years of Excellence in Environmental Services