SITE INFORMATION AND CORRESPONDENCE

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Arts&Artists -2- 2/23/98 San Joaquin County 1 interpretation of the data is that the pumping prior to 1992 also caused the plume of contamination to plunge downward beneath MW-6,MW 7, and MW-8 as shown by the occasional detection of contamination. Once the two municipal wells were'taken"off-line" and one municipal well was abandoned, the ground water flow reversed direction towards the south and southwest causing the plume to spread southward at depth. This I: interpretation of the data is based on the occasional detection of petroleum contamination in MW-5, the t deepest well on-site, from June 1992 through January 1995, and the consistent detection of contamination from April 1995 through January 1996. Neither scenario can be proven with the current data. Rather than using wells with long screens;wells with short screens should have been used to assess the vertical extent of contamination and determine'whether or not the plume was plunging beneath the wells. From July 1995 to February 1997, soil vapor extraction and air sparging was used to remediate the soil and ground water contamination. The air sparging�wells w_ ere completed at 75 ft bgs and the soil vapor extraction well was completed at 50�ft bgs. N-o pump and treat`wells were�-insWled.�This system moved approximately 4,680 kg of hydrocarbons. However, data from two soil probes, advanced to 55 ft bgs; was used to determine that 668 kg of hydrocarbon compounds remain in the soil. The amount of contamination remaining in the ground water was not determined. Since February 1997,residual ground water contamination has been detected in MW-I and MW-4,with the highest concentrations occurring in MW-4. The highest concentration of ground water contamination was originally in MW-1, and the remaining soil contamination is near MW-1. Therefore, the higher concentration of ground water contamination at MW-4 (430 µg/1 TPH-g, 0.92 µg/1 benzene, 23 gg/1 ethylbenzene, and 4.9 gg/l xylene) is unexpected. In addition, ground water samples analyzed using EPA Method 8260 had 4.1 Rill methyl-tertiary butyl ether(MTBE), and 6.3 to 17 gg/l 1,2-dichloroethane (1,2-DCA} in MW-1,MW-4 and MW-5. The California Maximum Concentration Limit is 0.5 µg/1. Conclusions: Based on the above data,the plume of contamination spread in a northward direction and probablyplunged beneath MW-6,MW-7, MW-8. Once the municipal wells were abandoned or taken"off- line"the''ground water flow reversed and the plume of contamination began spreading southward at depth. The highesfconcentrations ofcontamination since the remediation system was activated occurred in MW4 and MW{I,,which are the deepest wells on-site. The vertical extent of contamination was not determined prior to designing the remediation system. Therefore,this system may not have completely captured the ,._....,_�.�..plume fil w is.shon by,the_detection of contamination in M)K-5.aftenthe remediation system was activated. Because the vertical extent of the residual contamination has not been determined;the concentration of petroleum contaminants, methyl tertiary butyl ether(MTBE), and 1,2 DCA may be higher than has been detected. ,7 Recommieixdations: The well design and operating history of the nearby municipal wells needs to be reported: Based on this information,the lateral and vertical extent of the remaining ground water contamination needs to be determined by collecting ground water samples to the depth of the municipal wells or to the depth at which ground water samples are nondetect for petroleum contaminants. Once this is determined,the mass of the remaining ground water contamination needs to be calculated. This information will be use to determine the potential for the remaining contaminated ground water to impact the nearby municipal wells, if they are ever returned to operation. If the potential is significant,then additional remediation will be required at this site. Ground ;water, collected during the purging of the wells, needs to be analyzed and o erI disposed. dn