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Jeffery Wong 7h3a&*d1&Rd10 San Joaquin County Environmental Division 22 December 2003 Page 3 PROPOSED ADDITIONAL WORK The following data suggest that the identified former UST may not be the source of the groundwater contamination identified at monitoring wells former MW-4 and MW-5: 1. Groundwater is reported to have consistently flowed in a northeasterly direction at the site,which would place monitoring wells MW-4 and MW-5 cross gradient from the identified former UST (Figure 1). 2. Groundwater samples were collected from air sparging test wells AS-1, AS-2 and AS-3 after installation, and they did not have detectable concentrations of TPHg or benzene. The air sparging test wells are located 10 to 20 feet from monitoring well MW-4, between MW-4 (and MW-5) and the former UST location. If the identified UST was the source of contamination at MW-4 these wells would likely have contained detectable TPHg or benzene. 3. The proportions of benzene, toluene, ethylbenzene and total xylenes (BTEX) to TPHg that have been detected in MW-1, adjacent to the identified UST, are very different from the proportions of BTEX to TPHg in monitoring wells MW-4 and MW-5. For example, since monitoring began in MW-1 and MW-4 in 1992, TPH-g and benzene has always been detected in MW-4, and the average TPHg to benzene ratio is approximately 15:1. However, in MW-1, TPH-g has been detected in 22 of 27 samplings,but benzene has only been detected twice, at an average TPHg to benzene ratio of approximately 450:1. Chromatographs of groundwater samples collected in 1996 indicate that the contamination in monitoring wells MW-4 and MW-5 appear to be from the same source, but it does not match the source in MW-1. This data suggest that another source in the vicinity of MW-4 could be present. We propose to advance up to 6 borings in the vicinity of monitoring well MW-4 to determine if another source, such as an unidentified UST, may be present. Figure 1 shows the proposed soil boring locations. The actual locations of some borings may vary depending upon field conditions. Samples will be collected using a direct push method with a cased hole. Groundwater samples will be collected from a temporary well casing in the borehole. Soil samples will be collected at 5-foot intervals and changes in lithology. The borings will extend to approximately 25 feet below ground surface (bgs) to obtain soil and groundwater samples. Immediately after collection, samples will be labeled, and placed in an iced cooler. All groundwater samples and selected soil samples will be submitted to a California state certified laboratory for analysis using chain of custody protocols. All samples submitted for analyses will be analyzed for TPH-g and volatile organic compounds (VOCs) using EPA Method 8260.