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Mr. Soon Kim <br /> Page 4 <br /> December 19, 1995 <br /> elevations are provided in Table 1 for each well measured since September 1991. An <br /> interpretation of shallow groundwater elevation contours are shown on Figure 2 for the <br /> October 18, 1995 monitoring event. As indicated by the groundwater elevation contours, the <br /> I groundwater is flowing in a southerly direction which is consistent with the previous quarter. <br /> BTEX Analytical Results <br /> Analyses of groundwater samples from monitoring wells are presented in Table 2. <br /> Laboratory reports and chain of custody forms are attached. Two sets of results are given for <br /> MW-DIS-2, because this well was sampled by two different methods and analyzed by <br /> different laboratories. <br /> Analytical results for MW-DIS-2 are suspect. Sample collected by rapid well purge and <br /> bailer sampling gave high concentrations that are inconsistent with the historic trend for that <br /> . Sample collected per the ITB emonstratio , minimum aeration technique, gave much <br /> ' lower concentration values. This result is the opposite of the expected outcome, as the ITB <br /> method is intended to minimize loss of BTEX due to volatilization. <br /> Possible sources of discrepancy are: <br /> 1. Analytical method EPA 602 was used for the"bailer method" sample with a 1:200 <br /> dilution factor, and analytical method SW 8020 was used for"minimum aeration" sample <br /> with a 1:10 dilution factor. Laboratory dilution can introduced significant error. <br /> I 2. Cross contamination is always a possibility in environmental sampling. Samples <br /> collected by the bailer method were taken in a sequence with the least contaminated wells <br /> first and the most contaminated wells last. This procedure is designed to minimize <br /> positive error. <br /> 3. The field method for rapid purge sampling involves the purging of several wells in series <br /> i followed by bailer sampling of several wells in series. The sequence of purging and <br /> sampling, and the amount of time water is allowed to sit in the well after purging, may not <br /> i have been consistent. <br /> 4. The field method for the ITB sampling protocol involves continuous low rate pumping <br /> with sample collection directly from a flowing stream with very little exposure to the <br /> atmosphere. During sample collection, small bubbles were observed in the discharge line <br /> that were possibly formed by the release of dissolved carbon dioxide as pressure <br /> decreases and temperature increases through the system. Hydrocarbon vapor could be lost <br /> in this degassing process. <br /> 7 • <br /> { <br />