SITE INFORMATION AND CORRESPONDENCE

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possible presence of a previously unidentified well in the area just downgradient of the apparent drawdown. 5. In Section 3.1.4.4, the vertical hydraulic gradients between the middle and lower horizons are said to be mostly downward, with the exception of an upward gradient in the vicinity of wells LM86 and LM90. Since there was a time lapse of 19 to 21 days between the measurement of the groundwater elevations in these two wells (from January 18 or 20 to February 8, 1993), the calculations of the vertical gradient based on this well pair cannot be considered valid. ANALYTICAL RESULTS 6. In Section 4.1.2.4 (page 4-16), the statement is made that the appearance of TCE in the Rose well could indicate the leading edge of the TCE plume in the middle horizon "because this well is assumed to be screened in this horizon." No explanation is provided as to the reasoning behind the assumption that the Rose well is screened in the middle horizon. This assumption should be supported with a reference to well construction logs or field investigation data. 7. Also in Section 4.1.2.4 (page 4-16), the assumption that the Pombo well is cross- gradient to DDRW-Tracy is made. This assumption is unsupported based on the following: • the anomalous water table drawdown in the northeast corner of the site (nearest to the Pombo and Rose domestic wells); • the possibility of an east-northeasterly curving paleochannel in that area supported by the current groundwater data and by the previously documented direction of movement of the volatile organic compound (VOC) plume in an easterly direction; and • the lack of sufficient available and reliable groundwater elevation data in all three horizons in the Upper Tulare formation in this area, with the exception of the limited upper horizon monitoring well coverage. 8. In Section 4.3.1.1., the statement is made that "most of the measured concentrations of pesticides are near the detection limit, an indication that contamination is possibly due to upgradient, non-point sources." This is an unverified inference and should either be removed or supported with data. The background wells LM11 and LM12 were not analyzed for pesticides and, therefore, cannot be used to support this statement. CONCLUSIONS 9. The reviewer agrees with the conclusion, as stated in Section 6.1.2.1 (page 6-2), that the current monitoring well coverage is not sufficient to define the eastern boundary of the VOC plume. EPA/R9 18]un 93 2