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L & M OPERABLE UNIT- FIRST QUARTER 2012 SITE STATUS, REMEDIAL SUMMARY, AND GROUNDWATER MONITORING REPORT Groundwater Monitoring and Sampling April 30, 2012 I:\Tidewater\TW Stockton (P2A)\QMRs\2012\1Q12\1Q12 QSSR QRSR QMR Stockton_FINAL.doc 2.12 the stated QC limits for percent recovery, due to the analyte concentrations already present in the un-spiked sample. This may indicate a possible bias for the reporting accuracy. The LCS percent recovery was within QC limits indicating accuracy in reporting. RPD values were within QC range in the MSD indicating precision in reporting. The reported values are considered valid. The MS associated with sample MW-16 for the analyte tertiary butyl alcohol was outside of the stated QC limits for percent recovery, due to the analyte concentrations already present in the un-spiked sample. This may indicate a possible bias for the reporting accuracy. The LCS percent recovery was within QC limits indicating accuracy in reporting. RPD values were within QC range in the MSD indicating precision in reporting. The reported values are considered valid. The surrogate 2,4,6-tribromophenol was out of QC limits for sample MW-24 due to matrix interference. This may indicate a possible bias for the analysis. The method blank surrogate spike was within QC limits; therefore, the sample outlier is not anticipated to affect the reported values. It should be noted that the MRL for chloromethane was raised in samples MW-42 and MW-59 and the MRL for chloroethane was raised in sample MW-59 due to the presence of an interfering compound. Validation issues with report number 80125: The RPD values associated with sample MW-51 for the analyte dissolved chromium was outside of the stated QC limits by approximately 0.3 percent. This may indicate a possible bias for the reporting precision. The LCS and LCSD percent recoveries were within QC limits indicating accuracy in reporting. Since there is no RPD value to confirm precision in reporting, the reported dissolved chromium may be biased; however, dissolved chromium values are within historical limits. 2.5 FIRST QUARTER 2012 C-ZONE GROUNDWATER RESULTS 2.5.1 C-Zone Groundwater Gradient Data C-Zone groundwater elevations at the Site ranged from 5.83 ft below MSL in well MW-64 to 5.44 ft below MSL in well MW-65 (Table 3 and Figure 17). Groundwater gradient during 1Q12 in the C-Zone was calculated for three sections of the Site (central, north, and east). In the central portion of the Site, groundwater flow direction was to the west with an approximate hydraulic gradient of 0.001 ft/ft. In the northern part of the Site, groundwater flow direction was to the south-southeast with an approximate hydraulic gradient of 0.0009 ft/ft. In the eastern part of the Site, groundwater flow direction was to the northeast with an approximate hydraulic gradient of 0.001 ft/ft (Figure 17). Historical groundwater flow direction and gradient data are summarized in Table 8, and vertical gradients summarized in Table 9. The C-Zone groundwater elevation contour map for the January 17, 2012 monitoring event is included as Figure 17.