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C v <br /> ATC <br /> A S S O C I A T E S I N C <br /> r <br /> 1 4.2 Groundwater Analytical Results <br /> Chromium VI was detected in groundwater at low levels in the samples from borings SB 1, SB2, <br /> SB3, SB4, SB5 and SB9. Concentrations ranged from 0.5 micrograms per liter (ug/1) to 5.1 ug/l. <br /> The reported concentrations were below the California Primary Maximum Contaminant Level <br /> (MCL) for Total Chromium of 50 ug/l. Chromium VI does not have an established MCL or <br /> Action Level. <br /> Nitrate concentrations ranged from I1 mg/1 to 140 mg/l. Nitrate concentrations in groundwater <br /> samples from SB 1, SB2, SB 7, SB 8, and SB9 exceeded the MCL for nitrate of 45 mg/l with the <br /> highest level in off-site boring SB9. <br /> Sulfate was detected in all nine soil borings.at concentrations ranging from 12 mg/1 to 64 mg/l. <br /> Sulfate concentrations in groundwater did not exceed the secondary MCL for sulfate of 250 <br /> mg/L. <br /> Oxidation reduction potential (redox) measurements were consistent in all groundwater samples, <br /> ranging from 124 millivolts (mV) to 191 mV, with the exception of the sample from boring SB5 <br /> where redox was measured at 44 mV, <br /> Dissolved oxygen readings ranged from 0.75 mg/1 to 3.61 mg/l_ <br /> The analytical results are summarized in Table 2. Copies of laboratory analytical results are <br /> contained in Appendix C. <br /> 5.0 CONCLUSIONS <br /> • Low levels of chromium VI have been detected in soil and groundwater samples collected <br /> during the investigation. <br /> a <br /> • The highest concentrations of chromium V1 in soil were detected in the samples from off-site <br /> boring SB9. Chromium VI concentrations at this location were well below the TTLC for <br /> chromium VI and well below concentrations previously detected in soil boring B8-A which <br /> was advanced beneath Vault 8 (Appendix A). <br /> F • Chromium VI was detected in groundwater at low levels in the samples from borings SB I,. <br /> SB2, SB3, SB4, SB5 and SB9. Concentrations ranged from 0.5 micrograms per liter (ug/1) to <br /> 5.1 ug/l. The reported concentrations were below the MCL for Total Chromium of 50 ug/1 <br /> t + and well below concentrations detected in monitoring well MW-3, which historically have <br /> been the highest detected levels at the site (Appendix A). <br /> l <br /> w:lwordpro162583\reportslSubsurl_report.doc 5 <br /> r <br />