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ASSOCIATES E NC <br /> Total hardness ranged from 280 mg/I to 7,600 mg/l in groundwater samples collected fiom wells <br /> MW 1 through MW8 and the facility well Hardness does not have an established MCL or <br /> ' Action Level <br /> Dissolved oxygen readings ranged from 0 75 mg/1 to 5 44 mg/1 <br /> ' The groundwater analytical results are summarized in Table 3 Copies of laboratory analytical <br /> results are contained in Appendix D <br /> ' 50 CONCLUSIONS <br />' • Hexavalent chromium was not detected in soil samples collected during this investigation <br /> • The highest concentrations of hexavalent chromium in groundwater were detected in the <br />' groundwater samples from well MW3, the shallow water bearing zone, and from boring <br /> CPT-1, the deeper water bearing zone, each Iocated adjacent to vaults 7 and 8 <br /> Concentrations of hexavalent chromium decrease rapidly with distance from the source <br /> • The reported concentrations from wells MW3, MW6, and CPT-1 were above the California <br /> Primary MCL for total chromium of 105 mg/l Hexavalent chromium does not have an <br /> established MCL or Action Level <br /> • Hexavalent chromium was detected in groundwater at low levels in the samples from wells <br />' MW2, MW4, MWS, MW6, MW7, and CPT-2 <br /> • Hexavalent chromium was not detected in groundwater samples from wells MW I, MW8 and <br />' the facility well <br /> 6.0 RECOMMENDATIONS <br />' Based on the results of this investigation, current regulatory guidelines, and the judgement of <br /> ATC, the following recommendations are presented for consideration <br /> • The vertical extent ofim roundwater acted with hexavalent chromium can be further <br /> g p <br /> defined by the installation of a groundwater well in the vicinity of Vaults 7 and 8 where <br />' concentrations of hexavalent chromium are reportedly highest One new monitoring well, <br /> screened in a deeper aquifer is recommended, north of MW-3 Based upon the results of <br />' boring CPT 1, ATC suggests installing a well with a screened interval from 102 to 112 feet <br /> below ground surface Additionally, ATC recommends using mud rotary drilling methods to <br /> install the well in order to minimize migration of hexavalent chromium to the deeper water- <br /> bearing zone <br /> • The construction details of the facility well are unknown It is recommended that the facility <br /> well be video Iogged to obtain details on the well depth and construction If it is determined <br /> wlwordpro1625831reports\Well_mstall_rpt doc 7 <br /> I <br />