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ENSR I - <br />' below the screened interval The system was shut off because the groundwater analytical data suggested <br /> that contaminant concentrations have reached levels at which the current system efficiency is minimal, and <br /> either the system needs to be upgraded or a new alternative needs to be evaluated <br /> While contaminant concentrations have been declining at the site,the rate of decline has not increased <br /> significantly subsequent to the installation of the ozone sparging system This could be attributed to system <br /> design which includes one fine of sparge points near the downgradient edge of the site and the small radius <br /> of influence that the sparge points can influence due to the low permeability sods (i e , clay and silt) <br /> Since the ozone sparging system became operational in August 2002, concentrations in several monitoring <br /> wells have declined The historical groundwater data from MW-2, MW-10, and MW-11 were graphed to <br /> evaluate changes in concentrations of TPHg and benzene before system startup and after system startup <br /> These graphs are included in Appendix D The graphs for MW-2 and MW-10 suggest that the ozone sparge <br /> system had a positive influence in decreasing concentrations in groundwater, while the data for MW-11 is <br /> not as conclusive <br /> 3.8 Data gaps and recommended activities <br /> The existing dataset indicate that groundwater data from downgradient, crossgradient, and upgradient <br /> locations have been below MCLs for several monitoring events Many of the wells have been non-detect or <br />' below MCLS for several years Concentrations of TPHg and BTEX have been declining in the source area <br /> wells (MW-2, MW-10, MW-11), but are still greater than the MCLs Dissolved oxygen concentrations have <br /> increased in two of these wells (MW-2 and MW-11)which are closest to the sparge points and appear to <br /> have had a positive influence in decreasing concentrations at these locations <br /> It is recommended that the applicability of the current interim remedy as an on-going measure or as an <br /> upgraded system needs to be assessed <br /> 3-5 <br />