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• - 5 - 14 September 2006 <br /> not been adequately defined. Additional groundwater investigation is necessary to <br /> characterize the lateral extent of the groundwater pollution. <br /> In summary, the soil and groundwater results from the USTs removal and quarterly monitoring <br /> show the following: <br /> • Groundwater monitoring results were compared from 1994 to 2006, and have not <br /> shown a declining concentration trend in all monitoring wells for all constituents. The <br /> lack of declining concentrations indicates that there is an ongoing discharge to <br /> groundwater from residual soil contamination. Residual soil contamination may <br /> necessitate additional active remediation to stop/slow leaching to groundwater and <br /> allow natural attenuation to cause a reduction in groundwater pollution concentrations, <br /> and concomitant declining concentration trends. A declining concentration trend is one <br /> requirement for site closure. <br /> • Stable plumes, and definition of vertical and lateral extents, are other closure <br /> requirements. The vertical extent of the hydrocarbon plume was determined by data <br /> from deep monitoring well MW-5. The lateral extent to the east is demonstrated by <br /> MW-4 and to the southeast by boring B-27. Groundwater monitoring and borings <br /> results show that the lateral extent of groundwater pollution has not been defined to the <br /> northwest (TPHg), and south (TPHg, benzene, MtBE, and 1,2-DCA) of the Site. The <br /> groundwater plume may not be stable, and may be moving offsite towards the south or <br /> southwest. An additional groundwater investigation and monitoring are necessary to <br /> define the lateral extent of the plume and evaluate the stability of the groundwater <br /> plume. <br /> • Constituents of concern detected in groundwater exceed WQOs. Case #1 residual soil <br /> concentration estimates showed that a considerable mass of TPHg and TPHd remains <br /> in soil. While Case #2 confirmation results after over-excavation were much lower than <br /> what would be expected from Case #1 residual mass estimates, the risk of soil vapor <br /> intrusion to Site and neighboring structures should be evaluated by collecting soil gas <br /> data to determine if additional measures are needed to mitigate this potential risk. <br /> Therefore, SCEHD's closure recommendation is not appropriate at this time, and I do not <br /> concur with the SCEHD's closure concurrence request. <br /> Attachments <br />