SITE INFORMATION AND CORRESPONDENCE FILE 2

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DawBarber ' Page 2 By using existing data and by generating additional data from the other existing wells or from the installation of additional wells, the most efficient system design must be determined and ultimately put into use. The system design must address all areas of impacted soil and groundwater. The data generated from the testing and/or operation of the system must.confirm the effectiveness of the system in influencing the entire contaminant plumes. ,j The'data collected from monitoring well 12 appears to show the extent of the plume in the south easterly direction. joil Monitoring wells 10, 11 and 13 show significant levels of contamination in the groundwater in these areas. Table 3 in the report erroneously reports the concentrations in milligrams per kilogram (weight per weight basis for a solid matrix) or parts per million. The units reported by the lab are in micrograms per liter (weight per volume basis for a liquid matrix) or parts,per billion. This error has a significant effect on the concentrations of contaminants that need to be addressed and also on the inferred radius of the groundwater contaminant plume as it exists now at approximately 80 feet BGS (versus the concentrations i of contaminants and the plume radius at 50 feet BGS, as in 1991). In addition, the laboratory detection limit of 0.05 and 0.10 parts per million for benzene and toluene, ethylbenzene and xylene, respectively, are too high. For future soil sample analysis a detection limit of 0.005 parts per million should be obtained. The groundwater plume in the area of monitoring wells 10, 11 and 13 has not been physically determined, but may be inferred using the data from these wells,.the historical data from monitoring well 4, and by known contaminant transport models. The groundwater in this area (around monitoring wells 10, 11 and 13), and the possible secondary soil contamination resulting from the drop in the water table, must be addressed by the remediation system. Historically, monitoring wells 3 and 5 have shown varying degrees of contamination at some time in the past before they went dry. As there are currently no monitoring points in this area (monitoring well 9 is obstructed), the quality of the soil in the deeper zones (at and below the screened interval of monitoring wells 3 and 5) and of the groundwater in this area is unknown at this time. However, the current contaminant concentrations and the plume radius of the soil and groundwater may be inferred in this area based on the historical data and the expected transport of the contaminants using established models. An attempt should be made to restore monitoring well 9, if possible,so that it can again serve a useful purpose as a groundwater monitoring point or as a possible vapor extraction point. :1 The soil and groundwater in this area (around monitoring wells 3, 5 and 9) must also be addressed by the remediation system. ' Additional vapor testing should be conducted, by either using additional existing wells or by using newly installed wells, to determine and document that all the above mentioned areas are being addressed in the remediation efforts. The initiation of the proposed,vapor extraction system can begin as soon as possible, with the additional efficiency testing and work being.scheduled concurrently or as new information warrants. The soil lithology and the soil contaminant plume data should be presented together in cross sections. This can be done using current information and may be updated, ifneeded,when additional information warrants. l II u , k