SITE INFORMATION AND CORRESPONDENCE

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Well Monitoring Program -6- 11 September 1992 DDRW, Tracy cluster sites are recommended for quarterly monitoring. Tracy should consider reducing the monitoring frequency of well cluster LM-49, LM-50 and LM-51 to a semi-annual basis. c. If it is determined that the elevated metals concentrations are associated with turbidity in the older wells, then LM-85 may need to be substituted for LM-25 for quarterly monitoring. Presently, LM-85 is only recommended for annual monitoring. d. It is unclear why the heavy metals analyzed using the Inductively Coupled Plasma (ICP) Method were primarily recommended for quarterly monitoring whereas, metals analyzed by Atomic Absorption (AA) were recommended for annual monitoring. The laboratory method should not determine the monitoring frequency. When a well is initially installed and ground water contamination by metals is suspected, analysis for all metals (Title 22 metals, for a total of 17) should be performed for one or two quarters, at a minimum. 3. Table 2-1 is a construction summary for monitor wells LM-1 to LM-92. This table has been extremely useful in the review of this Report and Woodward-Clyde should be commended for the preparation and presentation of this table. The table may be improved, however, if it were to be expanded to include on-site supply wells (both operational and abandoned) and the IRM wells, which would include the extraction and injection performance monitor wells. 4. The base well map needs to be revised to reflect the new numbering system for the IRM wells (LM-200 series) , and needs to depict the location of all wells including the on-site supply wells. 5. The Draft Report did not include concentration versus time graphs for any of the monitor wells sampled during the four quarters. These graphs should be constructed to depict the changes in concentrations for TCE and PCE over time. These graphs may not be necessary for all of the monitor wells but for selected wells. All historical sampling events (not just the four quarters summarized in this Report) should be recorded on the graphs. These graphs will help to understand the dynamics of the plume and should be compared to the hydrographs to determine if contaminant concentrations trends are affected by the hydraulics of the ground water system. 6. It is unclear why <1 µg/l was reported in the Table 3-7 instead of <0.5 µg/l . The higher limit does not appear to be related to matrix or other VOC contaminant interferences. It is assumed that 1 Ag/l is the Practical Quantitation Limit (PQL) rather than the detection limit. 7. Section 4.1.1 states that five up gradient domestic background wells were selected and sampled to establish background levels of contamination. Two of these wells may be affected by an off-site source and therefore would not meet one of the selection criteria (page 3-3 of the November 1991 Monitoring Report) . The Final Report should be revised to state that these wells were specifically selected to establish background for the OU-I Risk Assessment.