SITE INFORMATION AND CORRESPONDENCE

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• • David Wang Page 4 elevation measurements taken during the Phase I optional Task 30 investigation. " Apparently, this map was produced using hydraulic head data from wells screened at different depths and with different screen lengths. Potentiometric data should be plotted and contoured from wells screened at similar elevations with similar well screen lengths. Several potentiometric surface maps should be compiled. Each potentiometric surface map should be constructed using water level data from wells screened at similar depths with similar screen lengths. DHS suggests the following potentiometric surface maps be compiled: o A map utilizing only data from short screened wells screened across the water table. o A map utilizing only data from wells screened in the -10 to -40 foot mean sea level (MSL) elevation interval. o A map utilizing only data from wells screened in the -60 to -90 foot MSL elevation interval. o A map utilizing only data from wells screened in the -110 to -130 foot MSL elevation interval . The above recommended maps would allow for tracking the horizontal movement of groundwater at different elevations in the subsurface. In addition, flow net maps should be constructed to track groundwater movement in the subsurface. It is recommended that cross-sections be constructed perpendicular to equipotential lines (similar to the orientation of geologic cross-section A-A' ) . Flow nets constructed on cross-sections with such an orientation would be of great value in tracking groundwater flow pathways, and would be a key element in defining the groundwater contaminant pathways. SECONDARY CONCERNS: 1) The final RI report should be signed by a California Registered Geologist or a California licensed civil engineer. 2) Section 6. 0, Conclusions, page 6-2 , paragraph 6. 2 . 1, lines 16 through 20: The relationship between TCE and PCE should be considered in this paragraph. The following should be explained: 0 source areas