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Analysis of Unknown Materials Encountered During Field Investigations <br /> Unknown materials encountered during field investigations will be <br /> carefully logged and sampled. Field personnel will be alerted to be <br /> particularly on watch for a "white, chalky material ." <br /> - Calculations of Hydraulic Gradient and Aquifer Characteristics <br /> Water level measurements taken as part of the scoping investigation on <br /> March 28, 1991, confirm the "mounding" in the shallow groundwater detected <br /> in the 1987 investigation. As a further check, all well elevations were <br /> resurveyed in May. The new survey elevations, although slightly different <br /> than the 1987 elevations, do not substantially change the apparent <br /> mounding. Additional information regarding the hydraulic gradient will be <br /> provided by the RI investigation including monthly water level <br /> measurements in the five existing wells and six additional monitoring <br /> wells. <br /> PG&E agrees that the vertical and horizontal extent of residues in <br /> groundwater should be fully defined, and has designed a groundwater <br /> monitoring network to meet this objective. We concur that cluster wells <br /> can be useful tools in accomplishing this evaluation and have included <br /> three clusters in our Work Plan. We note however, that it has not been <br /> established that there exists a " . . .groundwater contamination plume <br /> migrating from the site." Slug testing will be performed in the <br /> supplemental wells to evaluate the characteristics of the hydraulic <br /> characteristics of the subsurface. The groundwater investigation plan is <br /> described in Section 4.1 .3 of the Work Plan. <br /> Well Survey and Abandoned Wells <br /> As stated in the July 31 letter, PG&E has performed a survey of wells <br /> located within one mile of the site as part of the preliminary remedial <br /> investigation. We are currently evaluating whether changes have occurred <br /> in the area in the last five years that merit updating this survey. <br /> PG&E records concerning the abandoned well on the property were reviewed. <br /> Apparently, the well was located in a well house approximately 75 feet in <br /> from the northern corner of the property, not near the southern corner as <br /> indicated in the April 1988 reports. Site personnel confirm this <br /> location. Records indicate that Delta Pumps of Stockton was contracted to <br /> abandon this well on March 3, 1973 by pulling 230 feet of pipe and sealing <br /> the well with concrete. Photographs are available documenting the <br /> destruction of the well . <br /> Well Construction <br /> We concur that excessively long screens create the possibility of vertical <br /> migration of contaminants, loss of depth-discrete data, and dilution of <br /> water quality samples. As outlined in the Work Plan, wells completed for <br /> the RI will have screens limited to 10 feet in length. <br /> PG&E agrees that mud rotary drilling is superior to hollow stem auger <br /> drilling in unconsolidated saturated sediments such as those found at the <br />