SITE INFORMATION AND CORRESPONDENCE 1991-1992

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Mr. Val Siebal • -2- ey 31, 1990 o Analysis of Unknown Materials Encountered During Field Investigations Unknown materials encountered during field investigations will be carefully logged and sampled. o Calculations of Hydraulic Gradient and Aquifer Characteristics As noted in the report, anomalous water level measurements, which indicated an apparent "mounding" of groundwater on site, were obtained during the 1987 field investigation. This effect may have resulted from a water line leak which was discovered and repaired in . March 1988. As noted in the report, two methods of estimating potentiometric surface slope were described in the report, in an attempt to correct for errors potentially attributed to the leak (least-squares linear regression analysis to fit the planar surface to all four wells, and fitting the planar surface, through matrix solutions of simultaneous linear equations, to the three wells least likely to be affected by the leak) . Both methods yielded gradients of approximately equal direction and magnitude. While it is true that these gradients do not agree with regional gradients as noted on published regional groundwater maps; it is quite possible for local gradients to run contrary to regional trends, due to a variety of hydrogeological and groundwater use conditions. Further, the report noted discrepancies in flow directions among the published maps themselves (including discrepancies involving nearby wells which show gradient a direction similar to that shown in the site report) . We expect that the future groundwater elevation measurements and aquifer testing planned for this site will yield more conclusive data regarding groundwater flow and direction, and concur that monitoring over seasonal fluctuations will be necessary to establish groundwater conditions at and in the vicinity of the site. Your comments note that ". . .PG&E must eventually define the vertical and horizontal extent as well as the migration velocity of the groundwater contamination plume migrating from the site. An acceptable method would be to construct cluster wells screened at various depths and the performance of aquifer tests. However, aquifer testing should be postponed until the need for a groundwater extraction system has been determined and prior to its design." While we agree that the extent of residues in groundwater, both on- and off-site, should be fully defined, we note that it has not yet been established that there exists a ". . .groundwater contamination plume migrating from the site." Further, we do not agree that aquifer testing should be performed only as part of the design of a ground water extraction system. Such testing yields significant information regarding flow magnitudes, potential migration rates and dispersion characteristics.