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Geosyntec D consultants 2.1.2 Site Hydrostratigraphy Existing documents were used to summarize the current hydrogeologic conceptual model for the site including both the on- and off-site areas. In general,the subsurface of the site, as described by others, consists of five hydrostratigraphic zones as follows: HydrostratierWhic A-Zone The hydrostratigraphic A-Zone can be divided into two general lithologic intervals. The first interval consists primarily of interbedded fine-grained sediments from the ground surface to approximately 50 feet below ground surface (bgs). The second interval consists primarily of a sequence of coarser-grained sandy sediments alternating with finer grained sediments from approximately 50 feet bgs to the top of the hydrostratigraphic A/B-Zone (approximately 75 to 85 feet bgs). These intervals vary in thickness and in lateral and vertical extent. The saturated portion of the A-Zone is approximately 35 to 55 feet thick and generally ranges from 30 to 40 feet bgs to the top of the A/B aquitard at approximately 75 to 85 feet bgs (LFR, 1999). Hvdrostratipyauhic AB-Zone The hydrostratigraphic A/B-Zone typically ranges between 10 and 30 feet in thickness, consists of dense clay and silt and when present and is encountered at an approximate depth ranging between 75 and 115 feet bgs. When present, the A/B-Zone can act as an aquitard. In some areas, the A/B-Zone is difficult to distinguish from fine-grained intervals within the lower hydrostratigraphic A-Zone and the upper part of the hydrostratigraphic B-Zone (LFR, 1999). An anomalous coarse-grained sand interval was encountered between approximately 92 to 100 feet bgs, which is within the A/B- Zone, in the vicinity of the Bank of Stockton east of the site. The similarity of groundwater elevations in the A- and B-Zone and the narrow distribution of elevated concentrations of hazardous substances in the B-Zone aquifer suggest that the A/B aquitard is incompetent across most of the site or absent in certain limited areas at the site. An example of this is the anomalous coarse-grained sand interval encountered at approximately 92 to 100 feet bgs in the vicinity of the Bank of Stockton, east of the site. In general, the A/B aquitard appears to be absent immediately downgradient (east) of Pacific Avenue that results in greater hydraulic communication between the A and B hydrostratigraphic units in this area. Pre-Design Investigation Work Plan 4 18.05.12