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Geological Technics Inc. `..t Page 3 Additional Pilot Test Work Plan Project No.770.2 October 30,2008 r points within the boundary of the sand channel area. It is possible the sand layer is laterally continuous across the site as shown by Figure 4. • Figure 5: Geologic Cross-Section B - B' illustrates the subsurface from the northwest to southeast sections of the subject area. The section also displays the predominantly fine grained soils at the site as in Cross-Section A—A'. The sand layer identified at a depth of 40 - 45 feet bgs in MW-7 was further defined during the November 2005 CPT investigation. The CPT investigation revealed that the r., sand layer appears to be ahannel (?) de-po with a southwest-northeast trend as depicted in Figures 4 & 6. The sand/gray�i;y,layer is present at depths ranging from as n , shallow as 38 feet in CPT-5'to as deep as4 feet in MW-1. The layer varies in thickness 46" from seven feet in CPT-1 to less than two feet in CPT-10, A visual analysis of the G subject depth soil samples retained in chip boxes at GTI's office affirm that the samples appear to be from the same unit(s). In a predominantly silty/clayey soil profile this feature has the potential to be a preferential pathway for contaminant migration. Other sand units with a thickness of less than a few feet to several inches were identified at the site but they were not shown to be continuous in lateral extent. `" • As documented in our April 20, 2006 "Site Conceptual Model (March 2006 Revision)" veins and/or fractures identified in soil samples appear to be a direct route for the vertical migration of the diesel. 2.2 Hydrogeology There are no surface water bodies near the area of sufficient size or volume to act as a hydrogeologic boundary. An irrigation ditch/canal is adjacent to the north of the parcel and it may act as a shallow aquifer recharge source. Regionally, groundwater is utilized as a source for domestic, agricultural, and industrial supply water. i The groundwater elevations observed in the water table wells ranged from approximately-28 to -44 feet mean sea&Wvel (MSL) since August 2000. This corresponds to a depth to groundwater of 51 - 67 feet below grade surface (bgs). The site's shallow aquifer displays «r highly variable shifts in groundwater direction. But the groundwater direction has been predominantly to the southeast since additional wells were installed in May 2002. The horizontal gradient measurements for the study area range from 0.0001 to 0.017 ft/ft during the 2000—2008 monitoring period. The deep well groundwater elevations also ranged from approximately -28 to -44 feet mean .. sea level (MSL) since August 2000. This corresponds to a depth to groundwater of 51 - 67 feet below grade surface (bgs). The deeper aquifer flow has been consistently to the east except for the April 28 and May 23, 2003 events, during which time the gradient reversed .� 180°from an easterly direction to a westerly direction. This suggests that the regional well pumping causes significant fluctuations in groundwater direction. The horizontal gradient measurements for the deeper aquifer level ranged from 0.001 to 0.007 ft/ft during the 2000 — �' 2008 monitoring period.