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CMIO fral Ted vacs Inc Page 2 g � 4�'Quarter 2004 Groundwater Monxtonng Report Project No 507 2 February 22,2005 on the pilot test was submitted to SJC EHD for review in November 2004 They have since commented on the field work and requested that a site conceptual model (SCM) be submitted prior to proposing further corrective action Since there is uncertainty regarding the soil profile at the site due to the 5-foot sampling ' interval, GTI will propose performing several continuous core borings The data generated by this effort will then be used to formulate a site conceptual model desired by SJC EHD After the site conceptual model issue is resolved GTI will propose further corrective action work ' 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site ' The geology of the site is predominately clays and sandy clays with lesser amounts of inter ' bedded silts and sands The average groundwater elevation was —1173 feet below msl on December 29, 2004, which corresponds to approximately 15 5 feet below grade surface (bgs) This average elevation represents an increase of 188 feet since the August 25, 2004 monitoring event Groundwater gradient for this monitoring event was 0 003 ft/ft flowing to the N58°E direction This site consistently exhibits a flat groundwater gradient ' The gradient direction for the August monitoring event is shown in Figure 2, and the historical groundwater directions and gradients are shown in Figure 3, Groundwater Gradient Rose Diagram The groundwater elevation data are summarized in Table 1, Appendix A ' Table 5 provides a summary of monitoring well construction The groundwater gradient is relatively flat and the direction of groundwater flow has varied ' during the course of the investigation As evident in Figure 3, groundwater exhibited a strong bifurcated flow regime in 2000, trending predominantly to the southwest or north-northeast at approximately 180-degree direction reversal Since the installation of wells MW-7 and MW- 10 in May 2001, the direction has been predominantly to the northeast The site is quite close to the San Joaquin River and tidal fluctuations may contribute to some of the observed changes in groundwater direction ' Vertical Groundwater Gradients ' Under vertical groundwater flow conditions, the water level in a monitoring well is a function of the length of the well screen and its depth or vertical position in the aquifer. As with horizontal flow conditions,the diameter of the well or piezometer is immaterial r