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' eo(o ica Tecfmus Inc Page 2 � 6 g Groundwater Momtormg Repo,t Project No 507 2 June 19, 2001 iThe Oxidation-Reduction Potential measurements show that all eleven of the monitoring wells are within the halo of reaction depleted water caused by biodegradation of the Contamination plume 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site ' The geology of the site is predominately clays and sandy clays with lesser amounts of silts and sands The average groundwater elevation was —7 61 feet below MSL on June 1, 2001 ' MSL is approximately 7 37 feet below ground sui face A groundwater gradient was calculated for the event and found to be flowing N77°E at 0 002 ft/ft Gradient direction for the June 2001 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 The groundwater gradient is relatively flat and the direction of groundwater has varied during the course of the investigation As evident in Figure 3, groundwater has a strong bifurcated flow regnne, trending predominantly to the southwest or northeast at approximate 180-degree direction reversal The site is quite close to the San Joaquin River and tidal fluctuations might be responsible for 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 ' The following procedure is used to calculate vertical groundwater gradient 9 Detennme vertical distance between the two measuring devices by the distance from the mid-point between the top and bottom seal of the deep well (MW-109) and the mid- point between the groundwater elevation and the bottom seal in the shallow well (MW- 9) a Measure the head in both wells used in the calculations • If the lateral distance between the well pair is greater than a few feet, then calculations must be made to correct the down-gradient piezometric head to account for the sloping ' water table between the wells The calculation considers the slope of the water table and the distance in a down-gradient direction between the two wells used in the calculations