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Results of Groundwater Pumping Tests - 152 East l I th Street, Tracy Page 10 ANALYSIS AND INTERPRETATION OF RESULTS OF PUMPING TESTS Hydrogeological Model Interpretation of groundwater pumping test data requires a hydrogeological model The geotechnical parameters of concern to the development of the model are the relative hydraulic conductivities and thickness' of the strata and their horizontal and vertical distribution The boring logs presented in Appendix A show that, in terms of the parameters of importance to interpretation of data from groundwater pumping tests, the hydrostratigraphy beneath and around the site can be represented in the simplified form shown on Figure 7 Silty clay of relatively low hydraulic conductivity, from the ground surface to a depth of approximately 14 ft overlies a layer of fine- to medium-course sand of higher conductivity This stratum is approximately 11 ft thick and is underlain by another stratum of silty clay, the top of which is at about 25 ft beneath the ground surface This hydrosrtatigraphy is essentially continuous over broad areas of the site The static water table is about 8 ft beneath the surface and both the pumped well, NAV 6, and the observation well, NAV 3, penetrate to the bottom of the sand layer at a depth of 25 ft They are screened from approximately one foot above the water table to a depth close to the bottom of the sand stratum During drilling, first water was typically encountered at 8 to 13 ft beneath the surface (i e above the top of the sand stratum) and after the wells were completed, water levels rapidly reached a static condition at a depth of approximately 8 ft beneath the surface Given the stratified nature of the geology, with a relatively high-permeability layer of sand occurring beneath a surface stratum of comparatively low-permeability silty clay and with a second stratum of silty clays occurring below the sand, it is appropriate, for the purposes of practical analysis, to treat the sand stratum as an essentially confined aquifer and apply the standard Cooper and Jacobs (1946) methods for computation of transmissivity and storativity from the groundwater pumping test data However, based on the evidence from the drilling logs that the clay material contains a significant fraction of silt and courser material, and that first water was encountered well above the top of the sand, it is reasonable to presume that there is some degree of hydraulic communication between the strata and that the degree of hydraulic confinement of the sand may be low These factors should be taken into consideration when evaluating hydraulic conductivity and derivative parameters Aquifer and Well Parameters Transmisslyity The drawdown data from the pumped well, NAV 6, obtained during the pumping tests performed on December 28th 1993 are presented on Figure 8 in the form of a plot of drawdown versus the logarithm of the elapsed time since the pump was started As shown