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.1 !r:x Lanimersville School District July 15, 1990 202/899.7004 a 3.0 REGIONAL GEOLOGY AND HYDROL0GY The Lammersvifle School District site is located in the northwest portion of the San Joaquin Valley, The San Joaquin Valley constitutes approximately the southern two-thirds of the Great Central Valley of California; to the north, the Sacramento-San Joaquin Delta and the Sacramento Valley beyond occupy the remaining third of the Great Central Valley. The San Joaquin Valley is bordered on the east and southeast by the Sierra Nevada geomorphic province and on the west by the Coast Ranges (Davis and others, 1959). The San Joaquin Valley floor slopes gradually toward the northwest and dips to a trough west of its center line. A low, broad and indistinct divide interrupts the lengthwise slope of the Valley at its midpoint and separates two hydrologic basins: the San Joaquin Basin to the north, which drains to the Pacific Ocean,and the Tulare Basin to the south, which has an outfet only when rare flood flows carry its water across the divide and into the San Joaquin Basin (State of California, Department of Watt Resources, 1981), or The subject site is located within the Tracy Groundwater Basin, a politically designated portion of the San Joaquin Groundwater Basin, The water quality in the Tracy area is being degraded by intrusion of saline connate groundwater as a result of overdraft of deeper aquifers in the Stockton and Tracy areas. Shallow groundwater is generally poor along the western boundary of the San Joaquin l~' Groundwater Basin. 4.0 BACKGR0UND i� One underground gasoline tank was formerly located on the site for fueling the school buses that serviced the Lammersville School District. The gasoline tank was removed and Lammersvilfe School District retained Groundwater Technology to conduct a preliminary sampling and analysis program to determine if leakage from the tank had affected the underlying soils and groundwater. Results of the initial investigation were presented in Groundwater Technolo Report', dated August 10, 1987. On June 18, 1987, five oringsu(MW-ibsurcth through MW-5)Hydrocarbon Investigation Report", adjacent to and around the tank excavation in an attempt to define the vertical and lateral extent-of -__petroleum hydrocarbons n the sof!and--- -- —- - - - - - - - T -- T groundwater Fi -ore 2). The initial work steps carried out in Juno - - GKOIJN'DIiATER TECHNOLOGY,INC.