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classification, which identifies areas subject to shallow flooding, ranging from I to 3 feet deep, during a 100-year flood. The existing drainage systems in and around the airport property are operating near capacity and occasional flooding occurs during major storms,particularly in areas just off the airport property (Reinard W. Brandley Consulting Airport Engineer 1997). The flood insurance rate map for the airport indicates that the main runway and taxiways and aprons at the airport would be flooded in a 100-year storm to depths ranging from I to 3 feet but that the depth in the airfield section of the airport would generally be 1 foot (Brandley pers. comm.) Figure 5-1 depicts the location of the 100-year flood plain. Construction records show that the surface of Runway 11 L-2R was built 2 to 3 feet above the surrounding ground (Brandley pers. comm.) Therefore, Runway I IL-29R, the aircraft holding apron to Runway 29R, the portion of Taxiway D leading to the air cargo apron at Farmington Fresh, and the air cargo apron at Farmington Fresh are probably above the 100-year floodplain(Brandley pers. comm.) All areas planned for development northeast of the runways, with the exception of the area ' in the vicinity of the old abandoned runways, are currently located outside the 100-year floodplain. 1 Groundwater Hydrology San Joaquin County is located over the central portion of California's Central Valley groundwater basin. This extensive aquifer extends some 400 miles from Red Bluff to Bakersfield { , and averages 40 miles in width. The Central Valley aquifer is a complex system of different groundwater basins composed of stratified sand, silt, and clay layers many thousands of feet thick. However, the average depth of strata bearing fresh water is 600 feet below the surface. These deposits have been accumulating over the last 100 million years and are underlain by impervious bedrock. In the eastern part of San Joaquin County, fresh water is found in a two-layer aquifer system (Brown and Caldwell Consulting Engineers 1985). The upper unconfined layer is.comprised of the Victor (approximately 150 feet thick) and Laguna (approximately 1,000 feet thick) formations, consisting of coarser and more permeable material on the surface and less permeable sands and silts in the deeper Victor formation. The lower layer is the underlying Mehrten formation, which is generally in an unconfined state east of Stockton and confined over the central part of the county. The Mehrten formation is composed of stream-deposited and semi-consolidated silts, sands, and gravel approximately 600 feet thick. The extensive and saline Valley Springs formation covers the majority of the region and underlies the freshwater systems. Long-term withdrawal of groundwater in the region of approximately 1.7 feet per year between 1947 and 1984 has lead to overdrafting of the available water and has resulted in intrusion of saline water into the freshwater aquifers. Groundwater levels are below sea level over extensive areas of the county. Water levels vary seasonally with the recharge from winter rainfall and summer withdrawals and range from 20 to 40 feet below sea level in the vicinity of the Stockton Metropolitan Airport. Stockton Metropolilan Airport Project Chapter 5. Drainage and Water Quality Draft Environmental Impact Report 54 March 20, 1998 f1 , r: