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Work Plan, PFAS Site Inspection Naval Computer and Telecommunications Station Stockton, California Background Information 2-2 DCN: TRBW-0202-5183-0010 feet in the Landfill Area. The drainage channels adequately control surface runoff and groundwater seepage (PRC 1997). Figure 1-2 illustrates the drainage channels at NCTS Stockton. The Administrative Area vegetation consists of introduced species such as ornamental trees, shrubs, and lawns in addition to recreational areas. The Warehouse Area is paved and contains minimal vegetation. The Agricultural Area consists of cultivated land including crops of alfalfa, corn, barley, sorghum, and safflower. The northern portion of the Landfill Area, which incorporates NCTS Stockton’s wildlife area and the duck pond, is a modified riparian community. The drainage channels located in the Agricultural, Administrative, and Warehouse Areas are cleared of sediment and vegetation as needed. 2.1.4 Geology and Hydrogeology This section describes the geology and hydrogeology at the former NCTS Stockton. 2.1.4.1 Geology As described in the Basewide Environmental Baseline Survey Report (PRC 1997), borings advanced at NCTS Stockton revealed a consistent surface layer of native material ranging from peat to organic or mineral-rich silts and clays to a depth of approximately 10 feet to 15 feet below ground surface. In some areas, especially those associated with development, silty and sandy fill materials may overlie this layer of organic matter. Beneath this layer of organic matter are clays and/or silts with little or no sand content. Within the silts and clays are sandier layers that range from clean fine to coarse‐grained sands to mixtures of silt and sand. These sandy layers are groundwater bearing zones that are generally confined and interconnected with distance and depth. Many groundwater monitoring wells throughout NCTS Stockton were drilled in one or more of the confining and/or water bearing zones (ERS 2012). 2.1.4.2 Hydrogeology Similar to other islands in the delta, parts of the surface of RRI are several feet below msl. A levee protects the island from the adjacent San Joaquin River, which is several feet above msl. The surrounding San Joaquin River exerts hydrostatic pressure on RRI’s shallow groundwater system. Consequently, unlike typical island groundwater hydrology, the shallow groundwater gradient in the delta islands generally flows from the surrounding surface water towards the island’s interior (ERS 2012). RRI ground surface is kept dry by a complex system of interior surface water drainage channels. These drainage channels are often deep enough to encounter the shallow groundwater zone, which results in vertically upward groundwater gradients. All the island drainage channels ultimately flow to the drainage channel at the southwestern end of RRI, where the water is then discharged under a National Pollutant Discharge Elimination System permit into Burns Cutoff (ERS 2012).