SITE INFORMATION AND CORRESPONDENCE

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7 Wendy L. Cohen • -3- 22 January 1996 Tracy Defense Depot Investigations at TDD have identified four hydrostratigraphic horizons in the area. These are the Above Upper, Upper, Middle, and Lower Horizons. The four horizons are water-bearing units of slightly higher permeability within a larger heterogeneous hydrostratigraphic unit. Depths and thicknesses of these geologic units vary laterally across TDD. The coarse-grained deposits are not correlative or continuous across the depot, but pinch out and interfinger with finer-grained deposits in some areas. The Above Upper Horizon is present across the TDD, but is saturated only in the northern portion of the depot and beneath the annex property. This horizon ranges from 10 feet thick in the southern portion of the annex to 25 feet thick in the southern portion of the depot and comprises the uppermost 10 to 35 feet of fine-grained sediments beneath the depot and the annex. The Upper Horizon is a coarse-grained zone occurring between depths of approximately 25 and 60 feet below ground surface (bgs). The top of this horizon is present between approximately 25 and 45 feet bgs on the annex and depot, respectively. Sands, gravels, and silty sands are the predominant sediments of this horizon. Interbedded layers and lenses of silt, silty sand, and clay are common. There are two highly-permeable buried stream channels that slope from the west and southwest to converge on the south-central portion of the annex, and then separate again to the north and northeast. The direction of the stream channels indicates the sediments were transported from the Diablo Range west-southwest of TDD. The stream channel deposits appear to act as a preferential pathway for ground water. A fine-grained unit underlies the Upper Horizon between approximately 50 and 80 feet bgs. This unit consists of silty clay, clayey silt, and silty sand which grades to the north into sandy and gravelly clay. The Middle Horizon, consisting predominantly of sand, gravel, and silty sand, occurs between approximately 55 and 85 feet bgs beneath the northern portion of the depot and between approximately 75 and 105 feet bgs beneath the southern portion of the depot. The horizon is fairly consistent across the TDD. Two stream channel deposits similar to the ones in the Upper Horizon are present in this horizon, but the stream channel deposits in the Middle Horizon are not as thick as in the Upper Horizon. A fine-grained unit underlies the Middle Horizon between approximately 90 and 120 feet bgs. This unit consists primarily of silt, silty, clay, and sandy clay with interbedded lenses of silty sands. The top of the Lower Horizon occurs at depths of approximately 110 feet bgs in the northern portion of the depot and 125 feet bgs in the southern portion of the depot. Beneath the southern portion of the depot, the Lower Horizon consists predominantly of gravel and sand. Beneath the northern portion of the depot, lenses of silty sand, clayey sand, and clayey gravel are common. The Lower Horizon sand layer contains buried channels like the Upper and Middle Horizons. Sediments consisting primarily of silty clays and sandy clays with thin layers of sandy gravels are present below the Lower Horizon at approximately 160 feet bgs. Horizontal ground water gradients in the Above Upper Horizon have historically been to the northeast and in the other three horizons to the north-northeast. The vertical gradients between each of the horizons are mostly downward.