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to decline to the east. Thus groundwater flows from the delta toward the Central Valley in the <br /> vicinity of RRI. <br /> 2.5 Site Groundwater <br /> There has been one shallow water-bearing zone, three aquifers, and five aquitards identified in <br /> the upper 75 feet of the soil underlying most of RRI. The aquifer system is complex and <br /> interconnections between aquifers are interpreted to be present. Groundwater occurs under <br /> confined conditions in three aquifers and in both confined and unconfined conditions in the <br /> shallow water-bearing zone. <br /> Sedimentary deposits at RRI are related to meandering river deposits in a back delta <br /> environment. Types of deposits include back swamp, flood plain, channel, and abandoned <br /> channel sequences. Groundwater elevations vary over time at RRI with seasonal rainfall and <br /> temperature variations significantly affecting groundwater elevations. Tidal fluctuations have a <br /> minor effect on groundwater elevations in some wells near the island boundary and wells located <br /> near interior drainage channels are affected by drainage channel water levels. Groundwater <br /> flows from the surface water surrounding the island through the aquifers and into interior <br /> drainage channels. <br /> Based upon Tetra Tech EMI's site investigations, groundwater occurs approximately 7 to 20 feet <br /> below ground surface. Recent measurements on October 7, 1998 taken from a piezometer <br /> located west of Building 1004 reported groundwater at 7 feet bgs. At the impacted area, <br /> groundwater is believed to flow south/southwesterly. <br /> Rough&Ready Island 7 Tetra Tech,Inc. <br /> Revised Draft Work Plan November 18, 1998 <br />