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Abstract A three-dimensional facies analysis of Cenozoic fluvial deposits in the vicinity of a federal Superfund site near Oroville, California, was conducted to characterize aquifer heterogeneity and to provide a basis for accurate modeling of the complex groundwater system. A network of 29 boreholes in the 6.5-km 2 study area penetrated four distinctive fluvial units, including the Laguna Formation (upper Pliocene-Holocene), Nomlaki Tuff (upper Pliocene), Mehrten Formation (Miocene-upper Pliocene), and lone Formation (Eocene). All four formations consist of pebble-cobble gravel, sand, and clay deposited by the Feather and ancestral Feather River systems at the margin of the Sacramento Basin near the western Sierra Nevada mountain front. Sediment gravity flows and waterlain andesitic and pumice-rich volcaniclastic facies derived from the southernmost Cascade volcanic centers episodically choked the ancestral Feather River system during late Pliocene upper Mehrten and Nomlaki Tuff deposition. Cenozoic fluvial aggradation was interrupted by three periods of valley cutting, which occurred 1) during the Oligocene, between deposition of the lone and Mehrten Formations; 2) during the late Pliocene, between deposition of the Nomlaki Tuff and Laguna Formation; and 3) during the middle Pleistocene, after most Laguna deposition was completed. The Oligocene and late Pliocene paleovalleys, 24 to 61 m deep, are filled with vertically and laterally interconnected channel pebble-cobble gravel of the Mehrten and Laguna sequences. The principal aquifer zones in the study area are >30-m-thick units of hydraulically continuous channel gravel and sand of the Laguna, Mehrten, and lone Formations. These aquifer zones are continuous across Oligocene and late Pliocene paleovalley margins through channel-fill gravel and sand of the various formations. Floodplain clay and volcaniclastic debris-flow units act as aquitards, which locally limit the interaction between permeable zones. A dissolved pentachlorophenol plume in local ground water originates at the Superfund site (a wood treatment plant) in the northern end of the study area, where it initially moves within the Laguna Formation. The plume has migrated into hydraulically continuous Mehrten and lone gravels that are in contact -425 m south of the plant site. The plume has also passed into juxtaposed Mehrten and Laguna gravels 1,400 m south of the plant site. Pumping-test data demonstrate that no marked changes in hydraulic conductivity occur across the paleo v alley- fu11 boundaries. Local changes in the hydraulic-gradient vector are caused by changes in the permeability of the aquifer material or in the thickness of the aquifer zones, both of which are associated with lateral changes in formations and facies.