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Geologic framework for the Puget Sound aquifer system, Washington and British Columbia

January 10, 2000

The Puget-Willamette study area is composed of two distinct subareas, the Puget Sound Lowland and the Willamette Lowland. The study area for this report is the Puget Sound Lowland, which is located in western Washington and in a small part of southwestern British Columbia, Canada. The lowland encompasses an area of about 17,610 square miles and contains about 2,615 square miles of saltwater. The unconsolidated Quaternary deposits that compose the Puget Sound aquifer system underlie about 7,183 square miles of the study area.

The extent of the Puget Sound aquifer system is delineated by its lateral and basal boundaries above the Tertiary and older rock units, called the basement confining unit, and the lateral extent of the glacial drift from the last glaciation, the Fraser Glaciation. The unconsolidated Quaternary deposits which compose the aquifer system consist of alluvium, glacial, and interglacial deposits. Of the unconsolidated deposits at the land surface, 1,570 square miles are covered by alluvium, 2,293 square miles are covered by coarse-grained deposits, and about 3,320 square miles are covered by fine-grained deposits. The alluvium and coarse-grained deposits generally compose the aquifer units, and the fine-grained deposits generally compose the semi-confining to confining units.

Four glaciations and three interglaciations are recognized in the Puget Sound Lowland. As a result, the unconsolidated Quaternary deposits consist of one to four regional drift sequences and as many as eight local ones. These sequences are generally separated by unconformities and by interglacial fluvial and lacustrine deposits. These sequences produce an alternating pattern of coarse- and fine-grained deposits from land surface to depths of more than 3,000 feet.

The unconsolidated Quaternary and Recent deposits within the lowland were grouped into local aquifer units and semiconfining and confining units based on their lithologic and hydraulic characteristics. These units were then grouped into regional aquifer units in order to assess the ground-water resources and describe the ground-water flow on a regional basis. The designated regional hydrogeologic units are the alluvial valley aquifers, the surficial semiconfining unit, the Fraser aquifer unit, the confining unit, and the Puget aquifer.