Water Resources Inventory Area 1 Watershed Management

Introduction to the Ground-Water System


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WRIA 1 consists of two hydrologic provinces: the upland areas where streams have steep gradients and cut through bedrock, and the lowlands where streams have low gradients and cut through glacial and interglacial sediments and alluvium (Generalized Pattern of Ground-Water Movement for the Puget Sound Aquifer System in the WRIA 1 Study Area). It is these sediments of the lowlands of WRIA 1, formed during cycles of glacial advances and retreats over more than the past 100,000 years, that form the ground-water system of interest in WRIA 1. The thickness of the lowland sedimentary deposits ranges from less than 100 feet to approximately 1,500 feet (Vaccaro and others, 1998). They consist of high-permeability, coarse-grained materials such as sand and gravels, and low-permeability clays, silts, and glacial till.

Most residents of WRIA 1 live in the lowlands on top of this sedimentary wedge. The land is used for a variety of purposes, including municipal development, agriculture, and industry. To sustain the various land-use activities, water is withdrawn from two sources: ground water and surface water. Ground water used in WRIA 1 is taken from aquifers in the sedimentary deposits (an aquifer is defined as a hydrogeologic unit that contains sufficient saturated material to yield significant quantities of water to wells and springs). This ground water is dynamic, moving at rates determined by a variety of factors including the permeability of the sediments, the rate of ground-water recharge, the vertical and horizontal distribution of sediment types, land-surface elevations, pumpage, and other factors.

The source of most of the recharge of the ground-water system in WRIA 1 is precipitation. When it rains, a portion of the precipitation infiltrates into soils and recharges underlying aquifers. If soils become saturated, additional precipitation runs off the land and enters surface-water bodies such as lakes and streams.

In a natural system, ground water moves toward discharge areas, which are lakes, streams, bogs, and Puget Sound. In a system modified by people, discharge also occurs from pumped ground-water wells. Some ground water has short flowpaths and only stays in the system for short periods of time (for example, water that recharges immediately adjacent to a stream and may discharge to that stream within days), while other ground water has much longer flowpaths and may remain in the system for thousands of years. Generally, the farther ground water is below land surface, the slower it moves and the longer it remains in the flow system.

On this web page, a subset of existing information related to the ground-water system of the lowlands of WRIA 1 is displayed in maps, tables, and graphs. Each piece of information may serve a purpose when computing a water budget for WRIA 1 during later study phases. For example, the hydrogeology of WRIA 1 (Surficial Hydrogeologic Units of the Puget Sound Aquifer System in the WRIA 1 Study Area) is important for estimating ground-water recharge and surface-water runoff characteristics, locations of wells (Locations of Selected Wells in the WRIA 1 Study Area by Primary Water Use) and basic well information are important to reconstruct the three-dimensional structure of the sedimentary wedge that makes up the ground-water system of the lowlands of WRIA 1, and aquifer test information ( Summary Information for Aquifer Tests in the WRIA 1 Study Area ) provides information about the ease with which ground water moves through the ground-water system at a particular location.

Reference Cited

Vaccaro, J.J., Hansen, A.J., and Jones, M.A., 1998, Hydrogeologic Framework of the Puget Sound Aquifer System, Washington and British Columbia: U.S. Geological Survey Professional Paper 1424-D, 1 plate, 77 p.