Puget Sound-Willamette Trough

Science Center Objects

More than 70 percent of the population of Washington and Oregon resides in the Puget Sound-Willamette Trough, which stretches north-south on the western side of the Cascade Range. The area is one of the principal regional ground-water systems in the Nation, but little is known about the quantity and quality of the ground water, the regional flow system, or the interaction of the system with surface water and local rock material. The USGS has included the Puget Sound-Willamette Trough in its Regional Aquifer System Analysis (RASA) program, which systematically assesses the principal regional aquifer systems in the Nation. The study provides to water resource managers and community planners comprehensive information on the region’s aquifer systems that is needed to make a wide range of management and planning decisions, both locally and regionally.

To obtain a better understanding of the regional ground-water system in the Puget Sound-Willamette Trough, the USGS is combining existing data and data collected during the RASA study to describe the geologic framework and hydrogeologic characteristics of the principal aquifers and to determine regional ground-water flow.

WA336 - Puget Sound-Willamette Trough Regional Aquifer System Analysis - Completed FY2001

Problem - The Puget Sound-Willamette Trough regional aquifer system is one of the 28 regional aquifers chosen for study under the USGS RASA program. The states of Washington and Oregon are very interested in this study because over 70 percent of their population resides within the study area boundaries. Within the project area, such information as quantity and direction of ground-water flow, lengths of flow paths, locations of ground-water discharges, stream-aquifer interaction, relationships with older rock materials, and continuity between aquifer units is largely unknown. All of these topics require better definition; lack of that information impairs the ability of managers to make knowledgeable decisions.

Objectives - The primary goal of this program is to obtain a better understanding of the regional ground-water system. To achieve this goal, the following objectives have been defined:

  1. describe the geologic framework of the principal aquifers,
  2. describe the geohydrologic characteristics of the principal aquifers,
  3. describe the regional flow system,
  4. estimate the water budget for selected areas of the aquifer system. Use this information to describe the regional water budget,
  5. determine the present water quality and variations of native water quality and water-rock interactions in selected areas, and
  6. use ground-water-flow models to synthesize the geohydrologic data and concepts on how the regional flow system operates.

Relevance and Benefits - This study was the final study of the U.S. Geological Survey's Regional Aquifer System Analysis Program, which systematically analyzed the principal ground-water systems in the nation. The study directly addresses the mission of the USGS and is national in scope and work. The study provided new information and methods on an important resource that transcended state and national boundaries. The information can now be put into a national framework for understanding the state of the nation's ground-water resources.

Approach - The first year of effort will consist of a planning stage or " pre-RASA" study. We will begin the gathering and analysis of existing data and studies. Available information will be mapped and analyzed on a regional scale. Trends and variations of hydrologic and water-quality information will be studied in conjunction with available geologic information. This initial analysis will attempt to see if the available information, including data on discharge, recharge, runoff, and hydraulic characteristics, and results from local studies allow construction of a conceptual model of ground-water flow. This element will identify where information on the regional system is grossly lacking. A plan of study will be written by the project chief identifying the timeframe, manpower, costs, and steps to be taken for completion of the study.