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Summary of Willamette Basin Ground-Water Study

The burgeoning population of the Willamette River Basin is putting unprecedented demands on the water resources of the region. Because surface-water resources are largely allocated, ground-water resources are being increasingly looked upon to meet the growing demand. The increasing demand on these finite resources poses a particular challenge to resource managers and planners, and to society as a whole. Developing a sound, quantitative technical understanding of the ground-water hydrology of the region is of paramount importance for the successful management of water-resources and providing for the variety of demands.

In its 1992 Willamette Basin Report, the Oregon Water Resources Department (OWRD) identified four major ground-water management issues in the Willamette Basin:

  1. Managing ground-water and surface-water conjunctively. A sound, quantitative understanding of the ground-water hydrology, the nature of the ground-water/surface-water connection, and streamflow characteristics is necessary for coordinated management of ground- and surface-water resources. This understanding has yet to be developed in the Willamette Basin.
  2. Controlling long-term ground-water declines. Declining ground-water levels were documented locally in the Willamette Valley as early as the 1960s. Water-level declines cause serious water supply-problems and hardship for rural home owners as well as for municipalities and irrigators, and threaten the long-term sustainability of the resource.
  3. Managing development of low-yield aquifers. Low-yield aquifers are a problem primarily in the marine sedimentary rock of the Coast Range, and volcanic rock of the Cascade Range foothills. In general, these materials do not readily transmit water and well yields are small.
  4. Identifying areas prone to natural ground-water quality problems. High salinity and high arsenic concentrations are the two major natural water-quality problems in parts of the Willamette Basin.

The study was designed to address the issues listed above by meeting the following objectives:

  1. Provide a quantitative understanding of the regional ground-water flow system of the Willamette Valley sufficient to effectively evaluate the hydrologic effects of land- and water-use policies and climate changes.
  2. Develop the understanding and tools necessary to quantitatively evaluate the timing, location and magnitude of streamflow depletion caused by ground-water pumping.
  3. Characterize the unique hydrology of basalt aquifers within the Willamette Valley, particularly as related to water availability and management of multiple water-bearing zones.
  4. Develop a better understanding of the relations between well-yield and factors such as geology, well construction, and siting in areas underlain by low-yield aquifers.
  5. Develop a better understanding of the origins and distribution of selected types of naturally occurring poor-quality ground water.

The approach to the study includes two phases that correspond to two levels of detail. During the first phase of study, the scale of analysis will be regional and data will be collected, compiled, and analyzed to determine the large scale water-budgets, ground-water flow directions, and relations between the streams and aquifers in the basin. Important hydrologic processes and areas of concern will be identified during the first phase for more detailed investigation during the second phase of the study. These detailed, or focus, studies will entail data collection and analysis focused on specific aspects of the hydrology of the basin in order to provide an understanding of hydrologic processes at the scale needed for management of the resource.

The first phase of the study will integrate and build upon the work by recent studies such as the USGS Regional Aquifer System Analysis (RASA), the cooperative surface-water study by the USGS and Oregon Department of Environmental Quality, the USGS National Water Quality Assessment (NAWQA), and the Portland Basin ground-water study. The primary goals of this phase will be to: (1) develop a quantitative, regional understanding of the hydrologic system, including the ground- and surface-water subsystems and their relation; (2) identify needs for further data collection; and (3) identify processes and areas for detailed study in the second phase of the study.

Focus studies during the second phase will concentrate data collection and analysis on specific processes and areas of the basin. Goals of this phase will be to: 1) gain a quantitative understanding of the hydrologic and geologic characteristics of the basin that determine the response of the hydrologic system to changes in water use and allocation, land use, and climate, and 2) develop models of the hydrologic system capable of predicting that response. The size of the Willamette River Basin does not permit detailed study of every area, however, the knowledge of processes gained from the focus studies will be applied to other parts of the basin with similar hydrologic and geologic conditions.

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