Oregon Water Science Center Hydrologic Studies

Portland Basin Ground-Water Study (completed 1991)

PROJECT DESCRIPTION

Ground water is an important resource for industrial, public-supply, and irrigation use in the Portland Basin. In the Clark County, Washington, part of the basin, water for most uses is derived entirely from the ground-water resource. In Columbia, Clackamas, and Multnomah Counties, Oregon, most water for public and industrial use is derived from the City of Portland's Bull Run watershed. For emergency purposes as a backup to the water from the Bull Run watershed, however, the City of Portland uses a well field just east of the city on the south shore of the Columbia River. Although this well field has not been used extensively since its completion in 1986, it can produce more than 100 million gallons per day for an extended period of time if necessary. Ground water is an abundant resource in the Portland Basin, but managers in both States are concerned about preserving the quantity and quality of the resource. Rapid urban growth in recent years and the associated increased demand for water has resulted in well interference problems, water-level declines, and water-quality problems in the basin. The need for a quantitative understanding of the ground-water flow system to allow for the best management of the resource led to the study described here.

The purpose of this study was threefold: (1) to describe and quantify the ground-water hydrology of the Portland Basin on a regional scale, (2) to quantify the ground-water hydrology to the extent necessary to predict the effects of existing and proposed wells on the basin, and (3) to improve the understanding of the ground-water hydrology to aid in development by the responsible agencies of a regional water-resources allocation plan and in development of a single-well permit process.

This study, begun in January 1987, was initially a cooperative effort between the city of Portland, the Oregon Water Resources Department, and the U.S. Geological Survey, and included the basin only as far north as Salmon Creek (fig. 1) in Clark County, Washington. Shortly thereafter, the Intergovernmental Resource Center, located in Clark County, Washington, became involved in developing a ground-water management plan for Clark County, Washington, and became aware of the Portland Basin study. The Intergovernmental Resource Center determined that the study of the ground-water resources in the basin could help guide their management plan and therefore became the fourth cooperating agency involved in the project. As a result, the study boundary was moved north to include all of Clark County.

Many aspects of the ground-water system were studied and several reports have resulted from this work. The reports include: (1) basic ground-water data (McCarthy and Anderson, 1990), (2) a description of hydrogeologic units (Swanson and others, 1993), (3) estimates of ground-water recharge from precipitation, runoff into drywells, and on-site waste-disposal systems (Snyder and others, 1994), (4) estimates of ground-water pumpage (Collins and Broad, 1993), (5) an overall description of the ground-water system (McFarland and Morgan, 1997), (6) a simulation model analysis of the ground-water system (Morgan and McFarland, 1997), and (7) an interface between a geographic information system and the U.S. Geological Survey modular ground-water flow model (Orzol and McGrath, 1992). More information on these and other reports is available.

Portland Basin Ground-Water Study Page
Oregon Water Science Center Hydrologic Studies Page

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