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Seven aquifer units were delineated for western Oregon by grouping aquifers according to hydraulic and geologic similarities. The bedrock aquifer units in the Klamath Mountains, Coast Range, and Western Cascade Range all have hydraulic conductivities generally less than 2 feet per day and generally yield less than 10 gallons per minute to wells. The Columbia River Basalt Group aquifer unit, which is present along the Columbia River and in the northern Willamette Valley, also has a low hydraulic conductivity of about 5 feet per day; however, the basalt does contain permeable interflow zones and scoriaceous flow tops that transmit water laterally. Well yields from the Columbia River Basalt Group are generally less than 100 gallons per minute. The most important aquifer unit in western Oregon is the Tertiary-Quaternary sedimentary deposits that occur in lowlands throughout the area and provide water for irrigation, industry, public supplies, and domestic and stock uses. Hydraulic conductivity of the most recent alluvium in the aquifer unit is generally 200 to 600 feet per day in the Willamette Valley, and well yields exceeding 2,000 gallons per minute have been reported. At shallow depths, the seven aquifer units generally contain water with low concentrations of dissolved solids. In the Tertiary rocks of the Coast Range, analyses of water from deep wells indicate that water with more than 10,000 milligrams per liter dissolved solids is probably widespread at depths greater than 2,000 feet. INTRODUCTION Objectives and Scope Under the authority of the Safe Drinking Water Act, the U.S. Environmental Protection Agency (ERA) is responsible for development and implementation of an Underground Injection Control (UIC) Program to protect the Nation's ground-water supply from contaminants injected into the subsurface. The U.S. Environmental Protection Agency (1975) regulations state that any aquifer containing water with fewer than 10,000 mg/L (milligrams per liter) of dissolved solids is to be protected. One of EPA's objectives in the UIC Program is to be able to estimate the size of the area around a proposed injection well that could be affected by increased pressures as a result of injection. EPA refers to the area as the "area of review" and defines it as "*** that area the radius of which is the lateral distance from an injection well pattern in which pressure changes resulting from the injection operation may cause the migration of the injection and (or) formation fluid into an underground source of drinking water" (U.S. Environmental Protection Agency, 1979). In Oregon, the U.S. Geological Survey was asked by the EPA to supply hydrogeologic information that will aid in the evaluation of proposals for underground injection. This report deals with aquifers in Oregon west of the Cascade Range crest. The three primary objectives of the project were to (1) delineate and describe the major aquifers, (2) describe the quality of water in those aquifers and identify geologic formations containing water with dissolved-solids concentrations exceeding 10,000 mg/L, and (3) evaluate methods by which the area of review may be estimated for proposed injection wells in western Oregon. During compilation of information for objectives 1 and 2, it became apparent that detailed hydrogeologic data for western Oregon's aquifers are inadequate for making reliable estimates of the area of review at any locality without first obtaining more site-specific data. However, rough preliminary calculations can be made to determine the order of magnitude of the size of the area of review using data tabulated in this report in conjunction with mathematical equations for predicting pressure buildup presented in an EPA publication "Radius of Pressure Influence of Injection Wells," by Warner and others (1979). These rough calculations can be improved using data obtained by test drilling and aquifer testing at the proposed injection site. For a detailed discussion of underground injection, the reader is referred to "An Introduction to the Technology of Subsurface Wastewater Injection," by Warner and Lehr (1977). This report is intended as a general guide for the EPA or other agencies involved in evaluation of proposals to inject waste into the subsurface in western Oregon. It is not intended for use as a sole source of information in evaluating specific sites for underground injection. Detailed information would need to be collected and analyzed on a site-by-site basis to perform such evaluations. 2 Method of Study and Available information Most of the Information in this report is compiled from published reports of the U.S. Geological Survey, Oregon Water Resources Department, and Oregon Department of Geology and Mineral Industries (see Selected References, p. 20-25). Additional data were obtained from western Oregon water well reports and lithologic and geophysical logs of more than 60 exploratory oil and gas wells. Geologic contacts shown on plate 1 and other plates in this report are primarily from the "Geologic Map of Oregon West of the 121st Meridian" (Wells and Peck, 1961). In some areas where more recent studies have been made, contacts from Wells and Peck (1961) were modified slightly. Generally, outcrops with an area I extent less than 1 mi 2 were not included in the plates. An important source of geologic background information is the publication "Geology of Oregon" (Baldwin, 1976). Description of the Area and its Climate Oregon is divided into western and eastern Oregon by the Cascade Range that trends north-south through the State. Approximately one-third of the State lies west of the crest of the range. The Cascade Range is an orographic barrier for eastward-moving oceanic weather systems and is a major drainage divide. Western Oregon has a mild climate and is rather humid with an average annual precipitation that ranges from about 30 in. in lowlands to more than 150 in. in the Cascade Range. Most precipitation occurs in fall and winter. Western Oregon is about 30,000 mi 2 in area and for this study is subdivided into four physiographic divisions: Klamath Mountains, Coast Range, Cascade Range, and WiIlamette Valley (fig. 1). Each division has a distinct geologic and hydrologic setting. Topographically high areas generally receive the most precipitation and, therefore, water available for recharge to the aquifers is greatest in those areas. The Klamath Mountains occupy most of southwestern Oregon and extend southward into northern California. They are rugged, have 2,000 to 5,000 ft of relief and, locally, receive more than 120 in. of precipitation annually (fig. 2). The Coast Range parallels the Oregon coast and extends from the Columbia River on the north to the latitude of Coos Bay, Oreg. on the south where it merges with the Klamath Mountains. The altitude of the crest of the Coast Range averages about 1,500 ft with peaks as high as 4,000 ft. Local areas of the Coast Range receive more than 200 in. of precipitation each year. The Cascade Range includes some of Oregon's highest peaks. The crest of the range averages about 5,000 ft in altitude and a few peaks along the crest rise to more than 10,000 ft. Some of the highest peaks receive more than 150 in. of precipitation annually, most of which falls as snow in the winter. Because of differences in underlying geologic formations, the Cascades are subdivided into two sections, the Western Cascades and the High Cascades. The Western Cascades form the western foothills of the range and are more deeply eroded and lower in altitude than the High Cascades, which are composed of geologically younger volcanic deposits (Peck and others, 1964). 3 0 60 MILES Modified from Dicken (1950) I . .'.i. .' . '. ' I Tl T I i r I 1 0 80 KILOMETERS FIGURE 1. Western Oregon and its physiographic divisions.

A description of aquifer units in eastern Oregon