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In 1982, the U.S. Geological Survey began a study of the Columbia Plateau aquifer system as part of its Regional Aquifer-System Analysis program. This report summarizes the results of the Columbia Plateau Regional Aquifer-System Analysis. The study area, for which most of the physiographic, climatic, cultural, economic, and geologic information were presented in a series of reports, covers about 50,600 square miles of the 63,200 square miles underlain by the Columbia River Basalt Group. The study area for the hydrologic analysis covers about 32,700 square miles of the study area. The Columbia Plateau aquifer system includes, from youngest to oldest: (1) the overburden, a collective term used in this study for all materials overlying the Columbia River Basalt Group; (2) a minor amount of sediment interlayered with the basalt; and (3) a large thickness of basalt belonging to the Columbia River Basalt Group, which is the most extensive and hydrologically important geologic unit in the aquifer system. This unit consists of five formations; the Saddle Mountains, Wanapum, and Grande Ronde Basalts are the principal formations of the aquifer system. Prebasalt rocks that underlie the plateau are the lower boundary to the aquifer system. The basaltic lavas make up more than 99 percent of the volume of the aquifer system. These basalts have an average thickness of 3,300 feet and an estimated maximum thickness of about 16,000 feet. The aquifer system has been divided into three confining units and four aquifer units. The three confining units are the Saddle MountainsWanapum interbed, the Wanapum-Grande Ronde interbed, and the prebasalt rocks. The four aquifer units are the overburden aquifer, the Saddle Mountains unit, the Wanapum unit, and the Grande Ronde unit. The overburden materials consist of fluvial consolidated to unconsolidated deposits of lacustrine, volcanic, and eolian origin whose lithology ranges from clay to gravels and shale to conglomerate. Most of these deposits generally have a much larger hydraulic conductivity than the basalts and, where saturated, make up a water-table aquifer called the overburden aquifer. The Saddle Mountains Basalt constitutes less than 2 percent of the volume of the basalts, and, depending on geographic location, it overlies the Saddle Mountains-Wanapum interbed, the Wanapum Basalt, or the Grande Ronde Basalt. The Saddle Mountains Basalt and its intercalated sediments is called the Saddle Mountains unit. A sedimentary interbed, the Saddle Mountains-Wanapum interbed, is commonly present between the Wanapum Basalt and the overlying Saddle Mountains Basalt and is considered a confining unit for this study. The Wanapum Basalt and its intercalated sediments (called the Wanapum unit) range in thickness from 0 to 1,300 feet and compose about 6 percent of the volume of the basalts. A sedimentary interbed, the Wanapum-Grande Ronde interbed, underlies part of the Wanapum Basalt and overlies part of the Grande Ronde Basalt. This unit is considered a confining unit for this study. The Grande Ronde Basalt underlies most of the plateau and constitutes more than 85 percent of the volume of the basalts. This formation and., its intercalated sediments is called the Grande Ronde unit. For modeling and descriptive purposes, this unit has been divided into the upper and lower Grande Ronde. The prebasalt "basement" rocks are considered the base of the regional flow system and are called the basement confining unit. For the overburden aquifer, lateral hydraulic conductivity values derived from specific-capacity data range from 2.9xlO~7 to l.ZxlO"1 foot per second, and regional values derived from the ground-water flow model range from 5.0xlO~7 to 1.3xlO~2 foot per second. Lateral hydraulic conductivity values derived from specific-capacity data for the basalt units range from 5.8x10"8 to 6.1xlO~2 foot per second and values from the ground-water model range from l.OxlO"6 to l.OxlO"4 foot per second. Model-derived vertical hydraulic conductance values range from 4.5xlO~13 to l.Oxl0~5 foot per second per foot. Estimates of vertical hydraulic conductivity were calculated using the vertical hydraulic conductance values and thickness of units. The estimated median value of the vertical hydraulic conductivity of the overburden aquifer is about 2.1xlO~5 foot per second, and of the basalt, about 1.5x10 foot per second. Calculated ground-water velocities range from 10"4 to 20 feet per day. The median velocity in the overburden aquifer is about 2 feet per day, and in the basalt units about 0.2 foot per day. Ground-water movement in the overburden aquifer is from uplands to discharge points along stream courses. The potentiometric surface in the basalt units, where they are not overlain by younger units, roughly parallels the land surface, and movement of water is toward surface-drainage features. The water movement in the basalt units also follows the regional dip of the basalt, and in structurally complex areas the water moves downward from the axis of anticlines toward structural lows in synclines. Water in the uppermost basalt flow is unconfined and becomes semiconfined-to-confined with depth. Over most of the plateau, the vertical component of flow is generally downward, except in the proximity of discharge areas. The long-term estimate of recharge for predevelopment (1850's) conditions is about 6,566 cubic feet per second (4,750,000 acre-feet or 2.72 inches per year) and for current (1980's) conditions is about 10,205 cubic feet per second (7,385,000 acre-feet or 4.24 inches per year). Discharge from the ground-water system, excluding pumpage, is mainly

Summary of the Columbia Plateau Regional Aquifer-System Analysis, Washington, Oregon, and Idaho