Geologic formations in Oregon, east of the crest of the Cascade Range, have been grouped according to similarities in their hydrogeologic and geologic properties into six major aquifer units. Two of the units, the Mesozoic-Paleozoic and the John Day-Clarno aquifers, are low-permeability aquifers, have hydraulic conductivities generally less than 1 ft/d (feet per day), and are generally capable of yielding only a few gallons per minute to wells. These are important aquifer units, nevertheless, because they are the only economical source of domestic water present in east-central Oregon where they outcrop. Four of the aquifer units contain beds or zones of high permeability materials with hydraulic conductivities that commonly range between 5 and 50 ft/d. In many localities where these units are present, they are capable of yielding 200 gallons/min or more to wells. These productive aquifer units are the Columbia River Basalt, the Cenozoic volcanic and sedimentary , Cenozoic sedimentary, and the Quaternary sediment aquifers, respectively. North of the Blue Mountains, the Columbia River Basalt aquifer is a major aquifer of regional extent and, in that area, heavy withdrawals, chiefly for irrigation, have resulted in regional groundwater level declines. South of the Blue Mountains, the basalt underlies rugged terrane, is not developed, and little is known about its hydraulic properties. Other major aquifer units are heavily developed in localized areas or in basins throughout eastern Oregon. (USGS)
