Hydraulic continuity of the portland basin deep aquifer system

The Portland Basin is 900 mi(2) in size and includes portions of Oregon and Washington. A deep aquifer system of the Troutdale Formation is used for m unicipal ground-water supply in both states over an area of approximately 3 00 mi(2). Although lateral sedimentary facies variations are evident in the lithologic record, data from multi-well aquifer tests show that the aquife r forms a continuous hydrostratigraphic unit in both states. The aquifer is synclinal and in the center of the basin is well-confined by low-permeabil ity layers. On the eastern and western margins of the basin, the aquifer is in hydraulic continuity with a buried Columbia River paleo-channel, The pa leochannel forms a boundary to the aquifer and relative head differences be tween the aquifer and the river cause ground water to flow up the formation dip and against the Columbia River gradient in the eastern portion of the basin. The paleochannel acts as a discharge sink during low-pumping periods and a recharge source during extended pumping. Municipal use of ground wat er in the deep aquifers includes: a) continuous use which has increased fro m approximately 1 to 2 mgd in 1984 to 13 mgd in 1998 and, b) emergency use which has caused total use peaks of 55 mgd, The increased ground-water use has resulted in a modified flow regime causing the aquifer to receive induc ed recharge through the paleochannel deposits, The induced recharge availab le from the Columbia River may allow for further efficient development of g roundwater supplies on the eastern and western margins of the basin.