Abstract

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 municipal ground-water supply in both states over an area of approximately 300 mi 2 . Although lateral sedimentary facies variations are evident in the lithologic record, data from multi-well aquifer tests show that the aquifer forms a continuous hydrostratigraphic unit in both states. The aquifer is synclinal and in the center of the basin is well-confined by low-permeability layers. On the eastern and western margins of the basin, the aquifer is in hydraulic continuity with a buried Columbia River paleo-channel. The paleochannel forms a boundary to the aquifer and relative head differences between 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 water in the deep aquifers includes: a) continuous use which has increased from 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 induced recharge through the paleochannel deposits. The induced recharge available from the Columbia River may allow for further efficient development of groundwater supplies on the eastern and western margins of the basin.

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