Description of the ground-water flow system in the Portland Basin, Oregon and Washington

Abstract

An increasing dependence on ground-water resources in the Portland Basin has made it necessary for State and local governments to evaluate the capability of the ground-water system to meet present and future demands for water. To evaluate this capability, the U.S. Geological Survey, Oregon Water Resources Department, City of Portland Water Bureau, and the Intergovernmental Resource Center engaged in a cooperative hydrogeologic study. This report describes the regional ground-water system and provides a conceptualization of the aquifer system. The extent, thickness, and boundaries of eight major hydrogeologic units in the basin were mapped. These units include (1) the unconsolidated sedimentary aquifer, (2) the Troutdale gravel aquifer in the Troutdale Formation, (3) confining unit 1, (4) the Troutdale sandstone aquifer in the Troutdale Formation, (5) confining unit 2, (6) the sand and gravel aquifer, and (7) older rocks. The eighth unit is an undifferentiated, fine-grained unit that is mapped where the Troutdale sandstone and sand and gravel aquifers pinch out. In those areas, the contact between confining units 1 and 2 cannot be mapped. Hydraulic-conductivity estimates from aquifer tests and single-well tests indicate that confining unit 1, confining unit 2, and the older rocks have median hydraulic conductivity values of 4, 1, and 0.3 feet per day, respectively. The aquifers have median hydraulic conductivities that range from about 7 to more than 200 feet per day. Recharge to the ground-water system in the Portland Basin is derived from three sources other than recharge that may occur from streams flowing through the basin: recharge from infiltration of precipitation, runoff to drywells, and on-site waste-disposal systems. Total average yearly recharge from sources other than streams is nearly 1,110,000 acre-feet or about 22 inches. Water levels in wells in the sedimentary aquifers and seepage measurements for streams indicate that streams are significant discharge areas for the ground-water system. Ground water generally flows from upland areas toward the major streams in the basin, but discharge also occurs from springs. Crystal Springs, located in southeast Portland, are the largest springs in the basin, with a total discharge of more than 5,000 gallons per minute. Springs north of the Columbia River in Washington between Vancouver and Prune Hill discharge about 6,000 gallons per minute. Withdrawals from wells in the basin also constitute a significant discharge from the ground-water system. In 1988, a total of 120,700 acre-feet of ground water was pumped from the basin for industrial, public-supply, and irrigation uses. Fifty percent of this water was used for industrial purposes, 40 percent for public supply, and 10 percent for irrigation. Short- and long-term water-level records from wells indicate that the ground-water system may not be in equilibrium in some areas of the basin. Short-term changes indicate that declines may be occurring in southern Clark County, Washington, and Clackamas County, Oregon, at rates generally ranging from less than 1 to 4 feet per year (1988-89).