Ground water in the Corvallis-Albany area, central Willamette Valley, Oregon

Abstract

The Corvallis-Albany area is part of the alluvial plain that lies between the Cascade and Coast Ranges in the central Willamette Valley in northwestern Oregon. As used in this report, the Corvallis-Albany area consists of approximately 210 square miles and includes a part of the lower foothills of the Coast and Cascade Ranges. Volcanic and marine sedimentary units exposed in the foothills range in age from Eocene to Oligocene or Miocene. The volcanic rocks are primarily pillow lavas and basalt flows, which yield only small quantities of water generally adequate for domestic and stock use. Marine-deposited sandstone, siltstone, and shale of the older sedimentary units are fine grained, poorly permeable, and generally yield small volumes of water to wells. In the valley plain the older units are overlain by Pleistocene and Holocene alluvial deposits. The alluvial deposits (sand and gravel) of the valley plain contain the most productive aquifers in the area and are considered to be the only units feasible for large-scale development of ground-water supplies. Aquifers in the area are recharged principally by direct infiltration of precipitation. Most of the precipitation (about 38 in. per yr avg) occurs during late autumn and winter. Ground water is discharged naturally from the area by seepage and spring flow to streams, by evapotranspiration, by underflow, and artificially through wells. During 1971 the seasonal decline of water levels from winter to late summer averaged about 10 feet for the alluvial deposits. The seasonal change of storage in that year was estimated to be about 130,000 acre-feet. Of this volume, about 14,000 acre-feet was pumped from wells; the rest (about 116,000 acre-feet) was discharged through seeps and springs by evapotranspiration. The difference between pumpage and natural discharge indicates that a great quantity of additional water is available for development. The storage capacity of the alluvial aquifers in the area is estimated to be about 750,000 acre-feet between depths of 10 and 100 feet. Ground water from the alluvial deposits is chemically suitable for all uses, as is most of the water from perched-water bodies in the older sedimentary and volcanic rocks. However, the mineral content of water from the older sedimentary rocks, particularly from deeper producing zones in the valley plain, is greater than that from the alluvial deposits. Locally, some of the water from the older rocks is too saline for general use. Analysis of water samples for coliform bacteria indicates that ground-water pollution exists in parts of the Corvallis-Albany area. Further study is necessary to document fully the nature and extent of pollution.