The Harrisburg-Halsey area lies between the Cascade and Coast Ranges in the southern Willamette Valley in northwestern Oregon. The area consists of approximately 350 square miles (910 km2) 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 Miocene. The volcanic rocks are primarily of dacitic and andesitic composition and yield only small quantities of water that are generally adequate only for domestic and stock use. The alluvial deposits (sand and gravel) of the valley plain contain the more productive aquifers in the area and yield most of the water that is pumped from wells in the area.
Aquifers in the area are recharged principally by direct infiltration of precipitation. Most of the precipitation, which averages about 40 in. (1,020 mm) per year occurs during late autumn and winter.
During 1974 the seasonal decline of water levels from winter to late summer averaged about 10 ft 13 m) for the alluvial deposits. The seasonal change of storage for 1974 was estimated to be about 170,000 acre-ft (210 hm3). Of this volume, about 14,300 acre-ft (17.6 hm3) was pumped from wells; the rest, about 156,000 acre-ft (190 hm3), was discharged naturally by seepage and spring flow to streams and by evapotranspiration. The difference between pumpage and natural discharge indicates that a large quantity of additional water is available for development. The storage capacity of the alluvial aquifers is estimated to be about 800,000 acre-ft (1,000 hm3) in the zone 10-100 ft (3-30 m} below land surface.
Ground water from the alluvial deposits is chemically suitable for irrigation and other uses, as is most of the water obtained from perched-water bodies in the older sedimentary and volcanic rocks. However, the mineral concentration of water from the older sedimentary rocks, particularly from deeper producing zones beneath the valley plain, is greater than that of water from the alluvial deposits. Locally, some of the water from the older rocks is too saline for general use. Water samples from domestic wells were analyzed for fecal coliform bacteria. Although these analyses did not indicate ground-water pollution, further study would be required to establish that none exists in the area.