Low-Yield Aquifers
Low-yield aquifers are a problem primarily in the marine sedimentary rock of the Coast Range, and volcanic rock of the Cascade Range foothills. The marine sedimentary rock primarily consists of fine-grained sandstone, siltstone, and mudstone with very low permeability. Rock in the Cascade foothills consists of sediments of similar grain size but of volcanic origin, and weathered or altered lava flows. In general, these materials do not readily transmit water and well yields are small. In some cases, wells that intersect permeable fractures within these rocks will have yields that are larger then typically found in the area. These fractures, however, generally have limited storage, and because the water stored in the fracture ultimately comes from the fine-grained rock, the large yields are not sustained. Marginal aquifers are particularly sensitive to climate effects, such as drought, and to interference problems between wells. Low-yield aquifers can cause serious water-supply problems, particularly for rural landowners that have no alternative sources of water.
The Willamette Basin Report outlines a policy to "identify low-yield aquifers and inform local agencies of probable insufficient ground-water flow for some uses." Geologic maps showing the location of rock types with low-permeability are available for all of the Willamette Basin at various scales. In most of the basin, however, the water-bearing characteristics of these rocks have not been characterized or systematically described.
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Last modified: Mon Jan 29 13:48:38 1996