Robert B. Perkins
Date of Award
Bachelor of Science (B.S.) in Geology and University Honors
Geology -- Oregon -- Columbia County, Groundwater -- Oregon -- Columbia County
Areas of Columbia County in northwest Oregon, particularly along U.S. Route 30 directly north of the Portland Metro region, are projected to see significant increases in population. However, development in areas not currently supplied by municipalities drawing water from the Columbia River could be limited by availability of suitable water resources. This is a concern for the area immediately west and upslope from Highway 30 between the cities of Scappoose and Saint Helens, a relatively flat area locally known as the "Warren Bench" that seems otherwise well suited for development. Groundwater resources there, and indeed in most areas of the county, are poorly understood and locating reliable zones with consistent and potable groundwater yields has been a challenge for land owners and developers.
In this study I analyzed 391 water well reports from the Warren Bench and surrounding area that were downloaded from the Oregon Water Resources Department (OWRD) website. I extracted locations, lithologic descriptions and well yield data from these records and correlated this information with previously published maps and reports of the local surface geology in order to identify and characterize the major water-bearing zones.
The major water-bearing zones were separated into five physically distinct hydrostratagraphic units: the early Eocene Goble Volcanic Series, a tholeiitic and alkalic basalt; the mid to late Oligocene Pittsburg Bluff Formation, chiefly a fine-grained marine sandstone; the late Oligocene to early Miocene Scappoose Formation, typically arkosic sandstones interlayered with siltstone and shales; the lower to middle Miocene Grouse Creek, Ortley, Winter Water, and Sentinel Bluffs Members of the Grande Ronde Basalt; alluvial deposits from late Miocene through the Holocene include fluvial and lacustrine deposits of the late Miocene and Pliocene Sandy River Mudstone and Troutdale Formation, Pleistocene sand and silt deposits from the Missoula Floods which may include surficial loess deposits, and late Pleistocene and Holocene alluvium locally deposited in fluvial channels. Of these, the members of the Grande Ronde are on average the most productive; however, water-bearing zones in the Grande Ronde Basalt are generally confined between individual flows. Generally disconnected vertically, these discrete water-bearing zones vary between 1-15 meters in thickness. Therefore, wells completed in the Grande Ronde are often screened or open across several members in order to achieve optimal yield.
Typically, water wells that are screened through the Oligocene and early Miocene sedimentary formations contain high levels of dissolved solids (Wagner, 2013) and unpredictable yields. However, several wells penetrating to the lower Scappoose Formation have been completed in a conglomerate water-bearing zone with exceptionally high yields. This zone is laterally discontinuous and poorly constrained and described by Kelty (1981) as incised channels of clast-supported conglomerates. These channels appear to be part of the paleotopography developed on the Pittsburg Bluff Formation. The oldest sediments of the Scappoose Formation and first to be deposited within these incised channels have been correlated with early flows of the Columbia River Basalt Group.
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Ahern, Eli Thomas, "Hydrostratigraphy of Warren Bench and Surrounding Area, Columbia County, Oregon" (2017). University Honors Theses. Paper 512.