Understanding Geohydrology - Research Goal 2

In many areas of the western United States, lava-flow dominated volcanic sequences contain significant, but poorly understood aquifers. These aquifers are becoming increasingly important sources of local groundwater for domestic, agricultural, and even municipal usage. The Miocene Columbia River Basalt (CRB), which covers large regions of the Pacific Northwest, now represents an important ground water aquifer as ground water development in places has moved beyond the alluvial basins. For example, groundwater supplies for the Willamette basin in northwest Oregon no longer are solely derived from the basin alluvial aquifers, but also from aquifers in the CRB in and around the basin.

The goal of this task is to understand the nature of groundwater aquifers in lava-flow dominated volcanic deposits. This will require determining, among many things, what zones of the volcanic sequences represent the aquifers and how they relate to the basic multiple-flow architecture of the deposit, how these zones develop in the context of the total volcanic process, how they evolve following initial formation, and what are their associated hydrologic characteristics and how these are influenced by external factors (such as faulting, folding, weathering, etc.). An important element of this investigation will be to establish effective methods for mapping the aquifers in exposed volcanic sequences and to extend this information to address how aquifers might be identified and delineated remotely through a combination of geological reasoning and inference, geochemical mapping, geophysical surveying, and other techniques. This aspect will be particularly important when confronting the practical problems associated with volcanic aquifers concealed beneath younger sedimentary deposits.

This work is coordinated closely with and benefits greatly from ongoing work in the Pacific Northwest Urban Corridor Project (http://geology.wr.usgs.gov/wgmt/pacnw/pnwmap.html), which already has conducted an extensive geologic mapping, paleomagnetic, and geochemical study of the CRB flows directed at resolving the stratigraphy within the stack of flows.

The task first concentrates on the Tualatin and northern Willamette basins, taking a northwest-southeast swath across the basin between and straddling the Mt. Angel and Canby-Molalla faults. Efforts will focus on (1) compiling and interpreting well logs for this area for control on the top and thickness of the CRB aquifer, (2) analysis of cuttings to determine the volcanic stratigraphy and hydrologic properties, and (3) mapping of the CRB within the Sherwood 7.5' quadrangle. Gravity data will be collected to augment the currently sparse station coverage. These data will be used in conjunction with the well data to "strip off" the effects of the overlying sediments and determine whether this method, in conjunction with the existing high-resolution aeromagnetic data, is effective at mapping the thickness on the CRB.

Preliminary Research Results

Langenheim, V.E., McPhee, D.K., Morin, R.L., Blakely, R.J., and Wells, R.E., A Three-Dimensional View of the Tualatin and Northern Willamette Basins, Oregon, from Inversion of New Gravity Data: Abstract submitted to the American Geophysical Union Annual Meeting (2004).

U.S. Department of the Interior - U.S. Geological Survey - Geology Discipline
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