Hydrogeology of the Columbia River Basalt Group in the northern Willamette Valley, Oregon

Abstract

ABSTRACT Basalt flows of the Columbia River Basalt Group (CRBG) host a series of regionally extensive aquifers between western Idaho and the Pacific Ocean that serve as an important source for domestic, municipal, agricultural, and industrial water supply throughout much of this area, and are the sole source for some communities in the Willamette Valley. Rapid growth and increased pumping have resulted in significant water level declines in some locales in the Willamette Valley, forcing some communities to develop other water sources, and/or develop aquifer storage and recovery projects to store water in CRBG aquifers. The CRBG generally consists of multiple concordant, tabular sheet flows. The primary water-bearing horizons within the CRBG are associated the vesicular and/ or brecciated flow top and flow bottom (pillow/hyaloclastite) structures that form the interflow zone between two flows. The interiors of the CRBG flows typically have limited vertical permeability and act as aquitards, creating a series of layered confined aquifers. The dominant groundwater flow pathway in the CRBG aquifer system is along these individual, laterally extensive, interflow zones. Tectonic structures may modify the dominant flow regime in the CRBG by offsetting or otherwise disturbing originally laterally continuous interflow zones. Faults result in a wide spectrum of effects on flow in the CRBG aquifers depending on the nature of the fault. The hydraulic properties inherent to CRBG aquifers, including high degree of confinement, low bulk permeability and limited recharge have led to overdraft conditions in many areas. Conversely, these characteristics create favorable conditions for aquifer storage and recovery system development in the central Willamette Valley and Tualatin Basin.