SOME GEOLOGIC FACTORS THAT INFLUENCE GROUNDWATER AVAILABILITY IN THE COLUMBIA RIVER BASALT GROUP (CRBG) AQUIFER SYSTEM IN THE WILLAMETTE VALLEY (WV), OREGON

The aquifers in the Miocene flood-basalt flows of the CRBG represent a significant potable groundwater resource in the WV. CRBG aquifers result from the interplay of intrinsic physical characteristics of the basalt flows and external pre-/post-emplacement factors. CRBG flows are giant sheet flows (flow volumes of 500 to >1,000 km3) which spread 300 to >500 km from their vents to reach the WV. Flows of the Grande Ronde Basalt comprise most of the CRBG section (up to 16 flows with a thickness of >300 m in northern WV and up to 9 flows with a thickness of <180 m in central WV) although locally the Frenchman Springs Member (Wanapum Basalt) can also be significant. Interflow zones typically host CRBG aquifers. Paleoenvironmental conditions during flow emplacement (e.g., presence of lakes/streams, general paleotopography, ground surface material (sediment vs. volcanic rock)) influenced the formation and extent of flow-bottom structures (e.g., pillow complexes, flow lobes) that can host aquifers. Flow dense interiors are essentially impermeable and act as aquitards. A sequence of CRBG flows can contain a series of individual confined aquifers. Post-emplacement geologic processes/events can significantly modified CRBG aquifer properties in the WV. Tectonic deformation in post-CRBG time has created faults that transect the CRBG section. Faults can be either barriers to, or pathways for, groundwater movement. Faults with stratigraphic offsets (>10 m) usually inhibit horizontal groundwater and in some areas (e.g., Stayton/Silverton/Salem) multiple faults create a series of hydraulically isolated CRBG aquifers. Tectonic fractures cutting the interior of CRBG flows can create limited secondary permeability with small yields (generally <10 gpm). Deep weathering of the CRBG in some areas (e.g., Salem, Eola, and Waldo Hills) has effectively destroyed the permeability in interflow zones, reducing the number of potential aquifers in the CRBG section. Hydrothermal alteration of CRBG interflow zones (e.g., Oregon City/Gladstone) associated with post-CRBG volcanism can locally eliminate permeability. Knowledge of both intrinsic physical characteristics of CRBG flows and external pre-/post-emplacement factors that modify them is essential for understanding the hydrogeology of the CRBG aquifer system in the WV.