EFFECTS OF FAULTS AND STRATIGRAPHIC VARIABILITY ON GROUNDWATER FLOW IN COLUMBIA RIVER BASALT GROUP AQUIFERS

Faults that transect Columbia River Basalt Group (CRBG) flows often create linear zones of low permeability that affect the lateral continuity of groundwater flow in aquifers hosted by CRBG interflow zones. Faulting has been shown to disrupt the principal directions of anisotropy in the CRBG by offsetting tabular, permeable interflow zones and creating linear features of low horizontal and high vertical permeability. A series of cross-cutting faults thus may create hydrologically isolated areas within the CRBG and/or provide vertical pathways for groundwater movement between otherwise isolated CRBG interflow zones. The hydrogeologic significance of faults was recognized by early CRBG researchers on the basis of often dramatic head differences within the same stratigraphic interval on either side of a fault, and sharp departures from the ideal Theis response to pumping, indicative of a linear “impermeable” boundary.

Recent changes in long-term pumping, implementation of ASR pilot testing, and detailed geologic mapping of the CRBG in the Tualatin Basin and Central Willamette Basin have provided opportunities to evaluate the hydraulic characteristics of faults. Several recent studies have documented varying degrees of hydraulic continuity within individual CRBG units across some faults, including several of those with significant vertical and/or lateral displacement. A preliminary evaluation of hydraulic response across faults within the CRBG suggests a range of hydraulic behavior depending on the nature and age of faulting and the magnitude and duration of the hydraulic stress.

Complicating interpretation of hydraulic test data in the CRBG is lateral stratigraphic variability of individual basalt flows, which greatly affects the hydraulic properties of interflow zones. These “facies” changes result in hydraulic responses that are similar to “positive” (higher permeability) or “negative” (lower permeability) boundaries, and they may lead to an erroneous interpretation of the existence of a fault. The spectrum of hydraulic characteristics of faults illustrates the importance of developing a thorough understanding of the geologic framework of the CRBG aquifers to accurately interpret and predict responses to stresses.