GEOLOGIC FEATURES IN THE COLUMBIA RIVER BASALT GROUP (CRBG) AQUIFER SYSTEM THAT FORM VERTICAL FLOW PATHWAYS AND SUBDIVIDE THE REGIONAL GROUNDWATER FLOW SYSTEM: EXAMPLES FROM THE COLUMBIA BASIN GROUND WATER MANAGEMENT AREA (GWMA) OF SOUTH-CENTRAL WASHINGTON

Studies in the 21,500 km² Columbia Basin GWMA are identifying geologic features in the regional CRBG aquifer system that impact recharge and groundwater flow. CRBG lava flows typically are laterally widespread, covering large areas (>12,900 km²). Aquifers within the CRBG generally are associated with intraflow structures at the top and bottom of individual sheet flows. Intraflow structures such as breccias, pillow lava complexes, and coarse sedimentary interbeds form interflow zones that often have excellent reservoir properties and can be exceptional aquifers. The interiors of thick, undisturbed sheet flows have very limited permeability, and form aquicludes and aquitards. The net hydrologic result of this is that CRBG water-bearing zones typically occur as a series of layered, planar-tabular, confined aquifers that extend many kilometers, and the dominant groundwater flow within these strata is parallel to the dip of individual, laterally extensive, interflow zones.

Groundwater movement in this stratiform system can be disrupted by geologic features. Groundwater movement between interflow zones occurs naturally where: (1) sheet flows pinch out creating limited (single flow) vertical connections where the flow interior ends; (2) erosional windows that connect one or more interflow zones; and (3) active faults provide open fractures across multiple, impermeable, layered, flow interiors.

Several of these features also act to limit lateral continuity within the CRBG aquifer system, including: (1) canyons and coulees (erosional features) which act as hydraulic drains in the shallow system, truncating the portions of the aquifer system they cross-cut and (2) faults and associated folds (such as Yakima Folds and regional monoclines) form major barriers to groundwater movement as deformation destroys primary porosity. Also, CRBG dikes which cross-cut the stratiform CRBG aquifer system break the lateral continuity of permeable interflow zones. The cumulative effect of these features is to form areas in the CRBG aquifer system where natural recharge may occur, and to compartmentalize the aquifer system, potentially forming sub-regional aquifer systems with limited interconnection.