Challenges to Interpreting Hydrogeologic Conditions in the Columbia River Flood Basalt Province (Pacific Northwest, USA)

The Columbia River Basalt Group (CRBG) is a sequence of more than 300 continental flood basalt flows that collectively range from several hundred to over ten thousand feet thick and cover more than 59,000 square miles in Washington, Oregon, and western Idaho. The stratiform CRBG aquifer system consists of vertically discrete interflow zones that are separated by layers of dense, massive, low-permeability basalt (the flow interior).  While most individual flows are present over large areas, mapped and inferred structural controls appear to laterally compartmentalize the aquifers. Most high-yield CRBG wells are many hundreds of feet deep, but can be 1,000 to over 2,000 feet deep in many areas within the flood basalt province.

 Difficulties understanding this deep hydrogeologic system arise from three challenges: (1) obtaining information and access from well owners, (2) mapping buried structures and evaluating how they affect groundwater flow, and (3) interpreting water level data given the prevalence of open-borehole wells that artificially connect multiple CRBG aquifers. The latter challenge is particularly acute because (1) each open-borehole well measures a composite pressure head from all contributing aquifers, and (2) groups of production wells in a localized area often are completed at different depths and span different aquifers. Accordingly, these wells provide only a generalized indication of groundwater conditions because the head measurements contain little useful information for quantifying the water budget of a given area. More valuable information can be obtained from pump tests and hydrological age tracer sampling at wells with known construction and known geology, and from observation wells that have water level records spanning many years. Nested observation wells targeting specific interflow zones are especially valuable in the basalt aquifer system because they provide vertical resolution in a manner that a group of uncased wells open to multiple interflow zones (aquifers) cannot provide.