INFLUENCE OF WELL CONSTRUCTION ON WATER LEVEL INTERPRETATIONS IN AQUIFERS OF THE COLUMBIA RIVER BASALT GROUP

The Columbia River Basalt Group (CRBG) hosts a series of laterally extensive, vertically stratified confined aquifers throughout its regional extent. In the CRBG, a fundamental geologic control is the vertical compartmentalization of stratiform water-bearing zones, which are contained in discrete, laterally extensive interflow zones that are separated by thick, laterally extensive layers of dense, massive, low-permeability basalt (the flow interior). Not all CRBG interflow zones are saturated, and those that are can have significantly different pressure heads. Given the hydraulic characteristics of individual CRBG water-bearing zones, correctly interpreting and evaluating the condition of CRBG aquifers from groundwater elevation data (static water levels) requires an understanding of both CRBG geology and well construction details. Uncased water supply wells in the CRBG often span and cross-connect multiple interflow zones and have water levels that represent composite conditions (pseudoequilibrium) produced within the wellbore. CRBG wells that are open to multiple interflow zones, often within different stratigraphic portions of the CRBG, only provide a measurement of a composite pressure head and do not provide any meaningful data for regional resource evaluation. In contrast, meaningful CRBG water level data can be obtained from wells that are cased and sealed into individual interflow zones that have been stratigraphically identified. For example, in the Columbia Basin Ground Water Management Area in south-central Washington, a state-monitored network of observation wells completed in individual interflow zones shows that groundwater levels in the Wanapum Basalt have been relatively unchanged during the past few decades, while groundwater levels have been declining in interflow zones of the underlying Grande Ronde Basalt. This type of vertical resolution in water level trends illustrates how an observation network containing wells that target stratigraphically specific interflow zones for production and monitoring allows spatial and temporal evaluation of groundwater conditions in a manner that a group of uncased wells open to multiple interflow zones (and CRBG units) cannot provide.