AN EXAMINATION OF SELECT WELLS NOT FOLLOWING THE DECLINING GROUNDWATER TREND SEEN IN COLUMBIA RIVER BASALT GROUP (CRBG) AQUIFERS NEAR MOSIER, OREGON

Mosier Creek basin is located in the eastern foothills of the Cascade Range in north central Oregon and is within the Columbia River Flood Basalt Province. The Columbia River Basalt Group (CRB) is series of tholeiitic flood lavas which cover approximately 210,000 km² of Oregon, Washington, and Idaho. Individual basalt flows are characterized as having thick, dense, low porosity and low permeability interiors and relatively thin, permeable zones at the flow tops and bottoms. This results in a series of stacked, thin, tabular aquifers which are confined by dense flow interiors and often have unique water level heads. In many locations across Oregon, the unique water level heads is a primary characteristic used to define discrete CRBG aquifers. CRBG aquifers generally have relatively high transmissivity, but very low storativity (10^-4 – 10^-5), and as such can readily be overdrawn.

During the past 45 years groundwater levels have declined over 40 meters in the CRBG aquifers in Mosier Creek basin. This decline has triggered multiple investigations into the cause and extent, with commingling and overuse identified as the primary causes of the declines. Thirty years ago the Oregon Water Resources Department closed the upper three basalt aquifers to further irrigation and municipal uses, subsequent monitoring showed continuing declines . Recently, new Special Area Well Construction Standards were emplaced to prevent future wells from commingling the aquifers. Currently the Mosier Million project is abandoning and replacing commingling wells. Additionally, a deep well was constructed last year to relieve some of the pumping stress from the main aquifers used in the basin.

While the water level declines in Mosier Creek basin CRBG aquifers have been problematic and lead to the various investigations and control measures described above, a few CRBG aquifer wells have displayed anomalously stable or even increasing water levels. This talk examines possible reasons to explain the water level behavior of two such wells. Spoiler alert: specific location, well completion, and nearby pumping play significant roles.