THE POMONA AQUIFER NEAR MOSIER, OREGON

The Pomona aquifer occurs in brecciated rock near the base of the Pomona Basalt adjacent to the axis of the Mosier syncline near the town of Mosier. Although the Pomona Basalt is relatively extensive within the Mosier syncline, the aquifer is restricted to a limited area of about 4 square miles within the basalt. The original distribution of the aquifer was primarily controlled by the development of the Mosier syncline which influenced the distribution of the basalt and the wet sediments and water that were needed to enhance porosity during the emplacement of the basalt. Post-emplacement faults have created additional boundaries within the aquifer. These stratigraphic and structural controls limit the capacity of the aquifer.

The Pomona Basalt is a 12 million year old lava flow within the Saddle Mountains Formation of the Columbia River Basalt Group (CRBG). The Pomona Basalt is underlain by older CRBG flows and overlain by sedimentary and volcaniclastic deposits of the Dalles Formation. Along the axis of the Mosier syncline, the Pomona Basalt is locally underlain by sediments of the Selah interbed, a portion of the Ellensburg Formation. The Pomona Basalt is about 200 feet thick near Mosier but thins to less than 80 feet within 1.7 miles on the up-dip flank of the syncline. The underlying Selah interbed is approximately 100 feet thick near Mosier but is absent 1.7 miles up dip. This geometry indicates that the basalt and sediment emplacement were controlled by the Mosier syncline, part of the Yakima Fold Belt which developed concurrent with the emplacement of the CRBG. The Pomona aquifer is largely restricted to areas underlain by Selah sediments near the axis of the syncline. This suggests that wet sediments and streams were the key agents needed to produce interconnected pore space during the emplacement of the Pomona Basalt. Outcrop occurrences of peperites and hyaloclastites support this conclusion.

The presence of multiple, local low-permeability boundaries in the Pomona aquifer are supported by aquifer test results as well as stratigraphic and structural analysis. Limited aquifer extent, low aquifer storage, and relatively high transmissivity make the Pomona aquifer highly susceptible to overdraft. Declines of at least 190 ft have occurred over the last 49 years.