GROUND-WATER FLOW IN COLUMBIA RIVER BASALT GROUP AQUIFERS OF THE NORTHERN WILLAMETTE VALLEY, OREGON

Recent field studies, new geologic maps, and a new thickness and water-level map provide insight into the ground-water hydrology of Columbia River Basalt Group (CRBG) aquifers in the northern Willamette Valley, Oregon. The Miocene CRBG occurs in the northern Willamette Valley, Oregon, as a series of stacked flood-basalt flows that range in thickness from several hundred feet in the south to more than 1000 feet in the north. The CRBG underlies a thick sequence of sediments in the Portland, Tualatin, central Willamette, and Stayton structural basins and is exposed in uplands between the basins.

Physical factors uniquely related to flood-basalt emplacement and morphology create three conditions that control the hydraulic behavior of the CRBG aquifers: high horizontal permeability, low vertical permeability, and low storage capacity. Therefore, CRBG aquifers have moderate to high production potential but low recharge and storage capacities, factors that combine to make them susceptible to overdraft.

A generalized water-level map in the CRBG indicates that there is flow from upland recharge areas towards lowland discharge areas that coincide with the major basin stream. Local discharge in upland areas sustains streamflows in areas where stream drainages are incised through one or more of the uppermost basalt flows. Precipitation is the primary source of recharge to CRBG aquifers in upland areas. Water-levels in shallow upland wells show a direct response to precipitation, intermediate-depth wells show an indirect response, and deep wells show no apparent response. Vertical gradients in the uplands are steeply downward. Downward vertical gradients in the lowlands indicate that there is recharge from overlying saturated sediments. Hydrographs of most wells in the lowlands and deep wells in the uplands are dominated by seasonal pumping drawdowns and exhibit sustained recovery curves that peak in late spring when the annual pumping season begins. Progressive declines in water levels are common in many areas where pumping occurs.

Newly completed geologic maps show an abundance of faults that may compartmentalize CRBG aquifers by juxtaposing tabular permeable zones against relatively impermeable flow interiors. However, current data is inadequate to determine the effectiveness of faults as barriers to flow.