INVESTIGATING GROUNDWATER / SURFACE WATER INTERACTION IN COLUMBIA RIVER FLOOD-BASALT NEAR MOSIER, OREGON

Tholeiitic flood lavas of the Columbia River Basalt Group cover 210,000 km² of Oregon, Washington, and Idaho. Individual Columbia River Basalt Group flows are typically tens of meters thick, with aquifers mostly occurring at the boundaries between flows where fractured flow top breccia and occasional basal pillowed lava form permeable zones. Aquifers in Columbia River Basalt typically have relatively high transmissivity, but very low storativity (10^-4 – 10^-5), and consequently transmit head pressure changes very rapidly. Columbia River Basalt Group aquifers are sometimes exposed within stream drainages where folded basalt units have been partially eroded through stream development. This occurs near Mosier, Oregon and elsewhere within the Yakima fold belt in Oregon and Washington.

Groundwater-level trends near Mosier have shown a nearly linear decline rate of about 1.1 meters per year (m/y) from 1974 to about 2004, when the slope of the trend abruptly decreased to approximately 0.3 m/y. The groundwater elevation after the change in slope roughly corresponds with the land elevation of the Pomona Basalt and Priest Rapids Basalt contact exposed in Mosier Creek; reflecting the influence of the stream as an aquifer boundary.

A stream gage was installed in 2012 downstream of the Pomona – Priest Rapids contact to augment an existing gage located just upstream of the contact in order to measure the exchange of water between the stream and the Pomona and Priest Rapids aquifers across the contact. Preliminary results indicate the changes in water exchanged between the stream and the aquifers parallel variations in groundwater-level elevations (drawdown and recovery) from seasonal pumping of local irrigation wells. Specifically, the stream changes from gaining to losing (or vice versa) across the contact at about the time the groundwater elevation drops below (or above) the basalt flow contact elevation. Additionally, the maximum groundwater drawdown occurs at a similar time as the maximum reach loss; 40% of the base flow in Mosier Creek during 2012 and 2013. Some temporal differences between the stream/aquifer exchange and groundwater levels exist at a sub-weekly time scale, but those may be attributed to differences in well proximities and individual well pumping schedules.