The study uses new techniques to estimate recharge to the Columbia River Basalt Group (CRBG) groundwater system on the Umatilla Indian Reservation (UIR) in northeastern Oregon. The CRBG groundwater system on the UIR and across the larger 81,000-square-mile Columbia Plateau is an important resource for municipal drinking water and agricultural irrigation. The project supports ongoing Tribal water-right claims and settlement negotiations.

Recharge rates determined by this study are up to 10,000 times lower than prior estimates and were independent of variations in precipitation. The low rate of groundwater recharge is reflected by the ancient age of groundwater in wells sampled for the study – half contained water that was recharged at least 11,000 years ago, including wells in areas that were assumed to be in active recharge zones. Recharge is low due to thick, dense lava flows that make up the CRBG groundwater system.

The new recharge estimates indicate the CRBG groundwater system of the UIR is not recharged on meaningful human timescales and should be considered a finite resource.