TRACER-CONSTRAINED ESTIMATES OF GROUNDWATER RECHARGE TO THE COLUMBIA RIVER BASALT IN THE UPPER UMATILLA RIVER BASIN, OREGON

Multiple isotopic tracers were used to determine the recharge rate to the Columbia River Basalt (CRB) groundwater system in the 2,300 sq km upper Umatilla River basin (Oregon). Results of the study contradict several interpretations about the rate and mechanism of recharge to the CRB groundwater system that have been reported in the literature.

Across the study area, the CRB groundwater system is vertically stratified by age. The mean age of groundwater increased with depth, with a vertical gradient of about 60 years per meter. The vertical age gradient was similar across the study area and varied little with elevation, surficial geology, the amount of annual precipitation, or proximity to streams. The consistency of the age gradient across the study area indicates recharge largely occurs through vertical infiltration with minimal lateral input where interflow zones are exposed at land surface. Where interflow zones are exposed at the surface (primarily in the Blue Mountains), the ubiquitous horizontal lines of vegetation and springs indicate interflow zones largely discharge groundwater rather than provide routes for recharge.

Tracer-based recharge rates were less than 3 mm/yr, which is one to two orders of magnitude lower than most previously published estimates. Rates were calculated from the infiltration velocity at each well (depth of the well bottom / mean age of groundwater from the well) and a range of effective porosity values for oceanic and continental flood basalts obtained from the literature. Recharge rates were similar among all wells in the study despite differences in completion depth (16 m – 545 m), well construction, annual precipitation (400 mm – 1,500 mm), surficial geology, and proximity to perennial or intermittent streams. The recharge rate effectively was decoupled from the available water at land surface, i.e. precipitation and streamflow loss. This decoupling likely is due the thick, dense, low-permeability interiors of CRB flows which severely limit the vertical movement of water. The study found little evidence for fault-enhanced vertical infiltration but substantial evidence that major faults inhibit the lateral movement of groundwater.