GROUNDWATER AGE IN BASALT AQUIFERS OF THE COLUMBIA PLATEAU, OREGON, WASHINGTON, AND IDAHO

Groundwater withdrawals from Columbia River Basalt (CRB) aquifers underlying the Columbia Plateau (CP) support a multibillion dollar agricultural industry and serve as the primary drinking water supply for hundreds of communities across the region. Declines in groundwater levels are widespread across the CP. Among the principal questions surrounding the use and management of groundwater from CRB aquifers is the rate of recharge and potential to recover water levels in depleted aquifers. Natural recharge across the CP generally is less than 5 inches per year; substantially higher rates occur along the margin of the CP and in areas of irrigated agriculture. Recharge to the deep CRB units in many parts of the CP is likely slow due to the low or distant recharge, along with low vertical hydraulic conductivities of the CRBs. Extensive groundwater age dating in the northwestern portion of the CP has identified Pleistocene-aged water, as have limited samples from elsewhere across the region.

In 2016 a systematic sampling of 60 wells completed in CRB aquifers was undertaken by the U.S. Geological Survey National Water-Quality Assessment (NAWQA) project to better understand the distribution of groundwater age and quality across the CP. Multiple age tracers were collected to identify components of mixed-age water and dissolved noble gas samples were collected to constrain recharge conditions. Preliminary carbon-14 and tritium results indicate the widespread presence of premodern water (>70 years old) -- about 20% of the wells had results consistent with predominately pre-modern water, whereas fewer than 10% of had results consistent with predominantly modern recharge. Most contained varying mixtures of pre-modern and modern water. Age-tracer and water quality results will be incorporated into ongoing efforts by NAWQA to improve understanding of groundwater processes at a National scale and to improve management decisions at the local and regional level across the CP.