RETHINKING RECHARGE TO THE COLUMBIA RIVER BASALT GROUP AQUIFER SYSTEM

Basaltic lavas of the Columbia River Basalt Group (CRBG) large igneous province (LIP) underlie approximately 210,000 km² of Oregon, Washington, and Idaho. The thick, laterally extensive series of flows (up to 100 m each) are highly productive aquifers and support billions of dollars of economic activity in the region. Groundwater mainly is produced from thin water-bearing zones that occur in the rubbly margins of individual lava flows, which typically comprise < 10% of the flow thickness. These productive zones are separated by thick, dense flow interiors, which have substantially lower permeability due to little primary or secondary porosity.

Recharge to the CRBG groundwater system typically has been estimated as a water budget residual and assumed to scale proportionally to the regional precipitation gradient. Recharge has been assumed to occur principally in high-precipitation upland areas where water can infiltrate into the rubbly flow margins, either where lava flows lap onto older rocks or are exposed at the surface.

Recent work in northeastern Oregon has questioned these recharge assumptions. Isotopic and gas tracers were used to estimate recharge rates to the CRBG groundwater system in this high-elevation, high-precipitation area of the LIP. Recharge was determined to be < 3 mm/yr – 1 to 2 orders of magnitude lower than most prior estimates – and independent of precipitation rates. Recharge largely was a localized process with no evidence for long-distance groundwater movement. Pleistocene groundwater was identified throughout this presumed regional recharge area.

Persistent (no/slow recovery) declines in groundwater level from pumping are widespread across the CRBG LIP, as is the occurrence of late-Pleistocene and early-Holocene groundwater. These observations are consistent with the small recharge rates and localized recharge identified in our recent study, and indicate that a regional reconsideration of recharge to the CRBG groundwater system is warranted.