GROUNDWATER RECHARGE AND RESIDENCE TIMES IN THE COLUMBIA RIVER BASALT AQUIFER SYSTEM, WASHINGTON

As part of an ongoing study of the Columbia River Basalt Group (CRBG) aquifer system within the Columbia Basin Ground Water Management Area (GWMA) in south-central WA, selected irrigation, municipal, and domestic wells and surface waters were sampled for a suite of geochemical and isotopic tracers, including major and trace elements, stable isotopes (O, H, C), dissolved gases (He, Ne, Ar, N2), and age tracers (3H, 14C, CFCs, and SF6), to identify sources of groundwater and, in particular, to evaluate recharge relationships in the deeper parts of the CRBG aquifer system. Detectable CFCs, SF6, and 3H indicate relatively short residence times (ranging from a few years to a few decades) for groundwater from shallow sediment aquifers and the upper parts of the basalt aquifer system. Discordant recharge ages for multiple age tracers in samples from some supply wells that are open over large depth intervals indicate intra-borehole mixing of groundwater from different portions of the CRBG aquifer system. Apparent 14C ages in these wells range from recent to several thousand years, and likely represent a flow-weighted average (mixture) of the tracer concentrations in the aquifer system contributing water to the well. In the central part of GWMA, 14C, dissolved gas, and stable isotope data from deep (>500 m) irrigation wells completed and sealed into the Grande Ronde Basalt indicate that much of the deeper groundwater is more than 10,000 years old, and was recharged during the Pleistocene-early Holocene under cooler and wetter conditions than present. Traces of CFCs, SF6, and tritium detected in some of these wells suggest admixture of small amounts (<2 %) of modern water. While this could be partly due to sample contamination, the admixture is also likely indicative of a small degree of hydraulic continuity (natural and/or man-made) with the shallower portions of the CRBG aquifer system. This study demonstrates that natural recharge to the deeper parts of the CRBG aquifer system (i.e., Grande Ronde Basalt) throughout much of the GWMA is presently very limited, but was apparently higher in the past. This finding has significant implications for CRBG groundwater availability and renewability in the study area.