As part of an ongoing study of the Columbia Basin Ground Water Management Area (GWMA) of south-central WA, parts of which have experienced substantial groundwater declines in recent decades due to large-scale pumping for agriculture, selected irrigation, municipal, and domestic wells were sampled for hydrologic tracers including stable isotopes, dissolved gases, and age tracers (CFCs, SF6, 3H, 14C, 4He) to evaluate groundwater sources, residence times and recharge mechanisms of the basalt aquifers. Detectable CFCs, SF6, and 3H indicate relatively short residence times (a few years to decades) for suprabasalt sedimentary aquifers and parts of the Wanapum basalt. Multiple age tracers yield discordant recharge ages for supply wells that are open over multiple aquifers, indicating intra-borehole mixing of groundwater. Apparent 14C ages in these wells range from recent to several thousand years and represent a flow-weighted mixture of different age waters. 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 was recharged during the Pleistocene (more than 10,000 years ago) under cooler and wetter conditions. Traces of CFCs, SF6, and 3H 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 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.
See more of: Topical Sessions