Evaluation of Compartmentalized Aquifers in the Walla Walla Subbasin of Oregon Using Isotopic and Geochemical Tracers

The Walla Walla Subbasin (WWSB) in Oregon is underlain by formations of the extensive Columbia River Basalt Group (CRBG) which have been deformed by post-Miocene folding and faulting. Extensive irrigation with groundwater from these basalt groups, as well as sedimentary aquifers and surface water diversions from the Walla Walla River, has enabled the WWSB to become a productive agricultural region despite its semi-arid climate. Over the past 10 years water levels in the basalt aquifer have declined to the point where some senior groundwater right holders cannot access their full water allocation. Effective water management depends on an understanding of the flow and recharge pathways in this complex groundwater system. This research uses spatial analysis of geochemical characteristics of groundwater samples to test the hypothesis that local faults create barriers to flow and isolate compartments of groundwater. For this study 31 wells were sampled for analysis of oxygen isotopes, hydrogen isotopes, pH, temperature, and conductivity. At a subset of 18 of those wells, samples were collected for a more extensive array of tests which also included carbon-14, tritium, and ten major ions. These parameters provide information to evaluate the range and distribution of groundwater age, evidence of modern recharge, and the progression of chemical reactions along hypothesized flow paths. Results of these laboratory analyses were compared with a literature review of fault geology and hydrogeology in the CRBG, results of interference pumping tests of basalt wells in the WWSB, and known physical characteristics of the wells sampled. This information was used to test three conceptual models of groundwater flow through the basalt aquifers. The first conceptual model was based on groundwater movement in unfaulted, sloping CRBG terrain where water infiltrates in upland recharge zones where the basalt is exposed, and moves laterally through the sloping interflow zones of the aquifer to wells. The second conceptual model describes local recharge in the vicinity of a well flowing slowly downward through the aquifer until the depth at which is became hydraulically connected to the well. The third model added faults into the model as lateral barriers to flow and vertical conduits to recharge. Comparisons of the monovalent/divalent cation ratios, stable isotope values, and principal component models with well depth, well surface level elevation, and well locations provide insights that conflict with the first two conceptual models and supported the third model. Further comparisons of these parameters with geologic characteristics of these faults from the literature through the lens of the third conceptual model provide a basis for estimating fault hydrogeologic characteristics in the WWSB.