REGIONAL GROUNDWATER GEOCHEMISTRY OF THE COLUMBIA RIVER BASALT AQUIFER SYSTEM, SOUTH-CENTRAL WASHINGTON

The hydrochemical evolution of groundwater in Columbia River Basalt Group (CRBG) aquifers within the Columbia Basin Ground Water Management Area (GWMA) in south-central Washington was investigated using exploratory data analysis methods in conjunction with geochemical modeling to identify chemical signatures and compositional vectors that (1) serve as proxies for groundwater age (residence time) and (2) can be used to identify, map and distinguish potential recharge sources on both regional and local scales. Spatial hydrochemical patterns that reflect the relative residence times of groundwater were mapped in three dimensions using geochemical signatures including the cation ratio (Na+K)/(Na+K+Ca+Mg), nitrate, fluoride and carbon-14, as well as multivariate analysis (hierarchical cluster analysis, principal components analysis), and reveal regional flow patterns and the nature and spatial distribution of recharge to different portions of the CRBG aquifer system. The hydrochemical analysis identified evidence for several contrasting sources of present-day recharge in the area: (1) surface waters of relatively low dissolved solids content that likely enter the shallower basalt aquifers locally where interflow zones are intersected by drainage channels (coulees) with perennial or episodic water flows; (2) infiltration of waters with higher dissolved solids content associated with agricultural activities mainly within the Columbia Basin Irrigation Project on the western side of the GWMA; (3) regional recharge from the east and northeast; and (4) isolated occurrences of relatively unevolved water in areas characterized by chemically more evolved groundwater, indicating a local effect due to cross‐connecting wells, pumping from multiple aquifers, or leaky surface seals. With the exception of localized well effects, the deeper Grande Ronde aquifer system in the central GWMA shows little hydrochemical evidence for ongoing modern recharge.