Regional Hydrogeochemistry of Basalt Aquifers On the Columbia Plateau, South-Central Washington - Integration of Hydrologic Tracer and Mulivariate Statistical Studies

Hydrochemical zonation within an aquifer system, when evaluated in the context of the chemical evolution of groundwater, provides unique insights into regional flow patterns and the distribution and mechanisms of recharge. This approach was applied to the hydrologically complex setting of the arid Columbia Plateau basalt aquifers within the Columbia Basin Ground Water Management Area, using geospatial multivariate statistical analysis, geochemical modeling, and multiple hydrologic tracers (stable isotopes, helium-4, carbon-14, tritium) to reveal hydrochemical changes accompanying water-rock reactions and relate these to groundwater residence times. Spatial patterns of geochemical (cation ratio (Na+K)/(Na+K+Ca+Mg), nitrate, fluoride and carbon-14) and statistical (principal components scores) related to recharge sources/mechanisms and groundwater age/chemical evolution were mapped within the Wanapum and Grande Ronde basalt aquifers. Four sources of present-day recharge were identified: (1) surface waters with low dissolved solids and nitrate concentrations that locally recharge the shallower basalt aquifers where interflow zones are intersected by drainage channels (coulees) with perennial/episodic water flows; (2) areal infiltration of higher dissolved solids waters containing nitrate associated with irrigated agriculture; (3) regional recharge originating from the east and northeast boundaries of the aquifer system; and (4) isolated occurrences of chemically unevolved waters in regions characterized by more evolved groundwater, indicating a local effect due to cross‐connecting wells, pumping from multiple aquifers, or leaky surface seals. With the exception of these localized well effects, there is little hydrochemical evidence for modern recharge of the deeper Grande Ronde aquifers in the Odessa area, in the central plateau. This finding is supported by recently completed subsurface geologic mapping that indicates that the outcrop areas for these deeper aquifer zones are located along the northern boundary of the plateau, several hundred feet above the surface of the only potentially significant recharge source, Lake Roosevelt.