Reexamination of controls on hydrogeochemical evolution of groundwaters of the Columbia Plateau

Contemplated groundwater remediation efforts and proposals for new waste-handling facilities have refocused attention on the processes governing the chemistry of pristine groundwaters in the Columbia Plateau region of eastern Washington. Previous work has hypothesized that major-element chemistry and its spatial variability in these systems are controlled by gradual dissolution/precipitation reactions along regional flow paths through basalt. However gas- and aqueous-phase chemical monitoring indicates that groundwaters can be imprinted with the chemistry of deeper basalt units within the first few meters of their recharge flow path through overlying loessial sediments. Moreover new laboratory batch and column experiments suggest that major-element loading and attainment of equilibrium with respect to key phases could occur quickly in waters in residing in basaltic materials. Available field data suggest that 13C and 14C signatures of dissolved CO2 covary with major-element parameters which are, in turn, poorly reproduced in the laboratory experiments. This suggests that in situ microbial respiration exerts some control over hydrogeochemical variability