Solubility equilibria in basalt aquifers: The Columbia Plateau, eastern Washington, U.S.A.

Abstract

A speciation-solubility geochemical model, WATEQ 2, was used to analyse geographically-diverse groundwater samples from the aquifers of the Columbia Plateau basalts in eastern Washington. The groundwater samples compute to be at equilibrium with calcite which provides both a solubility control for dissolved Ca and a pH buffer. Amorphic ferric hydroxide, Fe(OH) 3 (a), is at saturation or modestly oversaturated in the few water samples with measured redox potentials. Most of the groundwater samples compute to be at equilibrium with amorphic silica (glass) and wairakite, a zeolite, and are saturated to oversaturated with respect to allophane, an amorphic aluminosilicate. The water samples are saturated to undersaturated with halloysite, a clay, and are variably oversaturated with regard to other secondary clay minerals. Equilibrium between the groundwater and amorphic silica presumably results from the dissolution of the glassy matrix of the basalt. The oversaturation with respect to the clay minerals other than halloysite indicates that their rate of formation lags the dissolution rate of the basaltic glass. The modeling results indicate that metastable amorphic solids limit the concentration of dissolved Si and suggest the same possibility for Al and Fe. The results also suggest that the processes of dissolution of basaltic glass and formation of metastable secondary minerals are continuing even though the basalts are of Miocene age and presumably have undergone a long history of contact with groundwaters. The computed solubility relations are found to agree with the known assemblages of alteration minerals in the basalt fractures and vesicles, if account is taken of both the difficulty of identifying amorphic phases and the slow rate of formation of clay minerals at low temperatures. Because the chemical reactivity of the bedrock will influence the transport of solutes in groundwater, the observed solubility equilibria are important factors with regard to chemical-retention processes associated with the possible migration of nuclear waste stored in the earth's crust. Specifically, the occurrence of secondary minerals will enhance the sorption of dissolved radionuclides. Speciation-solubility geochemical models provide an important means of determining these solubility-equilibria relationships.