Hydrochemistry of intermediate flow systems associated with basin fill sediments derived from rhyolitic rocks [Hidrogeoquímica de sistemas de flujo intermedio que circulan por sedimentos continentales derivados de rocas riolíticas]
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An analysis of the reactions that characterize the chemical composition of an intermediate groundwater flow system was carried out. This system travels through basin fill sediments in a graven structure in central Mexico. Water-rock interaction in volcanic regions is usually related with the dissolution of primary minerals or of the glassy matrix of the rock. However, obtained geochemical calculations suggest that the water quality may be conditioned by the reactions with clayey minerals produced by the alteration of the volcanic rocks. Identified reactions include: a) incongruent montmorillonite and illite dissolution leaving kaolinite as a insoluble residue, b) exchange of sodium in clay minerals for calcium present in the groundwater, and c) calcite dissolution. A quantitative approximation was made through both inverse geochemical modeling and mass balance computations, which considered the established expected theoretical reactions. Additionally, laboratory experiments were carried out with rain water and basin fill sediments; the reactions identified in the laboratory through the analytical results agreed with those postulated for field conditions.
