Geochemical methods for evaluating the origin and evolution of ground water in volcanic rocks
A broad-based geochemical approach, including chemical and isotopic analyses of ground water, chemical analysis of aquifer materials, and laboratory water-rock experiments, was used to evaluate the origin and chemical evolution of ground water on La Palma, a volcanic island in the Canary Archipelago. Stable isotopes of oxygen, hydrogen, carbon and sulfur were successfully used to delineate recharge zones and identify solute sources. Laboratory study of water-rock interactions established the source of solutes and the nature of the chemical reactions that control ground-water chemistry. Most ground water on La Palma originates in a recharge zone that encircles the island from 500 to 1800 meters above sea level. Dry fallout and seaspray are minor sources of solutes, however, evaporative concentration during recharge may produce elevated chloride levels in some portions of the island. Laboratory water-rock experiments and ground-water analyses indicate that incongruent dissolution of aluminosilicate minerals is the dominant process of solute acquisition. The geochemical evolution of most waters is controlled by the availability of dissolved carbon dioxide gas. However, oxidation of pyrite enhances the reactivity of ground water in some portions of the island. Waters in the early phases of chemical evolution appear to be in equilibrium with a kaolinite alteration product, whereas more mineralized waters have compositions consistent with smectite equilibrium. Zones or compartments of ground-water flow were delineated by classifying sampling sites into geochemically distinct groups. Eight distinct zones of ground-water flow were identified using these criteria, including superimposed but hydrologically separate flow paths.
Hydrology., Groundwater., Hydrogeology -- Methodology., Geochemistry -- Methodology.
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