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Ground-water geochemistry of the Columbia Plateau aquifer system, Washington, Oregon, and Idaho

1996; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr95467

2332-4899

W.C. Steinkampf, P.P. Hearn,

Radioactive element chemistry and processing

Basalt aquifers of Miocene age are the major sources of ground water for agricultural, domestic, and municipal uses for the Columbia Plateau.Agricultural withdrawals and extensive surface-water applications in the Columbia Basin Irrigation Project have affected both ground-water chemistry and flow in some parts of the aquifer system.Ground waters primarily are dilute calcium-magnesium-bicarbonate and sodium-bicarbonate waters.The former generally are found in upgradient and recharge areas and the latter, downgradient and deeper in the system.Dissolved elemental concentrations, with the exception of divalent cations, increase downgradient.This general scheme is locally influenced by anthropogenic inputs and upward flux from deeper units.Spatial hydrochemical variations derive from water-rock interactions and the extent to which the aquifer system is isolated from the atmosphere.Solute sources are basaltic glass, plateau precipitation, soil zone and atmospheric carbon dioxide, and possibly interlayered organic material.Dissolution, by hydrolysis and carbonic acid, yields initially stoichiometric relative solute concentrations, but subsequent reactions, ground-water mixing, secondary mineralization, and possibly ion exchange alter initial relations.Dissolved oxygen oxidizes ferrous iron and aluminum, and ferric iron rapidly precipitates as amorphous aluminosilicates and iron oxyhydroxides.Both phases tend to become more crystalline over time.The aluminosilicate additionally contributes to the removal of calcium, magnesium, sodium, iron, and silicon from solution.As dissolution proceeds, the pH increases to a point at which, in the most evolved waters, silicon species are perhaps important in hydroxyl ion concentration buffering.Calcite precipitation controls or affects calcium, inorganic carbon, iron, and magnesium concentrations.Silicon appears to be controlled by the precipitation of a -cristobalite, and adularia and clinoptilolite have an additional effect in warmer and more evolved waters.Clinoptilolite also likely affects sodium and potassium concentrations, once they, together with silicon, reach sufficiently large concentrations.Data from a ground-water flow path in the Horse Heaven Hills in south-central Washington suggest that local mixing of upward-flowing older, warmer, and more evolved water with overlying water increases calcite oversaturation, with a resultant decrease in calcium and magnesium concentrations and an overall "freshening" of the mixed water.This is contemporaneous with glass dissolution.Mass-balance calculations along flow paths in the lowermost basalt unit in the central part of the plateau, and in the next uppermost unit in the Horse Heaven Hills, support the suggested conceptual model.Results also suggest that the relative importance of controlling phases varies with evolutionary progress and with position within the flow system.