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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Chemical Processes in Manganese Oxide and Carbonate Precipitation in Pinal Creek, Arizona

by

John D. Hem (U.S. Geological Survey, WRD, Menlo Park, Calif.) and Carol J. Lind (U.S. Geological Survey, WRD, Menlo Park, Calif.)

Abstract

Two water samples from well 503 near the head of the perennial reach of Pinal Creek and one sample of surface flow at the Setka Ranch crossing about 2 kilometers downstream from the well were used as the manganese-bearing solution in automated pH-stat titration with a 0.10 molar NaOH solution while bubbling CO2-free air into the mixture. The three samples had manganese concentrations ranging from 63 to 94 mg/L and dissolved solids concentrations ranging from of 3,500 to 3,570 mg/L, mostly consisting of Ca and SO4. The ground-water samples initially were about 5 x 10-3 molar in undissociated dissolved CO2 and had pH's near 6. The creek water had a pH near 7 and a CO2 concentration smaller than 10-3 molar. Titration of one ground-water sample done without removing the dissolved CO2 produced a mixture of hausmannite (Mn3O4) and kutnahorite (CaMn(CO3)2). The other two samples were treated to decrease the dissolved CO2 species before titration. One of these samples yielded mainly hausmannite during titration that altered on aging to manganite (MnOOH). The other sample yielded a mixture of oxides that altered on aging to a mixed Ca + Mn species similar to todorokite ((Mn2+, Ca) Mn4+5 O11 · 4H2O) in which most of the manganese is in the 4+ oxidation state. X-ray-diffraction studies of precipitates from Pinal Creek identified kutnahorite in carbonate cemented crusts near the head of perennial flow. The mixed Ca + Mn4+ oxides also were identified near the head of perennial flow and in black precipitates farther downstream.

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