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Interpretation of pH and Eh trends in a fluvial‐sand aquifer system
Author(s) -
Jackson R. E.,
Patterson R. J.
Publication year - 1982
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/wr018i004p01255
Subject(s) - aquifer , geology , biotite , calcite , dissolution , carbonate , sulfate , mineralogy , plagioclase , geochemistry , groundwater , quartz , chemistry , paleontology , geotechnical engineering , organic chemistry
Ground waters and aquifer materials from a shallow fluvial‐sand aquifer system (1‐km‐long flow path) were sampled and analyzed to determine the hydrogeochemical processes affecting pH and Eh (measured Pt electrode potential). The aquifer system is a fine‐grained sand (K ∼ 10 −3 cm/s) composed principally of quartz and plagioclase (70% of all grains) with minor biotite (partially altered to vermicuiite) and trace amounts of calcite. Acid neutralization capacity measurements and X ray diffractograms suggest that acid precipitation recharging the aquifer undergoes a two‐step neutralization process; the first step involves reactions with the surfaces of mineral grains, mainly with plagioclase and biotite, and the second step involves an irreversible neutralization involving biotite alteration to vermiculite and carbonate mineral dissolution. During migration through deep confined parts of the aquifer ground waters attain equilibrium with calcite. A thermodynamically based redox model of a closed‐oxidant system satisfactorily accounts for the sequential reduction of dissolved oxygen, ferric oxides, and sulfate in the aquifer by the oxidation of dissolved organic carbon and (for oxygen only) ferrous iron. Because of the problems of using Pt electrodes in ground waters it is recommended that Eh measurements be supplemented with dissolved oxygen and sulfide measurements.