Redox evolution in glacial drift aquifers: role of diamicton units in reduction of Fe(III) |
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| Authors: | Nathaniel A Barnes Alan E Kehew R V Krishnamurthy Carla M Koretsky |
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| Institution: | (1) GeoTrans, Inc., 21335 Signal Hill Plaza, Suite 100, Sterling, VA 20164, USA;(2) Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, USA |
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| Abstract: | High iron concentrations create water quality problems for municipal use in glacial drift aquifer units. The chemical evolution
of oxic groundwater in shallow aquifer units to anoxic groundwater in deeper aquifer units, in which soluble Fe(II) is stable,
is attributed to coupled reduction of Fe(III) on aquifer solids with oxidation of organic carbon. The objective of this study
was to characterize the distribution of organic carbon in aquifer and aquitard sediments to determine the availability of
potential electron donors to drive these reactions. To do this, four complete rotasonic cores in a glacial aquifer/aquitard
system were sampled at close intervals for analyses of grain-size distribution and organic carbon content. The results indicate
significantly higher organic carbon concentrations in diamicton (till) units that function as aquitards, relative to coarse-grained
aquifer units. In addition, readily reducible iron content in the diamicton units and lower aquifer unit materials is sufficient
to produce far more dissolved iron than is present in the aquifer. Groundwater evolves to the level of iron reduction as a
terminal electron-accepting process as it moves downward through aquitard units along flow paths from upland recharge areas
to downgradient discharge areas. Deeper aquifer units are therefore unlikely to contain groundwater with low iron concentration. |
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