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Abstract
Reduction/oxidation (redox) conditions in 15 principal aquifer (PA) systems of the
United States, and their impact on several water quality issues, were assessed from
a large data base collected by the National Water-Quality Assessment Program of the
USGS. The logic of these assessments was based on the observed ecological succession
of electron acceptors such as dissolved oxygen, nitrate, and sulfate and threshold
concentrations of these substrates needed to support active microbial metabolism.
Similarly, the utilization of solid-phase electron acceptors such as Mn(IV) and Fe(III)
is indicated by the production of dissolved manganese and iron. An internally consistent
set of threshold concentration criteria was developed and applied to a large data
set of 1692 water samples from the PAs to assess ambient redox conditions. The indicated
redox conditions then were related to the occurrence of selected natural (arsenic)
and anthropogenic (nitrate and volatile organic compounds) contaminants in ground
water. For the natural and anthropogenic contaminants assessed in this study, considering
redox conditions as defined by this framework of redox indicator species and threshold
concentrations explained many water quality trends observed at a regional scale. An
important finding of this study was that samples indicating mixed redox processes
provide information on redox heterogeneity that is useful for assessing common water
quality issues. Given the interpretive power of the redox framework and given that
it is relatively inexpensive and easy to measure the chemical parameters included
in the framework, those parameters should be included in routine water quality monitoring
programs whenever possible.