Spatial and temporal gradients in aquifer oxidation‐reduction conditions

The study was undertaken to identify principal oxidizing and reducing chemical species in groundwater with the goal of determining the utility of platinum electrode (Eh) measurements to characterize subsurface redox conditions. Serial measurements of Eh and groundwater analyses were conducted in oxic and suboxic environments over more than a 2-year period for major ionic, oxidized, and reduced species. Vertical gradients in measured Eh values in the oxic groundwater environments can exceed −40 mV/m depth. In the poorly poised, oxic groundwaters, the Eh measurements correlated reasonably well with calculated values based on analytical determinations of O2 and H2O2 which was detected persistently in the 10−8 to 10−9 M range. The equilibrium calculated Eh values from this and other redox couples did not correlate well with measured Eh values over the 2-year study period. In the suboxic range, the average calculated values based on the Fe3+/Fe2+ coupled correlated well with averaged measured values. The results support the need to determine redox pairs in groundwater as a supplement to either calculated “equilibrium” Eh values or Pt electrode Eh measurements.

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