Redox is fundamental to our understanding of nutrient and contaminant mobility and fate in natural waters. The solubility and toxicity of such species as As(III/V), Cr(III/VI) and Se(IV/VI) depend substantially on the dominant redox species of these elements. Unfortunately, the concept of aquatic reduction-oxidation processes is often misunderstood and misapplied despite warnings from Appelo and Postma (2007), Langmuir (1971, 1997), Stumm and Morgan (1996 or earlier), Berner (1981), Thorstenson (1984), Hostetler (1984), Lindberg and Runnells (1984) and others.

The key points are: (1) there is no such thing as a single redox potential for a natural water; (2) redox potential measurements (Eh), often do not reach a stable reading because of mixed potentials and because electroactive species are too low in concentration; (3) redox species of different elements do not coexist at equilibrium, disequilibrium is the rule; (4) there is no practical analogy between pE and pH (pE is a theoretical construct that has heuristic value, Truesdell, 1968); (5) analytically measuring redox species in solution records far more about redox chemistry of a water than any Eh measurement. Redox species of Fe, O₂, N, S, As, Sb, Se, and Cr, have been determined analytically for many decades and are known to quality laboratories, but not as much with those planning and directing water quality studies. That responsibility includes regulators, consulting firms, academicians, and industry firms, but it ultimately reflects on the education of those entering the field. Learning about pE-pH diagrams is useful in teaching, but not the tool to use for interpretation of water analyses. The science has advanced far beyond these diagrams of the 1960s as has analytical chemistry. Modern interpretive techniques have taken advantage of saturation indices, inverse modeling, isotopes, and element ratios that consider simultaneously the effects of temperature and complexing with a reasonably complete water analysis. Interpretation should include some guidelines for what minerals are likely to reach equilibrium or not based on knowledge of kinetics. All water chemistry and aqueous geochemistry courses should include both a field component and a lab component that puts into practice some of the principles learned in the classroom.