Groundwater flowing through an organic-rich aquifer transitions from oxic to anoxic conditions. The sequence of electron acceptors consumed is: O2, then NO3-, then Mn(IV), then Fe(III), then SO4 2-, then CO2 (methanogenesis). What governs this sequence?
AThe concentration of each electron acceptor
BThermodynamic favorability -- each successive electron acceptor yields less free energy per electron transferred when oxidizing organic matter, so organisms preferentially use the most energetically favorable acceptor available
CMicrobial species appear in this order by chance
DThe sequence is controlled by pH, not redox
The terminal electron acceptor sequence reflects decreasing free energy yield. Aerobic respiration (O2) yields the most energy, so O2 is consumed first. When O2 is depleted, denitrification (NO3-) is next most favorable, followed by Mn-reduction, Fe-reduction, sulfate reduction, and finally methanogenesis. This thermodynamic ordering creates predictable redox zones in aquifers, sediments, and stratified water bodies. Organisms using each pathway outcompete those using less favorable acceptors as long as their preferred acceptor remains available.
Question 2 True / False
The Eh (redox potential) of a natural water can be measured accurately with a platinum electrode in all geological environments.
TTrue
FFalse
Answer: False
Platinum electrodes give reliable Eh measurements only in systems dominated by electroactive couples that equilibrate rapidly with the electrode (Fe2+/Fe3+ in acidic waters, H2S/SO4 in some systems). In many natural waters, multiple redox couples are in disequilibrium with each other, and the electrode responds to a mixed potential that does not correspond to any single couple. Dissolved oxygen, for example, does not equilibrate with a platinum electrode at ambient temperature. Field Eh measurements are therefore interpreted cautiously and often supplemented with direct analysis of redox-sensitive species.
Question 3 Short Answer
Explain why arsenic contamination of groundwater is often associated with reducing (anoxic) conditions in aquifers.
Think about your answer, then reveal below.
Model answer: Under oxidizing conditions, arsenic is adsorbed onto iron oxyhydroxide minerals (ferrihydrite, goethite) that coat aquifer sediments. When conditions become reducing (due to organic matter decomposition consuming dissolved oxygen), these iron oxyhydroxides undergo reductive dissolution -- Fe(III) in the mineral is reduced to soluble Fe(II), releasing the adsorbed arsenic into solution. Additionally, arsenate (As(V)) is reduced to arsenite (As(III)), which adsorbs less strongly and is more mobile. This coupled iron-arsenic redox process is the primary mechanism for arsenic contamination affecting tens of millions of people in Bangladesh, India, Vietnam, and other regions with organic-rich, reducing aquifer sediments.
The arsenic crisis illustrates how redox chemistry controls element mobility: arsenic locked on iron oxides under oxidizing conditions is released when reducing conditions dissolve the host mineral.