4.2 Oxidation-reduction capacities

Natural attenuation sometimes relies on the capacity of an aquifer matrix to supply electrons (reduction of pollutant) or consume them (oxidation of pollutant) to drive pollutant transformations and attenuation (Barcelona and Holm, 1991). In the case of reduction capacities, examples of electron sources include natural organic material (NOM), or reduced elements in minerals making up the aquifer matrix. Examples of pollutants that can be abated by reduction reactions include nitrate (to dinitrogen gas) via denitrification, chromium via a conversion from the mobile Cr6+ state to the immobile Cr3+ state, or the dechlorination of solvent compounds such as trichloroethene. Prior to the introduction of a pollutant, an aquifer may be near geochemical equilibrium, with the reduction capacity essentially stable and at steady state. The introduction of the pollutant begins a progressive consumption of the reduction capacity, from the source area to regions down-flow, as the pollutant contacts the matrix material (Figure 17).

Figures showing progression of contaminant plumes
Figure 17Progression of contaminant plumes (in plan-view with time advancing down the table) behaving conservatively (left) and susceptible to transformation by oxidation or reduction in an aquifer with a redox capacity (right). The plume growth is retarded by the mass loss due to reaction, and the aquifer redox capacity consumed progressively from left to right over time (center).

The same reasoning applies to aerobic aquifers except that an oxidation capacity is established in the matrix and is progressively exhausted by pollutants initially in a reduced state (e.g., petroleum hydrocarbons).

The velocity of the groundwater is highly determinative of the time that the aquifer will maintain its capacity to attenuate a pollutant. This relationship follows from simple mass balance considerations. The problem, in a highly simplified form, can be understood with the following analogy: an unending stream of hungry children moves down a grocery store aisle stacked with cookies. The children eat the cookies as they encounter them, first at the aisle entrance and then progressively deeper into aisle over time. The store owner can quickly appreciate that his inventory will be depleted sooner if the children run down the aisle rather than walk.

Real-world aquifers are to some degree heterogeneous and are therefore unlikely to lose their redox capacity in the purely progressive way described above. Pathways of faster flow will interlace with pathways of slower flow leading to a comparatively complex distribution of redox capacity changes in most cases. This could lead to earlier breakthroughs of pollutants than might otherwise be expected. Predicting the timing of these breakthroughs in some heterogeneous materials, requires detailed characterization of flow patterns, at the centimeter to meter scale.


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