Multiple-Reaction Approaches - Transport and Fate of Chemicals in Soils: Principles and Applications - page 142

Calcite
growth

FeCO 3

CaCO 3

Qtz

HCO -

Fe ++

Qtz

Organics

HCO -

Fe ++

Biolm

OH + Fe ++

Ca ++ Fe ++

Fe(OH) 3

Qtz

Clay

Landll

O 2

Methanogenesis

O 2

SO 4 reduction

Aerobic respiration

Fe reduction

Denitrication

FIGURE 5.1
Schematic representation of the oxidation-reduction zones that may develop in an aquifer
downstream from an organic-rich landfill. Closest to a landfill may be a zone of methanogen-
esis, which is progressively followed downstream by zones of sulfate reduction, dissimila-
tory iron reduction, denitrification, and aerobic respiration that develop as the plume becomes
progressively oxidized through the influx of oxygenated water. Within the dissimilatory iron
reduction zone, a pore scale image is shown in which the influx of dissolved organics provides
electrons for dissimilatory iron reduction mediated by a biofilm. The dissolution of this phase
leads to the release of Fe 2+ , HCO 3− , and OH − into the pore fluid, potentially driving siderite
or calcite precipitation downstream, and thus reducing the porosity and permeability of the
material. Sorption of Fe 2+ may also occur on clays, displacing other cations originally present
on the mineral surface. Where reactions are fast relative to local transport, gradients in concen-
tration, and thus in reaction rates, may develop at the pore scale. (From C. I. Steefel et al. 2005.
Earth and Planetary Science Letters 240: 539-558. With permission.)

theoretical results. Urea hydrolysis, nitrification, and dentrification were
assumed to be governed by first-order kinetics as described above.
Unlike the N cycle, where transformation processes have been identified
for several decades, for other reactive chemicals, the governing processes of
sorption are at best ill defined. For example, reactive chemical transport in
soils may be complicated by precipitation reactions that act as kinetic sinks
for chemicals. Because of its agronomic and environmental significance, P
precipitation reactions during transport in soils have been most frequently
studied. Often P is a severely limited nutrient in terrestrial and aquatic
ecosystems, which adversely affects plant and fish production. Because
of the significance of P precipitation reactions in soil, the geochemical